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Optical waveguide chemical sensors

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Optical waveguide chemical sensors
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Weiss, Martin Neil
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ix, 161 leaves : ill. ; 29 cm.

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Subjects / Keywords:
Ammonia ( jstor )
Dyes ( jstor )
Fluorescence ( jstor )
Light refraction ( jstor )
Plasmons ( jstor )
Polyimides ( jstor )
Polymers ( jstor )
Sensors ( jstor )
Waveguides ( jstor )
Wavelengths ( jstor )
Chemical detectors ( lcsh )
Dissertations, Academic -- Electrical and Computer Engineering -- UF ( lcsh )
Electrical and Computer Engineering thesis, Ph. D ( lcsh )
Optical wave guides ( lcsh )
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bibliography ( marcgt )
non-fiction ( marcgt )

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Thesis:
Thesis (Ph. D.)--University of Florida, 1996.
Bibliography:
Includes bibliographical references (leaves 150-160).
General Note:
Typescript.
General Note:
Vita.
Statement of Responsibility:
by Martin Neil Weiss.

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Copyright [name of dissertation author]. Permission granted to the University of Florida to digitize, archive and distribute this item for non-profit research and educational purposes. Any reuse of this item in excess of fair use or other copyright exemptions requires permission of the copyright holder.
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OPTICAL WAVEGUIDE CHEMICAL SENSORS


By

MARTIN NEIL WEISS








A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL
OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF
DOCTOR OF PHILOSOPHY

UNIVERSITY OF FLORIDA


1996









This work is dedicated to my parents,
Edward and Catherine













ACKNOWLEDGMENTS


John Donne once wrote, "no man is an island, entire of itself". This dissertation

stands as a testament to those words. As I look back on the various experiments,

processing techniques (read "tricks"), and theoretical models devised herein, I cannot help

but recall the vast myriad of people who aided me in the completion of this study. Most of

all I would like to thank Dr. Ramakant Srivastava, my advisor. In addition to providing me

with the wide range of resources and training I needed for my research, Dr. Srivastava was

a constant source of encouragement and support. Without his leadership and guidance, I

doubt I would have been able to accomplish as much in this work. I would also like to

thank Dr. Peter Zory, the cochairman of my Ph.D. committee, and Dr. James Winefordner,

for giving me access to their laboratories for materials characterization. Thanks also go to

Dr. Toshikazu Nishida, Dr. Stephan Schulman, and Dr. Ewen Thomson for serving on my

Ph.D. committee. I would like to thank my friends Howard Groger and Dr. Peter Lo of the

American Research Corporation of Virginia who were in fact the people who first sparked

my interest in the chemical sensor field. I am indebted to James Chamblee, Tim Vaught,

Steve Shine, and Frank Tavano for maintaining the Electrical Engineering department's

cleanroom where I spent many an hour fabricating my various sensor devices. I am also

grateful to Dr. Ben Smith for help in characterizing the fluorescence spectra of dye-doped

polymers and Dr. Sheng Li for the HeCd laser used in the later chapters. Some of the more
obscure tips on material processing were provided by Dr. Drew Roza of OCG

Microelectronics Materials, Inc., Dr. Charles Sullivan of Sandia National Laboratories,

and Richard Steppel of Exciton.

A portion of my dissertation work was performed during a brief visit to the Federal
University of Pernambuco in Recife, Brazil. I would like to thank Dr. Cid de Araujo, and








his colleagues Dr. Ricardo Correia, Dr. Anderson Gomes, Dr. J. F. Martins-Filho, and

Breno Neri for making my wonderful stay very productive.

I am grateful to the Microfabritech program at the University of Florida, the

National Science Foundation, and the American Research Corporation of Virginia for

funding this work.

Lastly, I am deeply grateful to my beloved fiancee Vicki and my dearest parents,

Edward and Catherine, for their unending love and support over these long years of

research during my career at the University of Florida.













TABLE OF CONTENTS

ACKNOW LEDGM ENTS .................................................................................. iii

A B STR A C T .................................................................. ........................................viii

CHAPTERS

1. INTRODUCTION TO OPTICAL CHEMICAL SENSORS .................1
1.1 Overview ............................................................ ................ 1
1.2 Organization ...................................... ................. .......... ...... 2
1.3 Sensor Evaluation Criteria ................................... ............. 2
1.3.1 Sensitivity ................................... .......... .............. 3
1.3.2 Selectivity ................................... .......... .............. 3
1.3.3 R eversibility ......................................... .............. 3
1.3.4 Cost Effectiveness ................................... ............ 4
2. NUMERICAL MODELLING OF OPTICAL WAVEGUIDES ............5
2.1 Review of Waveguide Theory .............................................. 5
2.2 Modelling Graded Index Waveguides: The Transfer Matrix ....8
2.3 Numerical Solution of the Transfer Matrix .............................12
2.4 Summ ary ......................................... .........................................13
3. SURFACE PLASMON WAVEGUIDE SENSORS ...............................14
3.1 Overview of Surface Plasmon Sensors .................................... 14
3.2 Theoretical Formulation of Surface Plasmon Resonance ..........15
3.2.1 Modelling of Surface Plasmon Waveguides ............... 18
3.2.2 Integrated-Optic Surface Plasmon Waveguide
Structures ................................... ...........................24
3.3 Experimental Investigation of Surface Plasmon Devices ..........29
3.3.1 Device Fabrication ..............................................29
3.3.2 Experimental Measurement of Refractive Index ........32
3.3.3 Humidity Measurement ............................................40
3.4 Proposed Surface Plasmon Structures With Improved
Perform ance ................................... .....................................43
3.5 Application of Surface Plasmon Resonance to Monolayer
D election ............................................................................49
3.6 Conclusion ................................................ ......................... 51








4. FABRICATION AND CHARACTERIZATION OF POLYMER
WAVEGUIDES ...........................................................................52
4.1 Advantages of Polymer Waveguides ........................................52
4.2 Fabrication of Polyimide Waveguides .....................................53
4.2.1 Substrate Preparation ....................................... ...54
4.2.2 W afer Priming ................................................ ..... 54
4.2.3 Polyimide Deposition ............................................56
4.2.4 Photolithography ................................... ............ 57
4.2.5 Curing of Polyimide Films .......................................58
4.2.6 Doping (Optional) ...............................................58
4.2.7 Endfacet Preparation ........................................ ...59
4.3 Characterization of Polyimide Films .......................................60
4.4 Summary ....................................... ....................................66
5. EVANESCENT WAVE SENSING WITH POLYMER
WAVEGUIDES ...........................................................................68
5.1 The Evanescent-Wave Absorption Sensor ..............................68
5.2 Detection of Aqueous Ammonia .............................................74
5.2.1 Choice of Sensing Layer Materials ...........................74
5.2.2 Fabrication of Oxazine-Doped Nafion .....................77
5.2.3 Characterization of Oxazine-Doped Nafion ...............79
5.2.4 Bulk Ammonia Sensor Response .............................81
5.2.5 Demonstration of Reversibility .................................87
5.2.6 Selectivity of the Nafion Response ...........................89
5.2.7 Waveguide Issues in Evanescent Wave Sensor
D esign ....................................................... ............91
5.2.8 Evanescent Wave Sensor Fabrication .......................92
5.2.9 Performance of Evanescent Wave Absorption
Ammonia Sensors .............................................92
5.3 Summ ary ....................................................................................100
6. FLUORESCENCE-EXCITED EVANESCENT WAVE ABSORPTION
SENSORS ........................................ ................. ............................ 101
6.1 Introduction ................................... ........................................101
6.2 Principle of Operation ..............................................................102
6.3 Theoretical Formulation of Fluorescence Capture by Guided
M odes ...................................................................................... 102
6.4 Sensor Fabrication ................................................................... 108
6.4.1 M materials ................................................................... 108
6.4.2 Waveguide Fabrication ............................................116
6.5 Sensor Characterization .......................................................116
6.5.1 Laser Pum ping .......................................................... 116
6.5.2 LED Pumping ........................... ......................117
6.5.3 Conventional Evanescent Wave Measurement ........... 121
6.6 Limitations of Fluorescence-Excited Waveguide Sensors ........122
6.7 Summary ........................................................... ................. 122








7. OPTICAL GAIN IN DYE-DOPED POLYMER WAVEGUIDES .........125
7.1 Introduction ................................... ..........................................125
7.2 Optical Amplification ........................................................... 127
7.3 Characterization of Optical Gain in Dye-Doped Polyimide
Waveguides ..........................................................................128
7.3.1 Active Waveguide Fabrication ..................................130
7.3.2 Experimental Set-up ...........................................130
7.3.3 Measurement of Optical Gain ...................................132
7.4 Optical Amplifiers as Chemical Sensors .................................137
7.5 Conclusion .............................................................................138
8. CONCLUSIONS AND FUTURE WORK ............................................139
8.1 Summary ............................................................................... 139
8.2 Future Work ............................................................................... 140
8.2.1 Improved Fluorescence-Excited Evanescent
Waveguide Absorption Sensors ..............................141
8.2.2 Active Waveguides for Chemical Sensing .............. 145
8.2.3 Polyimide Waveguides as Selective Chemical
Recognition Elements ..........................................147
APPENDIX

LIST OF ACRONYMS........................................................................148

REFERENCES ..........................................................................................150

BIOGRAPHICAL SKETCH .................................................................. 161













Abstract of Dissertation Presented to the Graduate School
of the University of Florida in Partial Fulfillment of the
Requirements for the Degree of Doctor of Philosophy

OPTICAL WAVEGUIDE CHEMICAL SENSORS

By

Martin Neil Weiss

December 1996

Chairman: Ramakant Srivastava
Major Department: Electrical and Computer Engineering


Optical sensors convert information about the surrounding environment such as

temperature and chemical composition into changes in light intensity and phase. Compact,

lightweight waveguide and fiber-based sensors offer a high level of detection capability

with an inherent immunity to electromagnetic interference. Several types of waveguide

sensors for chemical detection have been explored, both theoretically and experimentally.

Primary emphasis has been placed on optimizing sensitivity through the design of the

waveguide structure, rather than the sensing material.

Using surface plasmon resonance, sensors comprised of metal-clad dielectric

waveguides have been designed with enough sensitivity to measure refractive index

variations on the order of 10-5 and adsorbed film thicknesses of less than 1 nm. The basic

surface plasmon resonance (SPR) waveguide structure has been modified to include a

dielectric tuning layer, which simplifies the design process and reduces the number of

materials needed in sensor fabrication. A humidity sensor was created by coating an SPR








waveguide with a thin layer ofNafion, an ion-exchanging fluoropolymer whose refractive

index varies with atmospheric moisture content.

Conventional endfire-excited evanescent wave absorption (EWA) sensors were

produced by coating waveguides with materials whose optical absorption changes upon
analyte exposure. Ammonia in aqueous solution has been detected at sub-part-per-million
levels using polyimide waveguides clad with Nile-blue doped Nafion. A novel variant of

the EWA sensor, which uses fluorescence generated inside a waveguide to probe the

cladding absorption, was also studied. This latter device, known as the fluorescence

excited evanescent wave absorption sensor provides a higher level of sensitivity and is

virtually free from the stringent alignment tolerances that often plague other EWA sensors.

Lastly, a sensing technique based on measuring analyte-induced perturbations in
the optical gain of a waveguide amplifier or laser was proposed. An optical amplifier, with

a gain of 14.4 dB at 670 nm, was achieved in a cresyl violet-doped polyimide waveguide.

Unfortunately, fluorophores were found to exhibit minimal analyte sensitivity when

immobilized in the polyimide matrix.













CHAPTER 1
INTRODUCTION TO OPTICAL CHEMICAL SENSORS

1.1 Overview

Modem society's recent trend toward increased environmental awareness has led

to a need for development of advanced chemical detection systems. Sensors which utilize

optical detection techniques, such as fluorescence excitation, have proven to be highly

effective in this regard. Highly sensitive, compact, and lightweight optical sensors have

been demonstrated for a large number of chemicals.

Optical chemical sensors employ either bulk or integrated-optical (IO)
components, such as fibers and waveguides, as sensing elements. Analyte interactions

with the sensing element are converted into optical information, such as light intensity or

phase, through a number of transduction mechanisms. A wide variety of optical

phenomena can be used for analyte detection, including surface plasmon resonance,

fluorescence excitation, optical absorption measurement, and refractive index

perturbation. The most common examples of optical chemical sensors are fiber evanescent

wave devices and bulk surface plasmon resonance devices.

Unlike their electronic counterparts, optical sensors neither produce nor are

affected by electromagnetic interference. In many cases, optical sensors can respond more

rapidly and with a higher sensitivity than electronic sensors. Although optical device

fabrication is in general not as well-established as semiconductor processing, IO sensors
still benefit greatly from optoelectronic technology developed for the optical

telecommunications industry.








Of the various types of optical sensors, waveguide-based devices are in general

more versatile than bulk ones. This is particularly true of integrated-optic sensors, where

multiple sensing and reference channels can be built on the same substrate. Processing
functions needed for conditioning of the sensor output signal, such as filtering,

polarization splitting, and wavelength demultiplexing can also be performed on-chip by

additional waveguide structures. In addition, integrated-optic chemical sensors are smaller
and lighter than bulk ones, and can be easily deployed in remote areas via optical fiber

delivery. Despite the promising outlook though, commercial development of integrated

optic sensors has proceeded rather slowly, due to concerns about durability in harsh

environments and high packaging costs.


1.2 Organization

We begin with a brief review of waveguide theory in chapter 2 to establish basic

concepts and terminology. A numerical simulator for calculating waveguide mode field

distributions and propagation constants is also introduced. In chapters 3 through 7, four

optical waveguide sensors are developed: the surface plasmon resonance waveguide, the

evanescent wave absorption sensor, the fluorescence-excited evanescent wave absorption
sensor, and the chemically-sensitive amplifier. The waveguide principles underlying the

operation of each device are presented, along with modelling predictions. Considerable

effort has been devoted to the sensor fabrication and experimental characterization. In this

work, the primary emphasis will be placed on the advantages and disadvantages

associated with each structure, rather than the actual sensing chemistry.


1.3 Sensor Evaluation Criteria

Each of the sensors presented in chapters 3 through 7 will be evaluated with regard

to the criteria of sensitivity, selectivity, reversibility, and cost effectiveness.








1.3.1 Sensitivity

The sensitivity of a device with respect to a given analyte is defined as the

derivative of the sensor output with respect to the concentration of that analyte.1 The

minimum amount of analyte which produces a measurable response (i. e. above the value

determined by the signal-to-noise ratio) is known as the lower detection limit (LDL). The

range of analyte concentration over which the sensitivity is non-zero is known as the

dynamic range. Sensitivity can be improved through optimization of waveguide

properties. However, as will be seen in chapters 5 and 6, certain waveguide structures are

inherently more sensitive than others.

1.3.2 Selectivity

Selectivity is the ability of a sensor to differentiate between multiple analytes.1

Devices with low selectivity are prone to mistake one chemical species for another.

Sensors which contain analyte-specific receptors, such as proteins or antibodies, offer

highly selective responses. As will be seen in chapters 4 and 5, polymer-based sensors can

also have some inherent level of selectivity when matrices with analyte-specific diffusion

properties are employed. In addition, individual sensor elements with generalized (i. e.

nonselective) but differing analyte responses can often be combined into an array, forming

a highly selective sensor system.

1.3.3 Reversibility

Reversibility is the extent to which a sensor returns to its initial state after exposure

to an analyte.2 We use the term reversible to specifically describe systems which

automatically return to their pre-analyte state when placed in an analyte-free environment.

In some cases, irreversible sensors can be restored to their initial state by application of an

outside influence, such as a rinse chemical. We refer to this latter type of sensor as

resettable. Only sensors which can be fully restored to their pre-analyte condition can be








reused effectively. Devices which are neither reversible or resettable are limited to a single

trial.

1.3.4 Cost Effectiveness

Optical waveguide devices are notorious for having high packaging costs. This

single aspect, perhaps more than any other, has hindered widespread commercialization of

integrated-optic devices. Not surprisingly, disposable waveguide sensors that are

discarded after only one use are not economically viable. Waveguide-based sensors must

either be fully reversible (or resettable), thereby ensuring long operating lifetimes, or

utilize inexpensive packaging schemes in order to be cost effective. Thus, packaging

issues will play an important part in the discussions of each of the sensors presented

herein.













CHAPTER 2
NUMERICAL MODELLING OF OPTICAL WAVEGUIDES

2.1 Review of Waveguide Theory

We shall begin with a brief review of basic waveguide concepts and establish a few
definitions and terminology in order to facilitate our later discussion of guided wave

chemical sensors. When light travelling in a dielectric medium of refractive index n2 is

incident on a second medium with index n, < n2 as shown in figure 2.1(a), total internal

reflection (TIR) occurs when the angle of incidence, 0, exceeds the critical angle, defined

by

0e = asin (nl/n2) (2.1)

By bringing two such interfaces into close proximity, as shown in figure 2.1(b), light can

be constrained to propagate only in the direction tangential to the interfaces. This structure

represents a one-dimensional waveguide. The higher-index central region is referred to as

the core, while the lower-index surrounding regions are known as the claddings. In many

waveguide sensor applications, it is the top layer of index n1 which is exposed to the

analyte. This layer is called the superstrate or the sensing layer. It is also useful to

characterize the refractive index mismatch between the top and bottom claddings in terms

of the normalized waveguide asymmetry parameter, defined as3

2 2 2 2
aE = (n- n / (n2 n3) (2.2)

For energy to propagate a significant distance in the waveguide, two criteria must

be satisfied: total internal reflection (at both interfaces) and constructive interference at all

points along the ray path. As a result of the latter requirement, only a discrete set of rays,














Yz


\ 0 0
n2,:


ni (cladding)


n2> nl, n3


Figure 2.1 Light incident on dielectric media. n2 > nl
(a) single interface
(b) two interfaces waveguidee)


z
z


d -








corresponding to waveguide modes, can be guided. For the guided light, two polarization

states are possible. When the electric field, E, is perpendicular to the plane defined by the
direction of propagation and the interface normal vector, the field is said to be TE-
polarized. Conversely, when E is parallel to this plane, the field is TM-polarized.4 For the
TE guided modes, transverse field distribution can be written as


Ae3, y < 0 (2.3)
E(y) = Be + Ce- ,0
De-, (y-d) > d
where


i= ko n2-N2

Y1 = k -n2

3 = ko N2 n
ko = 2n/A0 (2.4)

where A, B, C, and D are constants, Xo is the vacuum wavelength, and N is called the

mode index. The total field associated with the ith TE waveguide mode is

E1 (y,z) = RE E(y)e-ji2 (2.5)

where ki = (27x/X0)Ni is called the propagation constant. In lossy waveguides, ki becomes
complex-valued, with an imaginary term which characterizes mode attenuation:

ki = kreal jkma (2.6)

The mode indices and field distributions correspond to the eigenvalues and eigenfunctions
of the waveguide structure and are obtained by solving the eigenvalue (characteristic)
equation. TM mode fields have forms analogous to those presented above for the TE case,
with some minor modifications.








As we will see later, the performance of a waveguide sensor depends critically on

the amount of power propagating in the sensing region. For the ith mode, the fractional
power propagating in the region yl < y < Y2 is defined as5

Y2 0
ri = (Ei x H) dy/ (E x Hi) dy (2.7)
YI -00


and simplifies to



TE = E2 (y)dy/E2(y) dy (2.8)
Y1 -00

for the TE polarization.


2.2 Modelling Graded Index Waveguides: The Transfer Matrix

We have used the well-known transfer matrix technique for solving the waveguide

eigenvalue problem for each structure. In this approach, a continuous refractive index

profile, n(y), is quantized into a series of slabs, each with a constant index value, ni, as

shown in figure 2.2(a). Application of boundary conditions at the interfaces between

adjacent slabs allows the mode indices and transverse field distributions of the structure to

be determined.

Starting with the TE polarization, with Ey = Ez = Hx = 0, it is evident from figure

2.2(b), that the total electric field in the ith layer, Exi, is the sum of the components
transmitted through the i-1h interface and reflected off the ith interface. Defining Ei' and

Ei- respectively as the components travelling toward and away from the ith interface and

the total depth d'i as

n (2.9)
d'i = ,di
i=2










n(y)


-z


x


y'


El


E2+ E2-




E3d
n3

4E4

n4


b)




Figure 2.2 Determination of waveguide mode indices and field distributions by the
transfer matrix method.
a) discretized refractive index profile, n(yi)
b) multilayer stack formulation of the waveguiding problem








where n is the total number of layers. The total field confined in layer i may be expressed

as


E = + -jkiyY y (2.10)
EXi~=Ee +E e d'i-i

where k = jn -2. From equation (2.10), it can be seen that real values of ky (ni > N)

yield sinusoidal solutions while imaginary values of kyi (ni < N) result in exponentially

growing or decaying fields. We expect the field in the core to be characterized by an

oscillatory solution while fields in the surrounding media decay. To solve for the actual

mode indices and fields, the tangential electric fields as well as their derivatives along the

y-direction are forced to be equal at each interface. Thus,

Exi = Ex(i-_) y=di_ (2.11)

and
dExi dEx(i 1) (2.12)
S =YY d'(2.12)
dy dy A


Noting that aExi/oy = kyExi and inserting (2.10) into (2.11) and (2.12) yields



E+ e-jkyid'i-I jkyid-i- (2.13)
y = d'i-I
= E 1e + E kiie I Y = d' I


+- i e-jky(i-)d'i+ k E jky~ U d
-k(i 1) i-1e y(i-1) i-i
(2.14)
= kyiE e-jk d.i-I + kyiE-ejkid'i -y = d'


Finally, through equations (2.13) and (2.14) the fields in the top layer can be expressed in








terms of those of the bottom layer as



E = M En (2.15)



where
n
M= mi (2.16)
i=2
and



1+k2ky e(ky(i-)-kyi)d'i- 2kyi j(ky(- + kyi) d'i-I

= y(i 1) y ky(i- 1) (2.17)
I Ji ( +k )ky)d'i- 1 + P2ki j (ky(i-1) kyi)d'i-
ky (i-i) ) ky(i -1)

In (2.17), P is a constant which equals unity for the TE polarization. When radiation

couples into one of the guided modes of a waveguide consisting of n layers, El" and En+
must be zero. Thus, determining the mode indices of the structure is equivalent to solving
for the roots of the equation M,1 (N) = 0. Inserting the values of mode index into
equation (2.15) yields the corresponding field distribution. Under this algorithm, the
integrated intensity distribution of each mode has an arbitrary value:

JEi (y)12dy = Ki (2.18)

where the subscript designates the mode order. For convenience, we define the normalized
mode fields as

ei (y) -E(y) (2.19)
K.i








so that

Plei(y)12dy= 1 (2.20)

The derivation of the transfer matrix for the TM polarization is analogous to the
TE analysis presented above. In the TM case, the fields are expressed in terms of Hx,
rather than Ex, and an identical transfer matrix is obtained, with the exception that in
(2.17), P = ni_ /ni.


2.3 Numerical Solution of the Transfer Matrix

In order to solve for the roots of the transfer matrix, the Newton-Rahpson
algorithm is employed6'7. In this method, the function Mil(N) is expanded in a Taylor
series about an initial point as8

M, (N+8) = M, (N) +M', (N)8+M", (N) 82/2+... (2.21)

where 8 is the difference between the estimated and the actual values of the root. Setting

M11(N+8) = 0 and retaining only the linear terms in (2.21) yields

8 = -M, (N)/M'1, (N) (2.22)

This correction is then added to the current estimate of the root and the process is repeated
iteratively until 8 becomes sufficiently small. Starting values for each of the roots of

M11(N) are found from a course sampling of the function, during which the structure is
assumed to be lossless.9 When multiple roots exist, the algorithm generally converges to
the one which is nearest to the initial estimate.
The Newton-Raphson technique is attractive for root finding because it converges
very quickly. In fact, it can be shown that the magnitude of the error associated with the
difference of the actual value of a root versus its value after a given iteration decreases
quadratically.7 In practice however, stability issues, such as the presence of local minima
in the function of interest, often make it necessary to allow the algorithm to update the








current estimated value of the root with only a fraction of the full Newton step. Since the

Newton-Rahpson technique is easily extended to solving multidimensional roots, it is

useful for determining the roots of lossy structures, such as metal-clad waveguides. We
have been able to duplicate theoretical results given by Harris and Wilkinson'o in the

modelling of a surface plasmon waveguide structure, thus confirming the accuracy of our

technique.


2.4 Summary

A basic introduction of waveguide concepts and terminology has been presented.

We have introduced the concept of fractional power which will figure prominently in the

remainder of this work. A numerical approach to solving the waveguide eigenvalue

problem has also been described. In the following chapters, the simulator will be used

extensively for modelling and optimizing a number of waveguide-based chemical sensors.













CHAPTER 3
SURFACE PLASMON WAVEGUIDE SENSORS

3.1 Overview of Surface Plasmon Sensors

Optical surface plasmon resonance (SPR) devices provide a highly sensitive
means for detecting environmental changes involving small perturbations in refractive

index. Sensors of this type have been employed in many diverse fields, ranging from

pollution monitoring11 and humidity measurement12'13 to immunoassay14 and molecular

self-assembly studies.15,16

SPR sensors operate by measuring changes in the refractive index of their

surrounding media.17 As such, they are inherently generic devices with regard to what is
actually detected. An SPR sensor can be made to detect a specific environmental

characteristic by coating the device with a layer of material whose optical properties are

changed by the occurrence of a particular event, such as exposure to a certain chemical.

Such a medium is often referred to as a transducing layer. In immunoassay studies, for

example, this is commonly done by taking advantage of the natural affinity of

complementary protein-ligand pairs.15 An SPR device coated with one material from the
pair becomes a sensor for the other. Antibody-antigen complexation may be monitored in
the same fashion.

Most commonly used SPR sensors consist of a dielectric prism with a metal
cladding of typically either gold or silver on one face.14'15'16 This arrangement is known

as the Kretschmann configuration. In this setup, a collimated beam of light shines into the
prism through one of the clear facets, reflects off the metal film and exits through the
remaining clear facet.18 The incident beam must be TM-polarized with respect to the








metal film. Surface plasmon excitation can be observed by monitoring the reflected power

as a function of the angle the beam makes on the front of the prism. At a particular angle

of incidence, Osp the reflectivity drops to nearly zero. Osp is highly dependent on the
refractive index of the surrounding media. However, these systems tend to be somewhat

cumbersome and are better suited to laboratory settings than field conditions.

A considerable improvement in the functionality of such sensors can be achieved

by taking an integrated optic (10) approach and using SPR waveguides. The 10 format

enables the realization of devices which are compact and lightweight and, furthermore,

allows the possibility of additional signal processing, such as polarization and wavelength
filtering, to be performed on-chip. In addition, multiple sensing elements and reference

channels can be incorporated into a single device. Guided-wave SPR sensors have been

explored previously using a number of geometries, including D-fiber,17 tapered fiber,19

side-polished fiber,20'21 and ion-exchanged GRIN waveguides.11,13,22,23


3.2 Theoretical Formulation of Surface Plasmon Resonance

A surface plasmon is a lossy TM-polarized wave supported by a metal-dielectric

interface.4'24 Physically, the plasmon wave is an optically excited electron plasma

oscillation in the metal. When the metal is a film of finite thickness, individual surface

plasmon waves are supported on both metal-dielectric interfaces. Furthermore, if the

metal film is sufficiently thin (on the order of the penetration depth of the optical wave),

these two waves will couple to form the so-called symmetric and antisymmetric bound

and leaky surface plasmon modes.20

The simplest integrated optic SPR waveguide sensor consists of a dielectric single

mode waveguide overlaid with a thin layer of metal. A dispersion plot of the uncoupled

waveguide TM mode and the plasmon modes is shown schematically in figure 3.1. When

the propagation constants of the waveguide mode and an SP mode are equal, the two
couple to form a lossy normal mode whose attenuation is proportional to the fractional



















kWG


kSB


XSPR


wavelength


Figure 3.1. Qualitative dispersion of the individual (uncoupled) TM propagation
constants of the waveguide, kWG, symmetric bound SP mode, k"B, and
antisymmetric bound SP mode, kAB. The normal TM mode of the composite
(coupled) structure is shown by the dashed lines. Field distributions for the
individual plasmon modes are shown in the inset.


symmetric antisymmetric
bound plasmon bound plasmon

dielectricmta
metal

dielectric/


-t








power propagating in the metal layer. When this condition, also known as phase-matching,
is not satisfied, propagation loss is considerably lower. SPR excitation in these devices is

thus strongly wavelength dependent. Wavelengths less than XSPR cause the TM

waveguide mode to couple primarily to the symmetric bound plasmon mode, while

wavelengths greater than this mainly excite the antisymmetric bound plasmon mode. TE-

polarized waveguide modes do not interact with the surface plasmons and experience a

small, relatively wavelength-independent loss due to the presence of the metal layer.

The response of the IO SPR waveguide is characterized in terms of the

polarization extinction ratio (PER), defined at a particular wavelength as the ratio of the

propagation losses of TM to TE-polarized normal modes, or

PER = AT- ATE (3.1)

where ATM,TE are the propagation losses of the TE and TM modes in dB/cm. The losses

are related to the normal mode propagation constants as

TE,TM TE,TM
A = 10log{exp(-2ki z)}/z (3.2)

where ki = (2r/A0) Ni, X0 is the vacuum wavelength, z is the propagation length, and Ni is

the imaginary part of the mode index. Since plasmon resonance induces large losses in the

TM mode, while leaving the TE one relatively unaffected, PER generally takes on
negative values. For single-mode waveguides, the PER varies linearly with device length.

In multimode devices, it is necessary to account for interference between the normal

modes when computing the extinction ratio.25 By exciting SPR waveguides with

circularly polarized light, the TE signal can be used as an internal reference for signal

normalization since its loss is relatively independent of the excitation wavelength. We

define the peak resonance wavelength, XSPR, as the wavelength at which the magnitude of

the polarization extinction ratio achieves its largest negative value. Perturbations to the

refractive index of the region immediately surrounding the sensor, also known as the

superstrate, affect XSPR by producing unequal changes in the propagation constants of the







uncoupled SP and waveguide modes. To simplify further discussions, we will refer to the
largest negative value of the polarization extinction ratio as PERma.




Air/Water
superstrate adsorbedd film)
0

high index tuning layer overlay
t. low index buffer layer
waveguide --
input signal substrate output signal


Figure 3.2. Schematic representation of the basic surface plasmon waveguide sensor.

3.2.1 Modelling of Surface Plasmon Waveguides

SPR waveguides have been modelled extensively, using the previously described
numerical simulation. The first device considered here is based on a K+-Na+ ion-
exchanged planar waveguide in BK7 glass, as shown in figure 3.2. In order to excite
resonance, several dielectric films a thin metal layer are deposited on the top surface of the
waveguide. We refer to these films, including the metal one, as the plasmon overlay. For
reasons that will be discussed later in section 3.2.2, the plasmon overlay in this case is
comprised of thin films of SiO2 (buffer), TiO2 (tuning), and gold, at thicknesses of 500
nm, 55 nm, and 30 nm respectively. The K+-Na+ ion-exchange in BK7 glass is well
characterized and produces high quality waveguides with a graded refractive index
(GRIN) profile given by26


n (x) = AnTE, TMerfc ( (x t) /dx), x to


(3.3)








where dx is the waveguide depth, to is the total thickness of the plasmon overlay, and
AnTETM are the respective surface index changes for the TE and TM polarizations
respectively. In order to ensure single mode operation in the red to infrared portion of the

spectrum, dx was chosen to be 3 gim. Although the K+-Na+ ion-exchange process in BK7
glass is known to produce birefringent waveguides, with AnTE = 0.008 and
AnTM =0.0092, only the latter value was used for modelling purposes since the

attenuation experienced by the TE mode is small (< 1 dB/cm) and relatively independent

of An. Refractive index dispersion data for the materials comprising the structure in figure
3.2 are taken from references 27-30.

In order to best illustrate the function of a basic SPR waveguide as a sensor, we

first treat the superstrate region as a thin film of index 1.415 adsorbed onto the surface of

the device and calculate PER as a function of wavelength. By 'thin,' we mean that the

thickness of the adsorbed film is less than the penetration depth of the evanescent wave at

the surface. The medium surrounding the device is assumed to be air (n = 1). As shown in

figure 3.3, prior to film adsorption, the calculated PER spectrum is essentially flat, with

the exception of a small positive-valued peak in the green region. This positive peak is not
related to the surface plasmon effect but rather arises from coupling between leaky TE
modes in the tuning layer and the guided TE mode. Upon the adsorption of a thin film,

surface plasmon resonance occurs and is evident as a large, negative-valued dip in the
PER. In this case, the full-width half-maximum (FWHM) of the resonance is about 30 nm.
XSPR increases as the adsorbed layer becomes thicker. At the same time, the magnitude of

pERmax increases since the optical waveguide mode becomes less tightly confined at the

longer wavelengths and interacts more strongly with the metal film. This effect continues

until the thickness of the film exceeds the penetration depth of the evanescent wave at the

surface, at which point XSPR becomes constant. Thus, film thickness can be measured
simply by monitoring the extinction ratio at a wavelength in the vicinity of SPR




















E -20
S" '*\ \" /
0 200 nm \\ *
S-40- --r v^
o -40 I
C
0
235 nm
S-60
o 1
300 nm \ i
\i
-80 "
1000 nm


-100
0.5 0.55 0.6 0.65 0.7 0.75
wavelength (lim)








Figure 3.3. Simulated device extinction ratio as a function of wavelength with a thin
adsorbed film of refractive index 1.415 as the superstrate. Adsorbate
thicknesses are indicated next to the respective curves.








Intensity distributions corresponding to the cases from figure 3.3 are shown for the

TM and TE modes at 670 nm in figures 3.4(a) and (b), respectively. Plasmon excitation,

evidenced as an enhancement of the TM field at the air/metal interface (x = 0 in

figure 3.2), becomes stronger at 670 nm as the adsorbed film thickness approaches

1000 nm. As expected, the increase in the fractional power propagating in the metal layer

results in higher loss for the TM mode. In contrast, the TE mode shows no surface field

enhancement.

Extending this analysis to the case of an infinitely thick superstrate, theoretical

plots of PER as a function of superstrate refractive index are presented in figure 3.5, using

several different excitation wavelengths. For each wavelength, there exists a unique value

of superstrate refractive index which maximizes the polarization extinction ratio. This

variation of PER with superstrate index can be used to measure the latter. We define the

minimum detectable change in the superstrate refractive index as



Anin Mx PER (n)
min -
=LPER (?, n)
anc (3.4)
n =n


where M is the signal-to-noise ratio (as a percentage) and no is the nominal superstrate

index. If we assume a superstrate index of 1.42 and a signal-to-noise ratio of 20 dB

(1% measurement precision), using a 670 nm excitation source yields Anmin 7x10-5

As is usually the case, there is a trade-off between sensitivity and dynamic range. In

regions where the derivative term in equation (3.4) is large, and the sensitivity is high,

refractive index can only be monitored over a small interval. Conversely, when the

derivative term is small, refractive index can be measured over a wider range of values,

but Anmi is larger. Measurement of the actual refractive index of the superstrate requires a

knowledge of the extinction ratio at two or more wavelengths.










0.50-
0.45
0.40 -
0.35
0.30
0.25
0.20
0.15
0.10
0.05
0.00
-0.5


0 0.5 1 1.5 2 2.5 3 3.5 4
position (um)

(a)


X-- = 670 nm








-------- Onm
300 nm -
6------ 1000 nm

0.5 0 0.5 1 1.5 2 2.5 3 3.5 A
position (um)


I.


Figure 3.4. Intensity profiles of the SPR waveguide at 670 nm with various thickness
superstrates of index 1.415.
(a) TM mode fields
(b) TE mode fields


0.50
0.45
0.40
0.35
0.30
-2
-2 0.25
' 0.20
(D
- 0.15
0.10
0.05
0.00

















20--
594 nm
-40 611 nm -- -



U \ "' I
-60 633 nm


c -80
SIV

-100 -670 nm I
I

-120

705 nm I
-140 i
1.34 1.35 1.36 1.37 1.38 1.39 1.40 1.41 1.42 1.43 1.44
superstrate refractive Index






Figure 3.5. Response of the surface plasmon sensor to changes in superstrate refractive
index. The superstrate is assumed to be infinitely thick. Excitation wavelengths
are shown next to their respective curves.








From a comparison of figures 3.3 and 3.5, it is apparent that in order to maximize

the derivative term in equation (3.4), the spectral width of the plasmon resonance must be

as narrow as possible. The resonance width is proportional to i/E2r, where Er and E, are the

real and imaginary parts of the permittivity of the metal layer, and in general decreases at

longer wavelengths.31 As we will see later, the resonance wavelength can be readily

controlled by the tuning layer. Thus, operating in the infrared region would improve

sensitivity. In addition, other metals, notably silver,32 support narrower plasmon

resonances than gold. Optimization of the sensor design parameters should allow Anmin to

be reduced to the order of 10.6 or less.

3.2.2 Integrated-Optic Surface Plasmon Waveguide Structures

The key to designing an SPR waveguide is to build the device in such a way that

resonance occurs at a convenient wavelength. Additionally, it is necessary to control the

PERmax, so that the TM output signal is measurable. The structures we have investigated,

shown in figure 3.2, address both of these issues.

As noted earlier, surface plasmon resonance is excited by placing a thin metal film

in close proximity to an optical waveguide, so that the TM-polarized waveguide and

plasmon modes are coupled. The thickness of the metal film needs to be on the order of

the penetration depth of the optical wave into the metal, typically about 20 to 30 nm, to

ensure maximum sensitivity to refractive index perturbations in the surrounding

environment. As seen in figure 3.6, changing the thickness of the gold layer used in the SP

waveguide design has a significant impact on both XSPR and PERmaX. As the metal

thickness is increased, XSPR moves to longer wavelengths, while PERmax decreases. At

the same time, the FWHM of the resonance broadens, resulting in decreased refractive

index sensing capability. From these calculations, it is evident that 20 to 40 nm of gold are

required for a useful device. Furthermore, it is evident that tight fabrication tolerances are

required to build SP waveguide devices with the desired resonance characteristics.

















-20


-40


-60


-80


-100


-120--


-140-
0.58


0.6 0.62 0.64 0.66 0.68 0.7


0.72


wavelength (m)






Figure 3.6. Extinction ratio as a function of wavelength for several different metal (gold)
layer thicknesses. The device is coated with a 1 pm thick adsorbed layer of
refractive index nc = 1.415.







In this configuration, propagation losses are controlled by the insertion of a low
index buffer layer (figure 3.2), placed between the waveguide and the metal film, as
demonstrated in figure 3.7. Since the ion-exchanged waveguide is slightly asymmetric, the
evanescent wave in the silica buffer layer decays rather rapidly and as a result, only a
relatively thin buffer layer of less than 1 pgm is required to achieve an acceptable PER.
Decreasing the thickness of the buffer layer from 750 nm to 400 nm increases PERm" by

an order of magnitude, from about -20 dB/cm to -200 dB/cm at 670 nm. In the quest to
minimize Anmin, losses need to be maintained at levels which are measurable. Metal-
induced losses to the TE mode, which are nearly 2 dB/cm when the buffer thickness is 400
nm also need to be minimized. An important consequence of this design is that XSPR is
virtually unaffected by buffer thickness.
The primary feature that sets these designs apart from others17'20,22,25 is the

inclusion of a high index dielectric tuning layer deposited either directly above or below
(figure 3.2) the metal layer, which provides a large degree of flexibility in choosing the
resonance wavelength. The function of this layer can be explained by a simple coupled-

mode argument: by virtue of proximity, the propagation constants of the SP modes are
strongly dependent on the thickness of the high index layer, while those of the waveguide

modes are not. Thus, in accordance with figure 3.1, changing the thickness of the tuning

layer shifts the resonant wavelength. In figure 3.8, the variation in XSPR is shown against
tuning layer thickness. The peak resonance wavelength can be easily tuned over a 130 nm
range, while maintaining a reasonable level of TE loss. )SPR is very sensitive, changing

by over 2 nm per nanometer of tuning layer thickness, which again underscores the need
for close tolerances during fabrication.
Conventional SPR waveguides lack the tuning layer and require that XSPR and the
PERmax must be set by simultaneously adjusting the metal and buffer layer

thickness.17'20,22'25 Waveguide parameters, such as maximum index change and depth
which determine mode index, are also important in determining XSPR in conventional SPR













50 -



O


E

-50 -
o


o-100 -
-OO



51
-150 -


7!
-200 -
0.58


14 2



10 -
C
0
8 5

6 u
l-


0.6 0.62 0.64 0.66 0.68


wavelength (pm)









Figure 3.7. Extinction ratio as a function of wavelength for several different buffer (SiO2)
layer thicknesses. The device is coated with a 1 ugm thick adsorbed layer of
refractive index nc = 1.415.














710

690

E 670

1 650

S630

610

1 590

* 570
0.
550

530


7.0


0 10 20 30 40 50 60 70 80
tuning layer thickness (nm)






Figure 3.8. Peak resonance wavelength and TE insertion loss at resonance as functions of
tuning layer thickness. The device is coated with a 1 pIm thick adsorbed layer
of refractive index nc = 1.415.








devices. In contrast, the tuning layer design allows PERma" and XSPR to be set relatively

independently of one another. Furthermore, the large flexibility in choosing XSPR in tuning
layer-equipped devices allows the waveguide characteristics and metal film to be chosen
fairly independently.
An alternative approach to tuning the resonance wavelength is to use electro-optic
(EO) materials, such as liquid crystal, in the design of the SPR device.33 In this case, an
EO material is positioned next to the metal layer of the plasmon device. Applying a
voltage across the EO material changes its refractive index which consequently alters
XSPR. Thus, incorporation of EO materials in SPR devices allows active control of the

plasmon response. Shifts in resonance wavelength of almost 200 nm have been achieved

by applying 30 volts across a liquid crystal layer in a bulk SPR device.33


3.3 Experimental Investigation of Surface Plasmon Devices

3.3.1 Device Fabrication

The SPR sensors were built on planar K+-Na+ ion-exchanged waveguides. BK7

glass substrates were cleaned by sequential immersion in trichloroethane, acetone, and
methanol for 10 minutes each. The substrates were then rinsed with deionized water,
blown dry with a filtered nitrogen gun, and baked in an oven at 90 C for 1 hour. The ion-

exchange was performed by placing the substrates in a bath of molten potassium nitrate
housed in an aluminum vessel at 375 C for 3.5 hours, as shown in figure 3.9. This

produces a birefringent waveguide with AnTE = 0.008 and AnTM = 0.0092, and dx = 3 pm.

For wavelengths above 600 nm, the waveguide is single-moded. A thermocouple and
temperature controller (Omega CN8500) were used to maintain a constant bath
temperature. The samples were then removed from the melt and cooled slowly to avoid

thermal stress-induced cracking and rinsed in deionized water. Next, thin films of SiO2,
TiO2, and gold were deposited sequentially on top of the waveguide in an electron-beam















Thermocouple


Aluminum
Container


Figure 3.9. Fabrication apparatus for constructing potassium-sodium ion-exchanged
waveguides.








evaporator, at thicknesses of 500 nm, 55 nm, and 30 nm, respectively. Lastly, the substrate

was cut into 1 cm squares, and endfacets were polished to permit endfire excitation.

In the polishing process, the sample was first waxed to a dummy piece of glass

with a low temperature wax to prevent chipping of the edges. The sample/dummy

composite was then mounted vertically with wax in a vise-like holder, with the endfacet

held parallel to the plane of polishing. A paste consisting of polishing powder (SiC or

A1203 from Buehler) and water was prepared on a clean, flat sheet of glass and the sample

was moved across this paste (by hand) in a figure-eight pattern for a few minutes. It was

polished sequentially with 400 grit until the edge was flat (typically less than 5 minutes),

20 gpm size for 5 minutes, and 5 pm size for 10 minutes. Between steps, the sample was

rinsed thoroughly with water. Next, the sample was placed on a mechanical polishing

wheel and ground on a 3 pm pad wetted with 1 p.m alumina paste for about 60 minutes. At

this point, the endfacet was generally free of major scratches. The sample received its final

polish on a soft TEXMET pad (Buehler), wetted with MASTERPOLISH 2 (Buehler), a

colloidal suspension of 0.06 p.m silica in a high pH solution. MASTERPOLISH 2

provides a chemical/mechanical polishing action, for optimum results. The final polish

typically took about an hour, after which the endfacet appeared completely free of

scratches under a 20X microscope lens. The sample was typically about 9 mm long after

polishing.

It was found that at first, the polishing process tended to cause extensive scratches

to the exposed and highly fragile gold film. To alleviate this problem, a 75 nm layer of

SiO2 was evaporated over the gold prior to polishing. The superior abrasion resistance of
this top SiO2 layer, referred to as the cap, allowed the devices to be polished with minimal

damage to the gold layer. The modelling shows that the addition of the cap will shift the

plasmon response to slightly longer wavelengths.








3.3.2 Experimental Measurement of Refractive Index


Figure 3.10 shows the experimental arrangement for the device characterization.

Light from a monochromator (Digikrom 240, dispersion 3.2 nm/mm) was collimated with

a 25 mm focal length lens, passed through a chopper (Rolfin-Sinar), and fed into a 100 p.m

core multimode fiber through a 20X microscope objective. The fiber was then butt-

coupled directly to the SPR waveguide. Light leaving the waveguide was collected by a

combination of a 40X microscope objective and a cylindrical lens, passed through a high

extinction (30 dB) Glan-Thompson polarizer to resolve the individual polarization

components and ultimately detected with a silicon photodiode. A slit aperture was placed

on the front of the photodiode to serve as a spatial filter and block light launched into

substrate radiation modes. A lock-in amplifier was used to improve the signal-to-noise

ratio. With the monochromator output slit at 2 mm, corresponding to a spectral resolution

of 6.4 nm, typical output powers collected from the waveguide were on the order of a few

tens of nanowatts at wavelengths off resonance.

The polarization extinction ratio of the SPR waveguide was measured by coating

the device with different index matching oils.34 The stated accuracy of the refractive

indices of these fluids is +/- 0.0002. Transmission spectra for the TE and TM modes were

measured separately by adjusting the output polarizer. In order to compute the PER, the

loss terms from equation (3.1), ATE and ATM, are expressed as dB per unit distance as

TE,TM (1 pTE,TM pTE,TM (3.5)
A = 10- log (Pin / out )


where L is the length of the waveguide and Pin and Pout are the power coupled into and out

of the waveguide. In this experimental configuration, only the output power, Pout, can be

readily measured. In our launching scheme, using incoherent light and a multimode fiber,

it is reasonable to assume that the fractional powers launched into the TE and TM modes










monochromator




lens (f= 25 mm)


SPR waveguide
I Szzzvgwe


OZZ~


chopper


20X lens



multimode fiber







40X lens


cylindrical lens


polarizer


slit (spatial filter)


photodiode


Figure 3.10. Experimental setup for characterizing the SPR waveguide sensor.


computer








are approximately equal. This allows equation (3.1) to be rewritten as
PER= TMiO log TE
PER = -10 log (Pout/P out)
L )(3.6)

In figures 3.11 (a) (d), the TE and TM transmission spectra and corresponding PER are
shown, for superstrate refractive indices of 1 (air), 1.340, 1.380, and 1.410, respectively.
Plasmon resonance, evidenced by a superstrate-dependant decrease in the TM

transmission over a narrow wavelength interval, is clearly observed. In accordance with
theoretical predictions, the resonance shifts to longer wavelengths as the superstrate index
increases. As expected, the TE transmission spectra are relatively unaffected by the

superstrate refractive index.
The wavelength dependance of the polarization extinction ratio is summarized and
compared against the theoretical model (updated to include the SiO2 cap) in figures 3.12
(a) and (b), with slightly better PER than shown in figure 3.11. This improvement was
realized by using a narrower slit over the detector. Experimental PER values are about an
order of magnitude less than those predicted by the model. In addition, the peak resonance

wavelengths are about 30 nm greater than expected. The large discrepancy in PERma
between theory and experiment is most likely due to imperfect spatial filtering of light
launched into substrate radiation modes. PER is computed from the loss of the guided

modes, in particular, from the guided TM mode which excites plasmon resonance.
Radiation modes do not significantly interact with the surface plasmon modes supported
by the gold layer, and as such, experience minimal polarization-dependant loss. TM-

polarized radiation modes which reach the detector can easily overwhelm the strongly-
attenuated signal from the guided TM mode, resulting in the appearance of a larger TM
signal at the detector and causing the TM loss term in equation (3.1) to seem considerably
smaller. While this problem could in principle be circumvented to some extent by using a
single-mode fiber to excite the waveguide, it was found to be extremely difficult to couple















040-
30







-10- 0


0 .0 -10
500 550 600 650 700 750 800
wavelength (nm)

(a)

80 -- 15



so-

505
70 ---- -------
0 / 10


o0 ,. .,-- .








S0 -0 550 600 650 700 750 8050
040








30 (b) = 1.340
20-

'" PER

600 550 600 650 700 750 800
wavelength (nm)

(b)


Figure 3.11. Transmission and polarization extinction ratio spectra for the capped SPR
waveguide sensor for differing superstrate indices
(a) nsup = 1 (air)
(b)nsup= 1.340
(c) nsp = 1.380
(d) nup = 1.410










60 -15
TE



0 /\ .TM --10


30-
0 0
70 -- -5_










60 -A- .--
420-







PER -






0-
TM








90 T. M.\... -8


10 0---- 1 -- ---- --



500 550 600 650 700 750 800
-2










wavelength (nm)

(d)



Figure 3.11. Transmission and polarization extinction ratio spectra for the capped SPR
waveguide sensor for differing superstrate indices
(a) nsup = 1 (air)
(b)np= 1.340
0-












(c) nup = 1.380
(d) np = 1.410










40



II 0
0 \
---- n=1------


-40

-60 .
n 1.310 :
-80 n 1.335
.. .----- n 1.380
-100 i .
50 50 600 0 60 650 700 750 800
wavelength (nm)

(a)



15.00-
Sn= = 1.000
10.00 --- n1.310
n n1.335
S5.00 -........ n -1.380


0.00- -\--







-15.00
500.00 550.00 600.00 650.00 700.00 750.00 800.00
wavelength (nm)

(b)

Figure 3.12. Polarization extinction ratio for the capped SPR waveguide sensor
(a) theoretical prediction
(b) experimentally measured response








a detectable amount of power into such a fiber, using the available equipment. In
particular, a tunable laser would be highly desirable.
To a lesser extent, defects in the gold film may have also contributed to some

degradation of the PER.21 From figure 3.6, it is clear that variations in the thickness of the
gold film significantly affect resonance characteristics. Metal films with thicknesses on
the order of a few tens of nanometers tend to be somewhat porous and exhibit grain

boundary-related roughness, which contributes to light scattering and possibly
polarization conversion. Some evidence of metal roughness is seen in the width of the

plasmon resonances shown in figures 3.12(a) and (b). The measured resonance width is

about 60 nm, roughly twice that predicted by the simulation. In addition, the refractive
index of metal films varies with both the film thickness and the deposition technique.
Refractive index data used in modelling were taken for a 50 nm thick gold film,24

compared to the 30 nm thickness in our experiment. This may account for the difference in
resonance wavelengths between theory and experiment. High temperature annealing could

have been employed to improve the quality of the gold layer, but would have altered the

waveguide characteristics, complicating the overall design of the device. From figures 3.6
and 3.8, it is also apparent that errors in the thickness of the gold layer, as well as that of

the TiO2 tuning layer would result in a shift in XSPR

To calibrate the SP waveguide for refractive index measurements, the polarization
extinction ratio was measured at 658 nm and 708 nm, as a function of superstrate index.

This data is shown in figure 3.13. This device is clearly useful in measuring refractive

index over a wide range, depending on the choice of excitation wavelength. Using
equation (3.4), a sensitivity, or minimum detectable index change of -7x105 had been
predicted for this device, at a signal-to-noise ratio of 13 dB. However, as noted earlier,

experimentally measured PER values were found to be about an order of magnitude less

than predicted. Consequently, the minimum index change which was experimentally
resolvable was only about 5x10 .
















S-2-



-C 0
l O _\ _








-12



1.3 1.32 1.34 1.36 1.38 1.4 1.42
Superstrate Refractive Index

0 658 nm, measured
] 708 nm, measured
658 nm, theoretical, +-10
Q -12 -------------








-708 nm, theoretical, +10


Figure 3.13. Theoretically predicted (lines) and experimentally measured (symbols)
polarization extinction ratio against superstrate refractive index for excitation
wavelengths of 658 and 708 nm. Theoretical curves have been scaled to be a
factor of 10 smaller for comparison purposes.








3.3.3 Humidity Measurement

Many materials, in particular organic polymers, exhibit a humidity-dependant

refractive index. By coating a thin layer of such a material onto the surface of an SP

waveguide, it is possible to create a device in which the PER at a given wavelength varies

with atmospheric moisture content. In this manner, the SP waveguide can be used as a

humidity sensor. It was observed that Nafion fluoropolymer exhibits a tendency to swell to

the point of cracking when immersed in water. As such, this material was deemed to be

appropriate for use as a humidity transducing layer.35 Nafion fluoropolymer was obtained

from Aldrich as a 5% polymer solution in a mixture of lower aliphatic alcohols and water

and diluted to 1% by the addition of methanol. A few drops of the dilute solution were

deposited onto the SP waveguide and allowed to dry at room temperature. This produced a

layer of Nafion on the order of 5 ntm thick, which covered the entire surface of the device.

Variations in the thickness of the Nafion layer do not significantly affect device

performance, since the film thickness is always greater than the penetration depth of the

evanescent tail of the plasmon into the polymer. The humidity-induced variation in the

PER of the Nafion-coated SP waveguide is shown in figure 3.14 over a range of 20% to

50% relative humidity (RH). Over this range, the sensor exhibits a reasonably linear

response, changing by 0.030 dB/%RH-cm for 658 nm excitation and -0.073 dB/%RH-cm

for 708 nm excitation. Humidity-dependent changes in the index of Nafion occur rather

rapidly, on the order of tens of seconds, and appear to be fully reversible. No attempt was

made to control the temperature, which varied from 220 C to 260 C over the course of

these measurements. Comparing figures 3.13 and 3.14, we find that the refractive index of

Nafion changes from 1.3460-0.0005 at 20% relative humidity to 1.3580.002 at 50%

relative humidity. For comparison, Fan and Harrison36 have measured the refractive index

of Nafion as 1.320.03 at 632.8 nm, using ellipsometry.

At first glance, it appears counterintuitive that the refractive index of a material

like Nafion which swells in water should increase for higher humidities. In the following,
















-3.0

-3.5

|-4.0

"-4.5

-5.0




0 .
1-6.0---

-6.5



-7.5


-8.0--
20 25 30 35 40 45 51
Relative Humidity (%)


S= 658 nm
E = 708 nm



Figure 3.14. Humidity response of the capped surface plasmon waveguide when coated
with a thin film ofNafion fluoropolymer.







we analyze this result theoretically. From the Lorentz-Lorenz relation, the refractive index
of a material may be related to its density, p, as37


(n2 1)
S Kp (3.7)
(n + 2)
where K is a constant. At zero humidity, the density of the film is

p = mF/VF (3.8)

where mF and VF are the mass and volume of the dry film, respectively. Thus, in principle,
humidity-induced swelling should reduce the density of the Nafion film, resulting in a
consequent decrease in index. However, in addition to swelling, Nafion also absorbs large
quantities of water from the surrounding atmosphere, resulting in a significant mass
increase. Sadaoka et al.38 find that the water-content of Nafion films can be as high as
-110 mg/g at 80% relative humidity, depending on the film processing conditions. To
examine the simultaneous effects of swelling and water gain, we take the derivative of

(3.7), yielding

2nAn[ 21 n2 = KAmF mFAV (3.9)
n 2+2 (n2+2)2. VF V
where AmF and AVF are the humidity-induced changes in film mass and volume
respectively, and An is the consequent change in refractive index. Rearranging terms in
(3.9) to solve for the index change gives

(n2- 1) (n2 +2) AmF AV,
=n 6n m I (3.10)
6n mF VF
Clearly, water absorption and swelling compete in the overall change in refractive index.
When (AmF/mF) < (AVF/VF), swelling dominates, and refractive index decreases with
increasing humidity. Conversely, and evidently in the case for Nafion in the humidity
range studied, mass increase due to moisture uptake dominates volume increase, and
refractive index increases with higher humidity, when (Amp/mF) > (AVF/VF).








The refractive index of polyimide, another material with a well-known moisture-

dependant refractive index, behaves similarly in that the index increases with humidity.

Moisture uptake in polyimide is on the order of 2%. Franke et al.9 report that the

refractive index of the polyimide SIXEF 33 changes from 1.5512 at 52% RH to 1.5525 at

96% RH, which is about an order of magnitude less than for Nafion.


3.4 Proposed Surface Plasmon Structures With Improved Performance

The SPR sensor discussed to this point offers an excellent means for detecting

extremely small index changes arising from changes in the environment. This high

sensitivity, in conjunction with appropriate transducing layers, makes these devices

appropriate for use in a wide range of applications. However, from a "real world"

viewpoint, large volume manufacture of these devices would probably not be possible.

The unfortunate drawback of integrated optics is that in general, packaging issues, namely

endfacet polishing and fiber coupling, may account for as much as 90% of the final device

cost. Thus, even though the ion-exchanged SPR waveguides are simple in design, it is

unlikely that a low-cost commercial product will be realizable through this route. Because

of this, we have also explored designs for SPR sensors based on SiO2/Si waveguides.

Silicon is perhaps the most mature processing technology, due to the large

competition in the semiconductor sector. Building devices on silicon wafers offers the

inherent advantage of batch processing, allowing the potential for high volume

manufacture. Silicon has a high quality natural oxide, SiO2, which can be grown either by

high temperature oxidation or a number of other means. The single-crystal nature of

silicon allows waveguide endfacets to be prepared simply by cleaving the substrate along

the appropriate crystal plane. Furthermore, through anisotropic etching with solutions of

either potassium hydroxide/isopropanol/water40 or tetramethylammonium hydroxide/

water,41 V-groove structures to facilitate passive fiber coupling can be built into the








substrate. Thus, packaging of silicon-based waveguides is considerably easier and less

expensive than that of glass ones.

Waveguides are deposited on silicon substrates by chemical vapor deposition

(CVD), flame hydrolysis, or plasma enhanced CVD (PECVD). These techniques allow

layers of silica to be deposited at micron thicknesses. Unlike thermal oxidation, these

techniques allow the addition of dopants to the oxide during deposition. Typically, the

oxide is doped with boron and/or phosphorous to reduce stress and the resultant glass is

known as a boron-phosphorous silicate (BPSG). Silicon oxynitride, SiOxNi.x, can also

be deposited. Both SiOxNi._ and BPSG have higher refractive indices than silica, and

through tailoring of the exact composition, substantial refractive index variations are

possible.42'43 As an example, a typical SiO2/Si waveguide could consist of an SiO2

cladding sandwiched between a BPSG core and a silicon substrate. The quality of CVD

deposited oxides is not as high as thermally grown ones. As such, the starting point for

SiO2/Si waveguides is generally a thermally oxidized silicon wafer.
We have designed and modelled an SPR waveguide based on the SiO2/Si

technology. Shown in figure 3.15(a), the structure uses an oxidized silicon wafer for the

substrate. The waveguide core, either SiOxNi.x or BPSG, is deposited next and has a

step-index profile. A second layer of silica, serving as the buffer layer is deposited on top

of the core, followed by a 35 nm layer of silver and an 86 nm Si02 tuning layer. The core

and buffer layers are 1.5 pm and 2.5 pim thick respectively. The refractive index of the

waveguide core is chosen to be 0.01 higher than that of pure silica, which is compatible

with existing deposition processes. The dispersion of the core is assumed to be equal to

that of pure silica. In principle, the waveguide core and buffer layers can be made during a
single run, simply by changing the gas chemistry. Figure 3.15(b) shows the proposed SPR

device on a substrate with an integrated V-groove structure for fiber coupling. In this case,

a cut from a wafer saw would be used to prepare a flat waveguide endfacet at the fiber

pigtail, rather than cleaving.










air/water


input signal


superstrate adsorbedd film), ns

tuning layer (SiO2, ZrO2)


SiO2 (top buffer)
doped SiO2 core (BPSG, SiOxNi.x)
SiO2 (isolation buffer)
silicon substrate


output signal


End View


waveguide
endfacet

>4


V-groove


fiber


silicon substrate


Figure 3.15. The SiO2/Si surface plasmon waveguide structure in the protected metal
configuration.
(a) device structure
(b) device with integrated V-grooves for fiber coupling








Note that several modifications have been made to the original SPR waveguide

design (figure 3.2). Silver has been used instead of gold as the metal layer, which, as will
be seen shortly, improves sensitivity. Since the waveguide is symmetric, a much thicker
buffer layer is required to achieve a reasonable PER. An advantage of using the thicker

buffer layer however, is that metal-induced TE propagation losses are reduced to less than
0.01 dB/cm, a considerable improvement over the previous GRIN waveguide designs

which had TE losses on the order of 1 dB/cm! In addition, the tuning layer has been

repositioned to be above the metal, which both protects the metal layer and increases the

range over which XSPR can be tuned. Furthermore, the accessibility of the tuning layer

makes possible post-deposition trimming to correct errors in the resonance wavelength

which may arise from fabrication tolerances in the thickness and index of the various

constituent layers in the plasmon overlay. This design offers a degree of durability which

is generally lacking in SPR devices and we aptly refer to it as the "protected metal

configuration."

As noted earlier, the advantage of substituting silver for gold in the SPR sensor lies

in the width of the spectral response of the PER. In figure 3.16, PER for the SiO2/Si SPR

waveguide is plotted against wavelength for various adsorbed films with thicknesses
ranging from 0 to 300 nm and an index of 1.415. The medium above the adsorbed film is

water (n = 1.33). The observed plasmon resonance has a FWHM of about 11 nm, roughly

one-third of that of the gold-based GRIN SPR device. Additionally, the shift in XSPR

produced by adsorption of the thin films is much larger than in the previous device.

Adsorption of a 100 nm thick film changes XSPR by 90 nm. An interesting consequence of

positioning the tuning layer above the metal film is that the positive-valued PER peak at

shorter wavelengths seen in figure 3.3 vanishes. In fact, the plasmon resonance dip is

observed even prior to film adsorption.

Extending this analysis to the case of infinitely thick superstrates, we next present

the PER as a function of superstrate index in figure 3.17. Excitation wavelengths of 633


















S-50--
U
m
*o
0 -100


-150--


S-200-
o


a-250-
o
Q.
B -


-300--
600


650 700 750 800


wavelength (nm)








Figure 3.16. Spectral response of the SiO2/Si silver-based SPR waveguide (protected
metal configuration) during the adsorption of thin films of varied thickness.
The adsorbed material has a refractive index of 1.415.


850

















-10


2 -30-

S-40-



S-50-
-s
-60 -
-

P-70


-80 -
1.31


1.32 1.33 1.34


1.35 1.36 1.37 1.38


superstrate Index








Figure 3.17. Refractive index dependance of the polarization extinction ratio for the SiO2/
Si silver-based SPR waveguide (protected metal configuration). The
superstrate is infinitely thick. The excitation wavelength is 633 nm.








nm and 670 nm are used. Again, the range of index values over which resonance is excited

is much smaller than the previous device. Using equation (3.4), at a superstrate index of

1.334, Anmin 4x10 assuming a signal-to-noise ratio of 20 dB. This represents a 40%

improvement in sensitivity over the gold-based sensor.


3.5 Application of Surface Plasmon Resonance to Monolayer Detection

An important area of interest for surface plasmon sensors is immunoassay, in

which extremely thin films must be detected.14'15'16 In such applications, the SPR device

is coated with a protein or antibody monolayer film, on the order of 1 to 2 nanometers

thick. This film serves as a transducing layer to catalyze various types of chemical

interactions, such as complexation and desorption, with analytes in the surrounding

environment. These reactions are monitored through changes in the plasmon resonance

characteristics.22 For instance, a common immunoassay study is molecular self-assembly,

which involves the accumulation of several monolayers of different materials. During the

process, the addition of each monolayer produces a discrete shift in the plasmon

resonance. Morgan et al.15 have used this technique to monitor the sequential deposition

of monolayer films of biotin, avidin, and bisbiotin onto a gold surface. The highly

selective nature of protein-ligand and antibody-antigen binding offers the potential for

realization of analyte-specific sensors.

In order to examine the applicability of the SiO2/Si SPR sensor to immunological

studies, we have calculated PER as a function of adsorbed film thickness, using a fixed

excitation wavelength of 633 nm. The adsorption of films of index 1.40 and 1.45 is

analyzed and the medium surrounding the device is water (n = 1.33). As shown in figure

3.18, the device is quite sensitive and is in fact able to detect films thinner than 1 nm!

Adsorption of a 2 nm film causes the PER to change from -40 dB/cm initially to

-33 dB/cm and -27 dB/cm for films of index 1.40 and 1.45 respectively. The plasmon

response is inherently nonlinear and the sensitivity, defined as the derivative of PER with















0-
I 633 nm n = 1.45


S-10 /

%-15 .*/
S2 /0 / nc= 1.40


1 /




-35
n.40

-4 5 .. .. .
0 2 4 6 8 10 12 14
superstrate thickness (nm)






Figure 3.18. Response of the SiO2/Si silver-based SPR waveguide (protected metal
configuration) to ultra-thin adsorbed films. The excitation wavelength is 633
nm. The tuning layer thickness is chosen so that XSPR initially (prior to film
adsorption) coincides with the excitation wavelength.








respect to adsorbed film thickness, decreases as the film becomes thicker. Nevertheless,
films of up to about 10 to 15 nm can easily be measured with this device, which is

adequate for detecting monolayers. Sensitivity is also clearly larger for high index films.
The improvement in the sensitivity of this device as compared to the design discussed
previously results from narrowing the spectral width of the plasmon resonance.


3.6 Conclusion

Surface plasmon resonance provides a highly sensitive means for detecting small

perturbations in the refractive index of the surrounding environment. Integrated-optic SPR
sensors, consisting of waveguides coated with thin metal and dielectric layers have been

modelled extensively. A dielectric tuning layer which simplifies the design process was

added to the basic SPR waveguide structure. Designs based on both GRIN and SiO2/Si
waveguides have been examined. A GRIN SPR sensor have been fabricated by depositing

a plasmon overlay with a thin layer of gold onto an ion-exchanged waveguide.

Measurements of the polarization extinction ratio of the GRIN SPR waveguide show

reasonable agreement with theoretical predications and a humidity sensor was produced

by coating the device with a thin film of the moisture-sorbing polymer Nafion.
Simulations of an SiO2/Si SPR waveguide which uses silver instead of gold in the design
show the potential for a 40% improvement in refractive index sensing capability over the

GRIN device. Furthermore, calculations show the SiO2/Si SPR sensor to be capable of

monitoring the adsorption of films thinner than 1 nm, making this device attractive in
immunological studies such as molecule self-assembly and protein-ligand binding.













CHAPTER 4
FABRICATION AND CHARACTERIZATION OF POLYMER
WAVEGUIDES

4.1 Advantages of Polymer Waveguides

Waveguiding in optically transparent polymers has been studied since the early
1970s. Today, polymers with a wide range of optical, chemical and mechanical properties

are available, offering numerous possibilities for novel optical devices. Extremely low

propagation losses have been achieved, using polymethyl methacrylate (0.12 dB/cm at
633 nm),44 polycarbonate (0.19 dB/cm at 830 nm),45 optical grade epoxy (0.3 dB/cm at
633 nm),46 deuterated fluoromethacrylate (0.1 dB/cm at 1300 nm),47 and others. A
number of companies, including DuPont, Amoco, Allied Signal, and others are presently
developing materials with even better performance. From an economic viewpoint,

polymer waveguide fabrication requires little specialized equipment and, within the
constraint of reliability issues, has the potential to be considerably less expensive than
other guided-wave technologies such as Ti:LiNbO3 and SiO2/Si.
The application which perhaps receives the largest benefit from the diverse nature

of polymers is the sensor field. Material properties, such as preferential adsorption of
specific chemicals and environmentally-induced swelling, can be exploited in polymer
waveguide sensors, allowing unparalleled levels of performance and versatility. A
significant advantage which polymer waveguides have over conventional integrated-optic
technologies such as lithium niobate and ion-exchanged glass is the relative ease with
which a chemically-sensitive dopant can be incorporated into the polymer matrix.46'48'49
This offers possibilities not readily achievable otherwise and allows the detection of a
wide range of analytes. Several integrated-optic chemical sensors based on waveguides








fabricated from a material known as polyimide will be presented in chapters 5 through 7.
However, in order to facilitate a better understanding of those devices, we will first
describe the polyimide waveguide fabrication process.


4.2 Fabrication of Polyimide Waveguides

Polyimides are a broad class of polymers which possess excellent chemical,

mechanical, and thermal stability. They are used extensively in the semiconductor industry
as dielectric materials in integrated circuits50 and are also used in planarization51 and

micromachining52 applications. Additionally, many polyimides may be
photolithographically patterned through commonly used photoresist processing
techniques. Recently, a number of companies, including DuPont, Amoco, OCG

Microelectronics Materials, Hitachi, and Hoest-Celanese have developed optically
transparent polyimides, laying the foundation for the use of polyimide in waveguide
applications. Propagation losses in planar polyimide waveguides as low as 0.2 dB/cm at
800 nm53 and 0.3 dB/cm at 1300 nm54 have been reported. Unlike many other types of
polymers waveguide materials, the glass transition temperature of these materials is very
high, often well in excess of 300 oC.55,56,57 Thus, polyimide waveguides can survive
elevated temperature environments. In contrast, the glass transition temperature of
polymethyl methacrylate is only 85 oC .44
We have experimented with the Probimide 400 series of photosensitive polyimide

from OCG Microelectronics Materials*. This material is obtained as a solution of fully
imidized benzophenone tetracarboxylic dianhyride-alkylated diamine (BTDA) polyimide,
dissolved in y-butyrolactone (GBL) solvent and is slightly amber in color. Probimide 400
may be deposited on various substrates by spin coating and behaves as a negative resist for
photolithographic purposes. Three products are available in this series: Probimide 408

OCG Microelectronics Materials, Inc.
200 Massasoit Ave., East Providence, RI 02914








(8.5% solids, 580 cs viscosity), which can be deposited from a thickness of 0.5 Ipm to 4.0

glm, Probimide 412 (12.5% solids, 3500 cs viscosity), which can be deposited from 3.0

pim to 12.0 im, and Probimide 410 (8.5% solids, 8200 cs viscosity), which can be
deposited from 5.0 pim to 20.0 pm.58 The glass transition temperature for this material is

356 oC. The procedure employed to produce polyimide waveguides is shown in figure 4.1.

In the following, we describe each step.

4.2.1 Substrate Preparation

Thermally oxidized silicon wafers are attractive as substrates for spin-cast polymer

waveguide, both because of the high optical quality of the SiO2 layer and the simple fact

that spin-deposition processes work best on round substrates. As will be seen shortly, the

refractive index difference between polyimide and SiO2 is rather large. Therefore, as seen

from equation 2.4, 73 is large and the field decays rapidly into the Si02 layer. As such,

only relatively thin buffer layers of the oxide are required to reduce the absorption due to

the underlying silicon. Oxidation is performed by placing clean, 2" diameter, <100>

oriented silicon wafers into a tube furnace at 1050 OC. Steam is pumped into one end of

the tube, creating a "wet oxygen atmosphere", which greatly enhances the rate of

oxidation. Under these conditions, the growth of a 2 pim thick layer of SiO2 takes 10
hours.59

4.2.2 Wafer Priming

Although Probimide adheres reasonably well to glass and silicon, it is generally

necessary to treat substrates with a silane-based adhesion promoter for optimum results,

particularly with respect to endfacet preparation (section 4.2.7). The adhesion promoter,

available from OCG as QZ 3289, is diluted at a ratio of 1 to 9 with a solution of 90/10 v/v

ethanol/water (QZ3290). The diluted adhesion promoter solution is stirred thoroughly and

allowed to sit for at least one hour prior to use to ensure proper mixing. During this time,

wafers are baked at 120 OC to remove moisture from the surface of the oxide layer. After










1. Oxidize 2" silicon wafer
10 hours at 1050 C under wet 02

2. Apply -0.8 mL 1:9 diluted
silane adhesion promoter,
spin 4000 rpm/20 sec,
bake on hotplate at 100 oC/20 sec



3. Dispense 0.6 mL P412
(large diameter syringe)


4. Spin wafer: 400 rpm/20 sec,
1000 rpm/10 sec, 2000 rpm/30 sec
Wait 3-5 minutes,
soft-bake on hotplate at 100 oC/15 min


5. Photolithographic patterning:


Sio2 I
silicon

silane
Si02
silicon

polyimide

Si02
silicon




Si02
silicon

UV light
photomask



Si02
silicon


UV exposure


cross-linked region



*-71


Development


polyimide

SiO2
silicon


6. High temperature cure


7. Dope with organic dye (optional)


(Same structure)


dye-diffused polyimidi

ISiO2


silicon


Figure 4.1. Schematic representation of the polyimide waveguide fabrication process.


7








the dehydration bake, the wafer to be primed is placed on the spinner and the dilute silane

adhesion promoter is applied in sufficient quantity to cover the entire wafer. For a 2"

wafer, typically about 0.8 mL of solution is required. The sample is then spun at 4000 rpm

for 20 seconds. This leaves a thin layer of silane on the wafer surface. The presence of

moisture on the wafer can interfere with uniform dispersal of the silane film and should be

avoided.

4.2.3 Polyimide Deposition

Deposition of a uniform polyimide film is the key to realizing high quality

waveguides. However, spin-coating of polyimide is somewhat more complicated than that

of conventional photoresists. Specifically, in order to prevent the inclusion of air bubbles

in polyimide films during deposition, a syringe with a large diameter tip needs to be used

to transfer polyimide solution onto the wafers. This is accomplished by cutting off the end

of a 3 mL plastic Luer-Lok tipped syringe (Becton-Dickinson), producing a tube with a

diameter of 8 mm. Butyrolactone was used to wipe off the ink markings on the bottom of

the syringe, so as not to contaminate the stock polyimide solution. The syringe plunger is

pushed down until there is no air trapped in the tube. Approximately 0.6 mL of Probimide

412 (12.5% solids, viscosity 3500 cps) is drawn into this modified syringe and held

vertically to maintain vacuum. The polyimide is then dispensed onto the center of the

wafer into a puddle approximately 2 cm in diameter. Any air bubbles which are visible at

this time must be removed, either by piercing them with a sharp, clean object, or simply by

letting them rise to the surface and break of their own accord. A three-step spin process is

then employed to distribute the polyimide solution. The spinner is set for 60 seconds at
400 rpm and activated. After 20 seconds, while the wafer is in motion, the spin speed is

increased to 1000 rpm. This rate is held for 10 seconds and then increased to 2000 rpm for

the remaining 30 seconds. This procedure is well-suited to uniform dispersal of the high

viscosity polyimide across the wafer. The wet film is left to sit for 3 minutes to settle and

allow any air bubbles trapped at the polyimide/SiO2 interface to migrate out of the film.








The polyimide-coated wafer is then placed on a hotplate at 100 OC and baked for 15

minutes. During this soft-bake, the silane forms a strong chemical bond with the

polyimide film, ensuring excellent adhesion to the oxidized silicon substrate.

It is important that the soft-bake step be performed on a hotplate, rather than a

conventional box-type oven. When polyimide films are soft-baked in an oven, the air-

exposed surface of the film dries first, forming a skin which impedes solvent removal. As

the solvent evaporates, this outer skin tends to crack, leading to large surface roughness.

At the same time, microvoids are formed inside the film. Both effects cause strong

scattering and increase propagation loss by about an order of magnitude. When dried on a

hot-plate however, polyimide dries at the substrate interface first and the solvent is

efficiently removed without formation of defects.

4.2.4 Photolithography

4.2.4.1 Planar Waveguides

While planar waveguides do not require photopatterning, a moderate UV dose

(365 nm) is nevertheless required to cross-link the polyimide chains and make the films

insoluble in organic solvents. The required UV dose varies with thickness. Typically, for

the process described above, a dose of about 0.8 J/cm2 is necessary. Moreover, cross-link

density can also be used to control the diffusion of an organic dye into the polyimide

matrix48 (section 4.2.6), and in some cases, UV doses of 2 to 3 J/cm2 may be required to

optimize concentrations. This topic will be dealt with more in chapter 6. In general, the

polyimide takes on a darker shade of amber when exposed to UV radiation, due to

increased absorption at shorter wavelengths.


4.2.4.2 Channel Waveguides

Photomasks with various channel waveguide patterns are placed between the

polyimide-coated silicon wafer and the UV light source, so that only the exposed regions

are cross-linked, much like a negative photoresist. The masks were designed with the








MAGIC and CADENCE software packages and fabricated on a model GCA MANN 3600

pattern generator. A UV dose of 0.6 to 0.8 J/cm2 is used. The film is then placed in a

developer solution (50/50 wt. butyrolactone/xylene) housed in an ultrasonic bath for 6

minutes. During this time, unexposed regions are dissolved. Ultrasonic agitation during

the developing stage improves pattern contrast.51 After developing, the sample is placed

sequentially in two identical solutions (50/50 wt. developer solution/xylene), referred to as

crossover baths, for 20 seconds each. Lastly, the sample is rinsed in xylene for 30 seconds

and blown dry with nitrogen. The crossover solutions are necessary to avoid precipitation

of the polyimide when going from the highly polar butyrolactone-based developer to the

non-polar xylene rinse. Precipitation causes the polyimide to turn opaque white, rendering

it useless for waveguiding purposes.

4.2.5 Curing of Polyimide Films

A high temperature cure is required to achieve good mechanical properties in the

polyimide. This needs to be performed under a nitrogen atmosphere, as the presence of

oxygen during curing degrades the mechanical performance and increases coloration in

these materials. Samples are placed in a vacuum oven with a nitrogen purge and ramped

up to 280 oC at a rate of 5 oC/minute. Peak temperature is held for 1 hour, after which the

samples are cooled back to room temperature at about 1 OC/minute. After the cure cycle,

planar polyimide films and wide (> 50 ptm) channel waveguides are approximately 5 pm

thick. Narrow channel waveguides tend to be up to 1 pIm thicker, depending on the

channel width.50

4.2.6 Doping (Optional)

A wide variety of dyes can be used to dope the polyimide, using a simple diffusion

process. Cross-linked and partially cured polyimide waveguides are soaked for 10 to 15

minutes in 1x10-4 M (or less) butyrolactone solutions of the desired dye. Butyrolactone

causes cross-linked polyimide to swell, allowing dye molecules to easily penetrate the








matrix. After the desired diffusion time, samples are quickly rinsed with methanol to

remove residual dye from the surface, immersed in water, blown dry with nitrogen, and

baked on hotplate at 100 OC for 30 minutes to remove residual solvent. After solvent

evaporation, the polyimide matrix contracts, effectively trapping the dye and preventing

clustering and migration. The peak dye concentration introduced into the polyimide films

by this process varies inversely with the degree of UV-induced cross-linking.48 The

uniformity of the doping mimics the quality of the polyimide waveguide. The laser dyes

cresyl violet 670, oxazine 720, nile blue 690, oxazine 725, oxazine 750, LDS 698, and

LDS 751 has been successfully introduced into the polyimide matrix using this technique.

Dyes can also be introduced into polyimide from methanol solutions, but this requires

longer diffusion times and results in lower peak dye concentrations than when

butyrolactone is used. It appears that the effectiveness of various solvents in transporting

dye into the polyimide matrix is related to the amount of swelling induced.

4.2.7 Endfacet Preparation

One of the main advantages of depositing polymer waveguides on oxidized silicon

wafers is that endfacets can be prepared by cleaving along the crystal planes of silicon,

provided the polymer exhibits sufficient adhesion. In this case, the <100> oriented wafers

are cleaved along <110> directions, producing rectangular samples. A small scratch is

made on the wafer in the desired cleave direction. Applying a small pressure to the wafer

so that it bends along the direction of the scratch will cause the wafer to cleave along that

plane. With practice, this process takes only a matter of seconds and produces quality

endfacets. In contrast, the preparation of good endfacets on glass ion-exchanged

waveguides by the polishing process described in chapter 3 takes several hours.








4.3 Characterization of Polyimide Films

The nominal value of the refractive index of Probimide 400, measured by prism

coupling, is found to be about 1.626 at 633 nm, for the TE polarization. Process
parameters, primarily the peak bake temperature and the UV dosage affect this value to
some extent. By measuring the index over the wavelength range of 594 nm to 780 nm, we
have fitted the TE refractive index to the second-order Sellmeier equation,

2 1.1413212 0.2113021 X2
nTE = 1 +----- + -- (4.1)
2 + 0.3670977 2 0.1570585
where X is in micrometers. TE refractive indices are shown in figure 4.2. The
birefringence, defined as

n = nTE -nTM (4.2)

was found to be very small (3x10 ) in Probimide 400 films. This is consistent with the
amorphous nature of the BTDA polyimide, which is comprised of short, flexible polymer

chains.60 As such, this material lends itself to use in devices such as splitters and switches,
where polarization-insensitivity is desirable. In contrast, the polyimides SIXEF 33 (Hoest-
Celanese) and Ultradel 9000 (Amoco), which are based on longer, stiffer polymeric
chains, are more crystalline in nature and have birefringences of 2x10-3 and 3.3x102,
respectively.39,61
The propagation losses of polyimide waveguides have been measured in the

visible and near infrared spectrum, using a tunable Helium-Neon laser (PMS Electro-
Optics, LSTP-0010) and semiconductor diode lasers operating at 677 nm and 780 nm.
Light is coupled into a guided mode using the prism coupler and an optical fiber is
scanned along the length of the guided streak to collect scattered light, as shown in figure
4.3. The scattered light is proportional to the power propagating in the mode and decays
exponentially along the length of the waveguide as given by

Is, i (z) = KIi (z) = KiIi (0) exp (-2k magz) (4.3)














1.640 -


1.635


1.630 -






1.620--






550
550


600 650 700 750


wavelength (nm)







Figure 4.2. TE refractive index of the Probimide 412 polyimide, fitted to a second-order
Sellmeier equation.


800



















photodetector



multimode
optical fiber



input beam coupling
prism scan
p uscane












s Z


pressure


Figure 4.3. Experimental set-up for measuring propagation loss in waveguides.








where Ki, Ii(O), and kimag are a constant, the initial power launched into the ih mode, and

the field attenuation coefficient of the ih mode, respectively. To a rough approximation, Ki

is proportional to the intensity of the mode field at the air/polyimide interface. A typical

measurement of scattered intensity as a function of position along the guided streak is

shown in figure 4.4, along with the values of the loss coefficients obtained by fitting data
to equation (4.3). Unfortunately, the technique tends to overestimate losses when used to

evaluate polymer waveguides. The pressure applied to the substrate to hold the waveguide

against the prism tends to physically deform the polymer film, causing light to be

launched simultaneously into several modes at once, instead of just one. Thus, the

detected scatter signal is actually the sum of the individual scatter signals from each

excited mode. Higher order modes scatter more strongly than lower order ones (a

consequence of having a larger intensity at the air/polyimide interface), and tend to be

more lossy. Thus, excitation of higher order modes while attempting to measure the

attenuation of a low order mode can make the loss appear to be larger. Signal-to-noise

constraints generally restrict the use of this method to modes with losses of greater than

1 dB/cm.

Propagation losses are shown in figure 4.5 for the TEo mode of a planar

waveguide. At 780 nm, the loss is less than 1 dB/cm. At shorter wavelengths, losses are

considerably higher due to increased absorption. In fact, a weak orange fluorescence is
observed in the material when 543.5 nm radiation is launched into the films. Loss was not

observed to depend significantly on the film thickness, which indicates that scattering

losses arise primarily from roughness at the air/polyimide interface rather than from

scattering sites distributed within the bulk of the polyimide film itself.

Propagation losses can be improved by optimizing the cure process. During

curing, the polyimide becomes darker in color and exhibits increased absorption. This

effect is more pronounced at higher cure temperatures.50 The use of a lower peak cure

temperature sacrifices some of the polyimide's chemical and mechanical resistance, but






































0 .5 .. . .
0 1000 2000 3000 4000 5000 6000 7000 8000
position (urn)





Figure 4.4. Variation in scattered power along the length of the waveguide for the TEo
mode at 670 nm. R is the correlation coefficient, which describes the degree to
which experimental data fall along the exponential fitting curves.













25
U

20






20
15


0
0


0
0
5w
I- 0--
540


590 640 690 740


wavelength (nm)





Figure 4.5. Propagation losses for the TE0 mode of a 3.6 pm thick polyimide waveguide
on a soda-lime glass substrate.


790








appears to be necessary to attain good optical properties.62 Slowing the heating rate during
the cure cycle to -1 OC/min also reduces loss.56,* Using a modified cure cycle, we have

been able to reduce propagation losses considerably. Under the new cure schedule,

samples are heated to 150 OC, at a rate of 5 C/min. After 15 minutes at 150 C, the
temperature is ramped to a final temperature of 250 OC at the same heating rate. Peak

temperature is maintained for 30 minutes, after which samples are cooled slowly to room

temperature over several hours. Using this modification, we have achieved propagation

losses of 3.1 dB/cm at 633 nm and 1.5 dB/cm 670 nm in the TMo mode of a 5 pm thick
planar polyimide waveguide.

Finally, it should be noted that the coloration of polyimide solutions becomes more

pronounced with age.** The quoted shelf-life of the Probimide 400 series is 1 year.
However, this specification is based largely on changes in photospeed, rather than

absorption. Most of the waveguides in chapters 5 through 7 were deposited from
solutions which were 2 to 3 years old and still showed good performance.


4.4 Summary

Polymer waveguide fabrication is less expensive than other guided-wave
technologies, such as Ti:LiNbO3 and Si02/Si, but requires careful attention to process

conditions in order to produce devices which are competitive with respect to loss. Using
the photosensitive polyimide Probimide 400, we have obtained propagation losses on the
order of 1 to 2 dB/cm at 670 nm in multimode waveguides deposited on oxidized silicon

substrates. This material is easily doped with a variety of organic dyes using a simple
diffusion process, making it particularly attractive for sensor applications. The refractive
index has been measured over a wide wavelength range and fitted to a second order


C. T. Sullivan, Sandia National Laboratories, private communication
C. T. Sullivan, Sandia National Laboratories, private communication
D. Roza, OCG Microelectronics Materials Inc., private communication





67


Sellmeier equation. We shall now present several chemical sensor designs which utilize

polyimide waveguides.













CHAPTER 5
EVANESCENT WAVE SENSING WITH POLYMER WAVEGUIDES

5.1 The Evanescent-Wave Absorption Sensor

The most common types of optical sensors are those which employ changes in the

optical absorption characteristics of an indicator material, such as a pH sensitive dye, as a
means of analyte detection. In sensors based on optical waveguides, the indicator is

immobilized in an analyte-permeable host material and used as either the core or the

cladding (defined in figure 2.1). The relative ease with which organic polymers can be

doped with a variety of analyte-sensitive dopants, makes this class of materials very

attractive in this application. We refer to the portion of the waveguide which houses the

indicator as the sensing region. When the sensing region is a cladding layer, as is

commonly the case due to fabrication requirements, only the evanescent tails of guided

modes interact with the indicator and the device is accordingly termed an evanescent wave

absorption (EWA) sensor.63'64 Table 5.1 lists a few examples of absorption-monitoring
waveguide sensors

Most absorption-monitoring integrated optic sensors (and in fact all of the

examples given in table 5.1) are based on multimode waveguides. Therefore, a brief
review of the formulation of wave propagation in multimode structures is in order. We

shall consider a simple two-dimensional multimode waveguide, consisting of a high index
media of index n2 bounded between two media with indices of n1 and n3 respectively as
shown in figure 5.1. For the sake of generality, we will also assume the each of the

materials forming the waveguide has a bulk absorption coefficient, Ki.








Table 5.1 Examples of Absorption-Monitoring Waveguide Sensors

Ref. core cladding indicator analyte

65 K+-Na IE PWG SiOz lutetium Chlorine
biphtalocyanine
66 multimode fiber silicone analyte hydrocarbons
67 Ag+-Na IE- CWG silicone analyte trichloroethylene
(multimode)
68 Ag+-Na+ IE- CWG sol-gel bromcresol purple NH40H
(multimode)
69 glass rod poly vinyl sodium picrate sodium cyanide
alcohol
70 AgClxBrl.x fiber none (*) fiber core SF6
71 sol-gel film on none (*) bromophenol blue pH
glass

IE CWG: glass ion-exchanged channel waveguide
IE PWG: glass ion-exchanged planar waveguide
(*): analyte-sensitive material located in waveguide core


A coherent light source is to be endfire-coupled into the waveguide. At z = 0, the power

launched into the ih order mode is


Pi P e(x) einput(x) dx
x


(5.1)


where Po is the input power, ei(x) and einput(x) are the respective transverse field
distributions of the ith order mode and the excitation source at z = 0. Thus, input power is

launched unequally into the various guided modes. Inside the waveguide, the power

propagating in a given mode can be written as


Pi (z) = Piexp (-2k imagz)


(5.2)


















lens
SInput field distribution, Einput


Z nl, K,
SX \(sensing layer)



Light n2, K
Source



n3, K3


field distributions, Ei




nl < n2 > n3








Figure 5.1 Launching light from a free-space beam into the guided modes of a waveguide
(endfire excitation).








where kimag is imaginary part of ki, the propagation constant of the ith mode. The total
power in the waveguide is thus

P (z) = XPiexp (-2kimaz) (5.3)

We define the transmission of a waveguide of length L as P(L)/P(0).
For a given mode, kimag depends on the fractional power, defined in equation
(2.7), travelling in each of the three regions shown in figure and the bulk absorption
coefficient of that region:

kimag = (riK + T,2iK + F3iK)/2 (5.4)

In an evanescent wave absorption sensor, losses occurring in the sensing layer (region 1)

must dominate all other forms of loss and thus, FliK1 ) r2iK2 + F3iK. Under this
assumption, inserting (5.4) into (5.3) yields

P (z) = XPiexp (-rliK1z) (5.5)
i
In the evanescent wave sensor, the absorption coefficient of region 1, K1, is expected to
change in the presence of analyte. Therefore, in order to maximize device sensitivity (i. e.
analyte-induced change in the transmission of the waveguide), we would like to have as
much power travelling in the sensing layer as possible.
We have used the waveguide simulator developed in chapter 2 to analyze the mode
fields of a simple three layer waveguide. In this model, we have chosen n2 = 1.625

(polyimide), n3 = 1.45 (SiO2), thickness d = 5 pim, and used a wavelength of 633 nm. In
figure 5.2(a), Fli, the fractional power carried by the ith mode in region 1, is shown for a
few values of n1. As expected, higher order modes are less tightly confined than lower
order ones and propagate a larger fractional power in region 1. This is particularly true
when (n2 nl) is small. From figure 5.2(a) and equation (5.5), we see that the launching
conditions at the input of the waveguide (z = 0) play a key role in determining the overall

propagation loss, and hence, sensitivity of a multimode waveguide absorption sensor. For









0.18
~0.16
S0.14
- 0.12
S0.10
S0.08
n 0.06
S0.04
S0.02
0.00


0 1 2 3 4 5 6 7 8 9
mode order
(a)


1.50
n,


Power distribution in a three-layer waveguide. n2 = 1.625, n3 = 1.45, and the
thickness of region 2 is 5 pm. The excitation wavelength is 633 nm.
(a) Fractional power carried in region 1 by each guided mode.
(b) Total fractional power (summed over all guided modes) and waveguide
asymmetry as a function of sensing region index, nj.


Figure 5.2








example, an EWA sensor with power launched predominately into lower order modes will
be less sensitive to analyte-induced changes in cladding absorption than an identical

sensor with more power launched into higher order modes.
Continuing the numerical example, we next calculate the total fractional power

propagating in region 1, defined as

1pirli
F1 = 1 (5.6)
XPi
i
as a function of nI. These results are presented in figure 5.2(b), along with the
corresponding waveguide asymmetry factor, aE, defined by equation (2.2). In these
calculations, each mode is assumed to carry equal power for convenience. As the core/

cladding index difference (n2 nl) decreases and the decay term in equation (2.4) becomes
smaller, the evanescent tails of the guided modes become longer and the total power in
region 1 increases. Sharp oscillations in the total power in region 1 occur as individual
modes go to cut-off. Decreasing waveguide asymmetry also results in more power in
region 1. Up to 10% of the light launched into the waveguide can propagate in region 1

when (n2 nl) is less than 0.1.

In contrast to the EWA sensor, many optical detection systems measure the
transmission of a free-space beam passing through a bulk absorption cell. In order to
establish a comparison between the EWA sensor and a bulk device, we define the effective
interaction length of light in an absorption sensor (either bulk or EWA) as


Lcf = FL (5.7)



In bulk absorption measurements, where a probe beam is passed through a dye cuvet, thin
film, etc., F1 = 1. However, as seen from figure 5.2, Ti is typically only a few percent in

EWA sensors, and is also dependant on the launching conditions at the waveguide input.








Examples of bulk and EWA sensors will be presented in the following sections 5.2.4 and
5.2.9, respectively.

Note that this analysis of absorption-based sensors is equally applicable to the

surface plasmon devices examined previously. Having established the basic operating
principles of evanescent wave absorption sensors, we shall now present a specific example

of an EWA device designed to monitor the concentration of ammonia in water.72


5.2 Detection of Aqueous Ammonia

Industrial pollution of rivers and lakes poses a serious hazard to wildlife in affected

areas. Increased algae blooms and the consequent red tides are common side-effects of

fertilizer run-off into swamplands and coastal waters. Pesticide and fertilizer run-off into

groundwater result in elevated ammonia and nitrite levels which can be hazardous to

wildlife at even very low concentrations. For example, total dissolved ammonia

concentrations (NH3 + NH4+) on the order of a few parts-per-million are harmful to

fish,73'74 on a time scale of hours.75'76 Whereas the ammonium ion is relatively

innocuous, the non-ionized form of ammonia is highly toxic to aquatic life and must not

exceed 40 to 400 ppb, depending on species, temperature, water chemistry, and pH.73,75'77

In order to monitor industrial pollution effectively, highly sensitive devices with short
response times are required. Ideally, in order to minimize cost issues, sensors also need to
be reusable. In this chapter, we focus on the development of an optical waveguide based-

sensor for the detection of aqueous ammonia which satisfies these criteria.

5.2.1 Choice of Sensing Layer Materials

Much of the efforts to date in the development of ammonia sensors have dealt with

the detection of vapor phase ammonia. Guiliani et al.78 were able to rapidly and reversibly

detect ammonia vapor concentrations of 60 ppm by monitoring the transmission of a
quartz rod coated with the pH sensitive dye oxazine 170. A similar system utilizing a








sensing layer comprised of ninhydrin immobilized in films of poly (vinyl alcohol) and

poly (vinyl pyrrolidone) exhibited a detection limit as low as 60 ppb, but required nearly
an hour to achieve full response and was not reusable.79 Oxazine 750-doped silicone and

bromcresol purple doped porous sol-gel films have been used as claddings on optical
fibers80 and ion-exchanged waveguides68 respectively, producing ammonia vapor sensors

with detection limits of less than 1 ppm with rapid, reversible responses. In most cases,
humidity has a significant effect on the performance of sensors based on pH indicators.

Unfortunately, the aqueous environment generally places more severe restrictions

on the materials used in EWA sensors. Structural degradation prohibits use of water-

soluble materials in the sensor design. Immobilization of the indicator in the sensing layer
is of paramount importance to avoid problems such as dye migration and clustering.

Aqueous-based ammonia sensors with rapid, reversible responses have been demonstrated
using bromophenol blue71 and oxazine 17081 entrapped in inorganic sol-gel matrices, but

some degree of leaching (loss of indicator) was observed. Oxazine 170-doped poly

(methylmethacrylate) films deposited on glass have also been investigated for ammonia
sensing, but were found to respond very slowly and exhibit poor substrate adhesion when
immersed in water for long periods of time.81 Thus, realization of a useful sensor for an

aqueous environment requires a careful optimization of the mechanical as well as
chemical characteristics of the constituent materials.

We have studied the performance of pH sensitive dyes in two polymers,

polymethylmethacrylate and Nafion. In the following, we describe their performance.


5.2.1.1 Polymethylmethacrylate

Polymethylmethacrylate (PMMA) is an extremely transparent polymer often used

in the fabrication of plastic optical fibers. Previous studies have explored the use of
PMMA as a host material for various laser dyes, such as rhodamine 6G,82 rhodamine B,83

and DCM.84 It was assumed that the immobilization of a pH sensitive dye in a PMMA







matrix would produce a sensor useful for the detection of ammonia. Following the
procedure outlined by Chernyak et al.81 a chloroform solution of PMMA (5% wt. solids)
was doped to 1x10 4M with oxazine 170. The dip-coating technique was used to prepare
a 1 gpm thick film ofPMMA/oxazine 170 on a soda-lime microscope slide. Unfortunately,
this material failed to exhibit a significant change in absorption when exposed to ammonia
vapor. In aqueous environments, the PMMA film separated from the substrate (though
remained largely intact). These results were not sufficiently encouraging to merit further
study of this material.

5.2.1.2 Nafion

We have investigated the use of DuPont's Nafion fluoropolymer, a copolymer of
polytetrafluoroethylene (PTFE) and an acid (S03) -terminated perfluorovinyl ether,85 as a
cladding material for evanescent wave sensing of aqueous ammonia. The chemical
structure of Nafion (1100 equivalent weight) is shown in figure 5.3. This material has been






[(CF2CF2)n (CF2CF)] -
I
0 CF2CFCF3



CF2CF2SO3H


Figure 5.3 Chemical structure of Nafton fluoropolymer.








investigated for this purpose previously by Churchill et al.,86 who produced a fast and
highly sensitive, albeit irreversible, ammonia vapor sensor based on Naflon films
containing the dyes oxazine 720, Nile blue 690, and bromothymol blue. Nafion has also

been used previously by Zen and Patonay87 for pH measurement and by Ballantine et al.88
for acid vapor detection.
Nafion has several properties which make it attractive for ammonia sensing. It is

expected that sulphonic acid groups of Nafion will react strongly to the presence of
ammonia, producing a large change in the pH of the polymer. Annealed Nafion films

exhibit ion-selective transport properties,89 which favor the diffusion of positively

charged species, such as NH4+, through the polymer network over that of negatively
charged ones like Cl .90 In addition, Nation swells considerably in water, allowing rapid

penetration of the analyte into the polymer matrix. We have successfully demonstrated an

EWA sensor for the detection of aqueous ammonia using Nation doped with pH-sensitive
indicator dyes from the oxazine family as a sensing layer.
5.2.2 Fabrication of Oxazine-Doped Nafion

Nation was obtained as a 5% solution of 1100 equivalent weight polymer in lower

aliphatic alcohols and water (Aldrich). Alcohol solutions of the oxazine dyes cresyl violet

670, oxazine 720, Nile blue 690, oxazine 725, and oxazine 750 were mixed with the
Nafion solution, producing mixtures that contained 1% polymer (by weight) and -0.1

mmol dye. The chemical structures of these dyes are shown in figure 5.4.91'* Dye

concentration was kept reasonably low to avoid dimerization. After thorough mixing, the
dye-doped Nation solutions were deposited on clean glass microscope slides using a
coverage of 30 pL/cm2 and allowed to dry at room temperature (24 C, 45% relative

humidity). The resultant films were reasonably uniform in color and thickness in the


*Exciton, private communication













--- N
H Ox
H'
Cresyl Violet 670


Oxazine 725


HS,
H0C2
H5C2


Oxazine 720


Oxazine 750


"/ N
H5C2 leBu 0
H5C2/
Nile Blue 690


Figure 5.4 Chemical structure of several oxazine dyes.


H5C2 /
H5C2/


C2HS








center of the samples, but were considerably thicker at the outer edges of the slide. The

films were on the order of 1 to 2 gm thick in the center of the slides.

During the course of this study, it was found that careful attention had to be paid to

subtle processing details to produce a high-quality film. Specifically, the choice of solvent

used for the dye, which forms 80% of the total solvent in the dye-doped polymer solution,

is critical to determining stability of the final Nation film in water.92,93 Films deposited

from methanol-based dye solutions showed poor mechanical properties, crumbling into

pieces and separating from the glass substrate when immersed in water. Baking had been

proposed as a means to improving the mechanical integrity of Nation films by inducing

some degree of crystallinity in the polymer matrix. However, baking alone was found to

be insufficient to improve the resistance of methanol-deposited films to water. The water

stability problem was finally solved by switching the dye solvent to isopropanol, which

produced films which were pliable and cohesive. When immersed in water, isopropanol-
deposited films exhibited excessive swelling and eventual substrate separation, but

remained largely intact. More importantly and unlike the case in methanol-deposited

films, it was observed that baking the isopropanol-deposited films at 120 oC for 60

minutes was sufficient to completely suppress these undesirable effects, resulting in the

realization ofNafion-coated glass slides which were stable in water.

It was also discovered that the dye-doped Nation solutions had shelf-lives of less

than one day in liquid form. Films deposited from "old" solutions showed different

absorption characteristics than ones deposited from fresh mixtures. Therefore, the dye and

Nation solutions were mixed only immediately prior to film deposition.
5.2.3 Characterization of Oxazine-Doped Nafion

Oxazine dyes like those shown in figure 5.4 respond to local pH variations by

reversible deprotonation of an amino endgroup (auxochrome).91'94 In dye-doped Nafion

films deposited by the above method, the polymer matrix is highly acidic in nature and the

dyes are fully protonated and generally lightly colored. However, when exposed to








ammonia, these films change color rapidly, corresponding to the deprotonated (basic) state

of the dye. Furthermore, the deprotonated state is retained after the ammonia source is

removed. We propose that in the presence of ammonia, hydrated ammonium ions rapidly
penetrate the Nation network and bind the polymer's acid groups by the reactions
NH3 + H20 + NH4+ + OH- (5.8)

SO3- + NH4+ -> NH4SO3 (5.9)
Based on the irreversible nature of the reaction of Nafion to ammonia, we may conclude

that the resultant ammonium salt is very stable at room temperature. With the side groups
in the salt form, the pH of the Nafion matrix is significantly higher than in its acid form,

causing deprotonation of the indicator. Sadaoka et al.95 have suggested a similar reaction
for poly (acrylic acid). Based on this model, Nation-based sensors can measure only
ammonium ion concentration. However, from this information, the non-ionized ammonia

concentration in a solution can be calculated, given a knowledge of the temperature, pH,

and other factors.96'97

Immersion of oxazine-doped Nafion films in pure water was also observed to

cause a color change corresponding to dye deprotonation, but only while the samples were

wet. When removed from water and dried, these films returned to the initial acidic state
whereas those exposed to ammonia did not. As might be expected, films in acidic state are
sensitive to atmospheric moisture and a study of the humidity dependency of the

absorption characteristics of oxazine-doped Nation has been performed separately.*
Bulk absorption spectra were measured in the range of 500 nm to 800 nm by

passing collimated light from a monochromator (Digikrom 240) through the dye-doped
Nafion-coated glass slides at normal incidence and monitoring transmission with a silicon

photodetector. The output slit on the monochromator was set to 1 mm, corresponding to a




G. A. Stewart, "Humidity dependant transmission characteristics of Nile blue
doped Nation," Independant study project, University of Florida, 1996.








spectral resolution of 3.2 nm. Light was also passed through an uncoated glass slide as a
reference. The absorption was determined as


A = 1 Tc (k) /Tr (,) (5.10)



where Tc(X) and Tr(,) are the transmissions of the Nafion-coated and reference slides,

respectively. Films were measured in the as-deposited state, exposed to fumes from an
ammonium hydroxide solution, and remeasured. We denote the films to be in the acidic
state prior to ammonia exposure and in the basic state after ammonia exposure. Figures

5.5 through 5.9 show the absorption spectra of the oxazine dyes cresyl violet 670, oxazine
720 Nile blue 690, oxazine 725, and oxazine 750, respectively, in Nafion films before and
after exposure to ammonia vapor. In all cases, the presence of ammonia causes a radical

change in the absorption spectrum. The primary concern here is the wavelength-
dependance, rather than the peak absorption value for each system, since dye
concentration and film thickness vary in each case. Interestingly, despite the relative
similarities in the structures of these dyes, two distinctly different types of absorption
response are observed. Cresyl violet 670, oxazine 720, and Nile blue 690 all exhibit weak
absorptions initially, but form strong and well-defined absorption bands in the visible
region when exposed to ammonia. Conversely, oxazine 725 and oxazine 750 show strong
initial absorption in the near-infrared and exhibit a 30 to 40 nm blue-shift when exposed to
ammonia. Nile blue was chosen as the most convenient indicator dye for the ammonia
sensing, since it showed a large ammonia-dependant change in absorption over the red
region of the spectrum where laser diodes and HeNe lasers are readily available.
5.2.4 Bulk Ammonia Sensor Response

Nile blue-doped Nation solutions were prepared by mixing 1 part Nation solution
to 4 parts of a 0.3 mmol mixture of Nile blue in isopropanol. Films were then made by
depositing 30 pL/cm2 of solution onto clean soda-lime glass microscope slides. The















0.800


0.700


0.600


0.500


0 0.400
0
m 0.300
I

0.200 --

0.100


0.000 45
400 450 500


550 600 650 700 750 800 850


wavelength (nm)






Figure 5.5 Absorption spectra of cresyl violet 670/Nafion (methanol solution) in the acid
and base forms. The film is yellow in the acid form and purple in the base
state.














0.450


0.400

0.350

0.300
base form
S0.250

0.200
Said form
0.150

0.100 ----
I "/ \ \
0.050- \

0.000 |. -
400 450 500 550 600 650 700 750 800 850
wavelength (nm)







Figure 5.6 Absorption spectra of oxazine 720/Nafion (methanol solution) in the acid and
base forms. The film is green in the acid form and blue in the base state.















0.40


0.35 -

0.30--


0.25

S 0.20 -
[- -
o
- 0.15-


0.10-


0.05 -


0.00--
400


450 500 550 600 650 700 750 800 850


wavelength (nm)







Figure 5.7 Absorption spectra of Nile blue 690/Nafion isopropanoll solution) in the acid
and base forms. The film is yellow in the acid form and blue in the base
state.














1.000 aj o I -
Naflon / Oxazine 725
0.900 \ \

acid form
0.700- -_-f--

S0.600 /
0.60 base form
S0.500
. 0.400-
U / \ / \
0.300 -/--

0.200-

0.100 --

0.000- ---- -yrr
400 450 500 550 600 650 700 750 800 850
wavelength (nm)


Figure 5.8 Absorption spectra of oxazine 725/Nafion (methanol solution) in the acid and
base forms. The film is black in the acid form and deep blue in the base state.
































500 550 600 650


700 750 800 850


wavelength (nm)






Figure 5.9 Absorption spectra of oxazine 750/Nafion (methanol solution) in the acid and
base forms. The film is aqua in the acid form and blue in the base state.


0.900

0.800

0.700

0.600

. 0.500
o0
S0.400

S0.300

0.200

0.100

0.000


Naflon I OxazIne 750



base form

/ ^I

acid form /



7 / I


....1-- -\


400


450








solution was allowed to dry at room temperature and was then baked at 120 OC on a

hotplate for 60 minutes. The final film was approximately 1.4 gpm thick and yellow in

color. The indicator-doped Nafion films prepared in this manner were found to be very

stable in aqueous environments. In fact, a film immersed in water for a period of 5 weeks

exhibited negligible leaching of the indicator and only minor swelling at the edges of the

substrate.

Measurements of the change in the bulk transmission of Nile blue-doped Nafion

films with ammonia concentration were performed at 632.8 nm using a HeNe laser. The

coated slides were repeatedly immersed in ammonia solutions in order of increasing

concentration, up to about 3 ppm, for 1 minute, blown dry with nitrogen, and baked on a

hotplate at 50 to 70 oC for 5 minutes. In this manner, the reversible reaction of Nafion to

water could be differentiated from the irreversible reaction to ammonia. Transmission was

measured after each immersion/drying cycle.

The response of the bulk device is shown in figure 5.10. For ammonia

concentrations below 1 ppm, the Nafion film is in the acidic state (yellow) and the Nile

blue indicator absorbs only weakly. In the range of 1 to 2 ppm, localized blue spots begin

to form on the film which persist after drying. With each subsequent immersion/drying

cycle, these spots grow in size until the entire film is blue. When one of these spots

intersects the HeNe beam, in this case at 2 ppm, transmission drops sharply.

Unfortunately, the step-like response observed here results in a sensor with an exceedingly

small dynamic range. Given the film thickness and taking into account the transmission of

the glass substrate, we have calculated the absorption loss of the Nile blue-doped Nafion

to be 16,000 200 dB/cm at 633 nm when the polymer is in the base state. In the acid

state, the absorption is too low to be measured reliably by this technique.

5.2.5 Demonstration of Reversibility

In order to allow Nile blue-doped Nafion films to function as reusable ammonia

sensors, a rinse technique was developed which allows ammonia-exposed films to be reset

















100


90 ----------- -- --------- --, ------ ..-.--



80 ----- ------ -- --"---- ---


0

E
(-
C



I I i i I
I \

50 ------.---- ---- -- ------- --



40 .
0 500 1000 1500 2000 2500 3000
ammonia concentration (ppb)








Figure 5.10 Bulk transmission of a Nile blue-doped Nafion film on a microscope slide to
various aqueous ammonia concentrations. The excitation wavelength is
632.8 nm. The film is approximately 1.4 jim thick.








back to the original acidic (yellow) state. In this process, base-state (blue) sensors are

immersed in 1:20 solutions of acetic acid/deionized water for 30 seconds, blown dry with

nitrogen, and dried on a hotplate at 90 OC. After the rinse, the films are again yellow in

color. To demonstrate the viability of this technique, we have monitored the transmission

of a bulk sample alternately exposed to a high concentration ammonium hydroxide vapor

followed by the dilute acetic acid process. As shown in figure 5.11, the rinse process is

extremely effective in restoring the acidic form of Nafion, cycle after cycle. Thus, this

technology has the potential to be economically viable, as sensors can be reused many

times.
We would like to emphasize that during the course of these experiments, no

problems with substrate adhesion were encountered.

5.2.6 Selectivity of the Nafion Response

As may be inferred from figure 5.3, the transmission-based Nafion sensor exhibits

little selectivity amongst strong bases. For example, exposure to aqueous NaOH converts

Nafion's acid groups into ionic salts in the same manner as NH4OH and produces the

same color change in the Nile blue indicator. The fact that Nafion responds irreversibly to
bases (prior to the acid rinse) but reversibly to water offers some level of discrimination.

In a sensor based instead on the diffusion rates of neutral species through a polymer

matrix, Nation's permaselective nature would provide some measure of selectivity.

However, in order to obtain a selective response to ammonia, the Nafion sensor would

have to be used in conjunction with several other sensors in a multielement array. Ideally,

each sensor element would exhibit different analyte response characteristics. The
composite array response could then be treated as a vector in a multidimensional pattern

space and analyzed by a neural network algorithm, providing both selectivity and

multianalyte detection capability.















90
acid acid acid acid
85


80


?75-
C
0
& 70
E

e65
I -


60

ammonia
55 ammonia ammonia ammonia


50
0 1 2 3 4

number of rinse cycles






Figure 5.11 Bulk transmission of a Nile blue-doped Nafion film on a microscope slide at
632.8 nm after sequential exposure to aqueous solutions of 5% acetic acid
and concentrated ammonium hydroxide vapor. The film is approximately
1 Jm thick.







5.2.7 Waveguide Issues in Evanescent Wave Sensor Design

An EWA sensor for the detection of aqueous ammonia has been developed by
coating a polyimide channel waveguide clad with a thin layer of Nile blue-doped Naflon.
An oxidized silicon wafer is used as the substrate. The entire device is shown in figure
5.12. An advantage of the choice of the materials in this structure is that refractive indices


output light



dye-doped Nafion

polyimide




SiO2




input light


Figure 5.12 Device structure of the evanescent wave absorption sensor.


of the SiO2 substrate (n = 1.45) and the Nafion cladding (n = 1.35, undoped35'36) are fairly
similar. This low degree of waveguide asymmetry increases the penetration depth of the
evanescent wave associated with each of the guided modes into the Nafion cladding,
thereby enhancing sensitivity (see figure 5.2(b)). Rectangular cross-sectioned ridge
waveguides were used instead of planar ones, in order to further increase the fractional
modal power propagating in the Nation cladding. Through numerical simulations, the
total fractional power travelling in the Nafion cladding of this structure is estimated to be




Full Text

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OPTICAL WAVEGUIDE CHEMICAL SENSORS By MARTINNEIL WEISS A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY UNIVERSITY OF FLORIDA 1996

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This work is dedicated to my parents Edward and Catherine

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ACKNOWLEDGMENTS John Donne once wrote "no man is an island entire of itself'. This dissertation stands as a testament to those words As I look back on the various experiments processing techniques (read tricks ), and theoretical models devised herein I cannot help but recall the vast myriad of people who aided me in the completion of this study Most of all I would like to thank Dr. Ramakant Srivastava, my advisor In addition to providing me with the wide range of resources and training I needed for my research Dr. Srivastava was a constant source of encouragement and support. Without his leadership and guidance I doubt I would have been able to accomplish as much in this work I would also like to thank Dr. Peter Zory the cochairman ofmy Ph.D. committee, and Dr. James Winefordner for giving me access to their laboratories for materials characterization Thanks also go to Dr. Toshikazu Nishida Dr. Stephan Schulman and Dr. Ewen Thomson for serving on my Ph. D committee I would like to thank my friends Howard Groger and Dr. Peter Lo of the American Research Corporation of Virginia who were in fact the people who first sparked my interest in the chemical sensor field I am indebted to James Chamblee Tim Vaught Steve Shine and Frank Tavano for maintaining the Electrical Engineering department s cleanroom where I spent many an hour fabricating my various sensor devices. I am also grateful to Dr. Ben Smith for help in characterizing the fluorescence spectra of dye-doped polymers and Dr. Sheng Li for the HeCd laser used in the later chapters Some of the more obscure tips on material processing were provided by Dr. Drew Roza of OCG Microelectronics Materials Inc Dr. Charles Sullivan of Sandia National Laboratories and Richard Steppel of Exciton. A portion of my dissertation work was performed during a brief visit to the Federal University of Pemambuco in Recife Brazil. I would like to thank Dr. Cid de Araujo and lll

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his colleagues Dr. Ricardo Correia, Dr. Anderson Gomes, Dr. J F. Martins-Filho, and Breno Neri for making my wonderful stay very productive I am grateful to the Microfabritech program at the University of Florida the National Science Foundation, and the American Research Corporation of Virginia for funding this work. Lastly I am deeply grateful to my beloved fiancee Vicki and my dearest parents Edward and Catherine for their unending love and support over these long years of research during my career at the University of Florida. IV

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TABLE OF CONTENTS ACKNOWLEDGMENTS ........ ... ............. ........... .......................... .................. .... .... .iii ABSTRACT ... ..... . ............. ...... ..... ..... .................................................................. ...... viii CHAPTERS 1. INTRODUCTION TO OPTICAL CHEMICAL SENSORS ................... 1 1.1 Overview ... ................................... .... ... ........... ......... ............... 1 1.2 Organization ............................................................................... 2 1.3 Sensor Evaluation Criteria .................... . .. ....... ......................... 2 1 3.1 Sensitivity ..... . ..... ............ ......... .......... .... ....... .... .... 3 1 3.2 Selectivity . ...... ... .... .................. ................. ...... ......... 3 1 3.3 Reversibility ................. ............. .... ... ..... ....... ..... ..... 3 1 3 .4 Cost Effectiveness ................................. .... ... ..... .... .4 2. NUMERICAL MODELLING OF OPTICAL WAVEGUIDES ............. 5 2.1 Review of Wave guide Theory ..................................... ............. 5 2.2 Modelling Graded Index Waveguides: The Transfer Matrix .... 8 2.3 Numerical Solution of the Transfer Matrix ............................... 12 2.4 Summary ........................................................ ............ .............. 13 3. SURFACE PLASMON WAVEGUIDE SENSORS ............................... 14 3.1 Overview of Surface Plasmon Sensors ........................ ............. 14 3.2 Theoretical Formulation of Surface Plasmon Resonance .......... 15 3 .2.1 Modelling of Surface Plasm on Waveguides ............... 18 3 2 2 Integrated-Optic Surface Plasmon Waveguide Structures ..... . ..... ........... .... ......................... .......... 24 3.3 Experimental Investigation of Surface Plasmon Devices .......... 29 3.3.1 Device Fabrication .... ......... .................. .......... .... ....... 29 3.3 2 Experimental Measurement of Refractive Index . ..... 32 3 3 3 Humidity Measurement .... .... .......... ........ . ................. 40 3.4 Proposed Surface Plasmon Structures With Improved Performance .......................................... .. ......... .................... 43 3.5 Application of Surface Plasmon Resonance to Monolayer Detection . .... ... .......... .. ........... ... .... .... .. ..... ............. ......... ..... 49 3.6 Conclusion ............................................................ ... .................. 51 V

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4. FABRICATION AND CHARACTERIZATION OF POLYMER WAVEGUIDES .... .... . ... .... ................................... ................ .. ........ ... 52 4.1 Advantages of Polymer Waveguides ........ ....... ................. .... .. 52 4.2 Fabrication of Polyimide Waveguides .. ............ ......... .. ............. 53 4 2 1 Substrate Preparation ............... ... ................ ..... .... . ... 54 4 2 2 Wafer Priming ... ... ..... ..... .... ..... . ... ..... .... . ............ .... 54 4 2 3 Polyimide Deposition ... . . ......... ... ........ . . ........ . . ... 56 4 2.4 Photolithography .... .............. ...... .... ..... . . . . . . ....... . 57 4 2.5 Curing of Polyimide Films .... . . ....... .... ... . .......... ...... 58 4 2 6 Doping (Optional) .... .... ........ ............ ...... ............ ... 58 4 2.7 Endfacet Preparation ........ .... .... .... . ...... .... .... ... . ... . 59 4.3 Characterization of Polyimide Films ..... ....... ......... ............ .... 60 4.4 Summary ....... .... ..... .. ......... ............ ... .. .... .... .. . ......................... 66 5 EVANESCENT WA VE SENSING WITH POLYMER WAVEGUIDES ...... .. .... ..... .. .. . ... .... .. .. ... .... .... .... ... ... .... ...... ......... .. 68 5 1 The Evanescent-Wave Absorption Sensor .. .. ............... ... . .... . 68 5.2 Detection of Aqueous Ammonia ..... .. ... ... .... . . . ...................... 74 5 2 1 Choice of Sensing Layer Materials ................ ..... ... .. .. 74 5.2 2 Fabrication ofOxazine-Doped Nafion . ... . ........ ....... 77 5 2 3 Characterization ofOxazine-Doped Nafion ........ . . . 79 5.2.4 Bulk Ammonia Sensor Response .... .... . ............ ........ 81 5.2 5 Demonstration of Reversibility ...... ............. .... ........... 87 5.2 6 Selectivity of the Nafion Response .............. .... . .... .... 89 5.2 7 Waveguide Issues in Evanescent Wave Sensor Design ........ . ... ................... . ........ .. ... .......... .... . ... 91 5.2 8 Evanescent Wave Sensor Fabrication ... ........ ..... ......... 92 5 2 9 Performance of Evanescent Wave Absorption Ammonia Sensors .. ... ..... ........... . .... ........... .... ... . ... 92 5.3 Summary .......... .......................... .................... ......................... 100 6. FLUORESCENCE -EXCITED EVANESCENT WA VE ABSORPTION SENSORS ......... .. ... . .. . ..... .. .... . .... .. ....... ...... .... ...... . .. . ........ ...... .. 101 6 1 Introduction ....... ..... ..... ....... ....................... ... ..... .................... 101 6 2 Principl e of Op e ration ....... . ... .... ..... ...... . ... ... .... ... ... ... ......... .. 102 6.3 Theoretical Formulation of Fluorescence Capture by Guided Modes .... ... ....... ... .......... ... ..... ........... .... .. . . ..... .. ......... .... ..... 102 6.4 S e nsor Fabrication ...................................... ........ ......... ... . . .... 108 6 4 1 Materials . ....... ................ ........ . ..... ..... .................. . 108 6.4.2 Waveguide Fabrication ... .................... .... ............... .... 116 6 5 Sensor Characterization ....... .. .. .... . . .. ... ........ .. ......... ..... .. . ..... 116 6 .5.1 Laser Pumping ... ...... .... .......... ............. ... ....... ... . ..... 116 6 5 2 LED Pumping .... .... ... ............... ................... ..... . . ... . 117 6 5 3 Conventional Evanescent Wave Measurement ... .... ... 121 6.6 Lim i tations of Fluorescence Excited Waveguid e Sensors ........ 122 6.7 Summary ........ ........ . . . ..... .. .... .. .. ... . .......... ... ....... .. ..... .... ... .. 122 Vl

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7. OPTICALGAININDYE-DOPEDPOLYMER WAVEGUIDES ......... 125 7.1 Introduction ........... .. ............ ...... .. .... . . . ... .. .... .. .. .. ........... ... . 125 7 .2 Optical Amplification .. .. ..... ....... .. ... .... ... .. .... ..... ... .... ... ..... .. 127 7.3 Characterization of Optical Gain in Dye-Doped Polyimide Waveguides .... .... .. .. .. .... . .. ........... ... ... .... .. . .......... ..... .. . ... . 128 7.3 1 Active Waveguide Fabrication .... ...... .... ........ ...... ... .. 130 7 3 2 Experimental Set-up ... .... . .... ...... . ... ... . . .... ......... . . 130 7.3 3 Measurement of Optical Gain . . ...... . ......... . .... .. .... . 132 7.4 Optical Amplifiers as Chemical Sensors .. . ... ... .. ..... ..... ....... 137 7.5 Conclusion .. ... .. ...... . ..... . ..... ... .. . ... .. .. ... ........ .. ............. . ....... 138 8. CONCLUSIONS AND FUTURE WORK ........... .. .... .. ... . .... ... ... ... .. .... 139 8 1 Summary .............. .... .......... .... .. ..... ......... .. . . . .... .. .... .. ............ 139 8.2 Future Work ......... . .. ........ .... ... ... ................ .. ..... . .. .... .. .... .... . .. .. 140 8 2.1 Improved Fluorescence-Excited Evanescent Waveguide Absorption Sensors .. . ... ........ .............. 141 8 2 2 Active Waveguides for Chemical Sensing ... . ... . ... .. 145 8.2 3 Polyimide Waveguides as Selective Chemical Recognition Elements ... ..... ........ .... . .... . . . . ... . . .. 147 APPENDIX LIST OF ACRONYMS . . .............. ........... ..... . .... ....... .... . . . ..... ...... 148 REFERENCES ............ .... ............ ............ . .... ... ................ .. ... ....... ..... ..... .... ..... ... 150 BIOGRAPHICAL SKETCH .... . ... .... .... . ... . . . . .... . ... ...... .... .... . . ........ .... ... . . ... 161 Vil

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Abstract of Dissertation Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy OPTICAL WAVEGUIDE CHEMICAL SENSORS By Martin Neil Weiss December 1996 Chairman : Ramakant Srivastava Major Department: Electrical and Computer Engineering Optical sensors convert information about the surrounding environment such as temperature and chemical composition into changes in light intensity and phase. Compact lightweight waveguide and fiber-based sensors offer a high level of detection capability with an inherent immunity to electromagnetic interference Several types of waveguide sensors for chemical detection have been explored both theoretically and experimentally Primary emphasis has been placed on optimizing sensitivity through the design of the waveguide structure rather than the sensing material. Using surface plasmon resonance sensors comprised of metal-clad dielectric waveguides have been designed with enough sensitivity to measure refractive index variations on the order of 10-5 and adsorbed film thicknesses of less than 1 nm The basic surface plasmon resonance (SPR) waveguide structure has been modified to include a dielectric tuning layer which simplifies the design process and reduces the number of materials needed in sensor fabrication A humidity sensor was created by coating an SPR Vlll

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waveguide with a thin layer of Nafion, an ion-exchanging fluoropolymer whose refractive index varies with atmospheric moisture content. Conventional endfire-excited evanescent wave absorption (EWA) sensors were produced by coating waveguides with materials whose optical absorption changes upon analyte exposure. Ammonia in aqueous solution has been detected at sub-part-per-million levels using polyimide waveguides clad with Nile-blue doped Nation A novel variant of the EWA sensor, which uses fluorescence generated inside a waveguide to probe the cladding absorption was also studied This latter device, known as the fluorescence excited evanescent wave absorption sensor provides a higher level of sensitivity and is virtually free from the stringent alignment tolerances that often plague other EWA sensors Lastly a sensing technique based on measuring analyte-induced perturbations in the optical gain of a waveguide amplifier or laser was proposed An optical amplifier, with a gain of 14.4 dB at 670 nm was achieved in a cresyl violet-doped polyimide waveguide Unfortunately fluorophores were found to exhibit minimal analyte sensitivity when immobilized in the polyimide matrix IX

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CHAPfER 1 INTRODUCTION TO OPfICAL CHEMICAL SENSORS 1.1 Overview Modern society's recent trend toward increased environmental awareness has led to a need for development of advanced chemical detection systems. Sensors which utilize optical detection techniques such as fluorescence excitation, have proven to be highly effective in this regard. Highly sensitive, compact, and lightweight optical sensors have been demonstrated for a large number of chemicals. Optical chemical sensors employ either bulk or integrated-optical (IO) components, such as fibers and waveguides, as sensing elements. Analyte interactions with the sensing element are converted into optical information, such as light intensity or phase, through a number of transduction mechanisms A wide variety of optical phenomena can be used for analyte detection, including surface plasmon resonance, fluorescence excitation optical absorption measurement, and refractive index perturbation The most common examples of optical chemical sensors are fiber evanescent wave devices and bulk surface plasmon resonance devices. Unlike their electronic counterparts optical sensors neither produce nor are affected by electromagnetic interference In many cases, optical sensors can respond more rapidly and with a higher sensitivity than electronic sensors Although optical device fabrication is in general not as well-established as semiconductor processing IO sensors still benefit greatly from optoelectronic technology developed for the optical telecommunications industry 1

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2 Of the various types of optical sensors, waveguide-based devices are in general more versatile than bulk ones. This is particularly true of integrated-optic sensors, where multiple sensing and reference channels can be built on the same substrate Processing functions needed for conditioning of the sensor output signal, such as filtering polarization splitting, and wavelength demultiplexing can also be performed on-chip by additional waveguide structures. In addition, integrated-optic chemical sensors are smaller and lighter than bulk ones, and can be easily deployed in remote areas via optical fiber delivery Despite the promising outlook though, commercial development of integrated optic sensors has proceeded rather slowly, due to concerns about durability in harsh environments and high packaging costs 1.2 Organization We begin with a brief review of waveguide theory in chapter 2 to establish basic concepts and terminology. A numerical simulator for calculating waveguide mode field distributions and propagation constants is also introduced In chapters 3 through 7, four optical waveguide sensors are developed : the surface plasmon resonance waveguide the evanescent wave absorption sensor, the fluorescence-excited evanescent wave absorption sensor and the chemically-sensitive amplifier The waveguide principles underlying the operation of each device are presented, along with modell i ng predictions Considerable effort has been devoted to the sensor fabrication and experimental characterization In this work, the primary emphasis will be placed on the advantages and disadvantages associated with each structure rather than the actual sensing chemistry. 1.3 Sensor Evaluation Criteria Each of the sensors presented in chapters 3 through 7 will be evaluated with regard to the criteria of sensitivity selectivity reversibility and cost effectiveness

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j 1.3.1 Sensitivity The sensitivity of a device with respect to a given analyte is defined as the derivative of the sensor output with respect to the concentration of that analyte.1 The minimum amount of analyte which produces a measurable response (i.e. above the value determined by the signal-to-noise ratio) is known as the lower detection limit (LDL) The range of analyte concentration over which the sensitivity is non-zero is known as the dynamic range Sensitivity can be improved through optimization of waveguide properties However, as will be seen in chapters 5 and 6, certain waveguide structures are inherently more sensitive than others 1.3.2 Selectivity Selectivity is the ability of a sensor to differentiate between multiple analytes .1 Devices with low selectivity are prone to mistake one chemical species for another. Sensors which contain analyte-specific receptors such as proteins or antibodies offer highly selective responses As will be seen in chapters 4 and 5 polymer-based sensors can also have some inherent level of selectivity when matrices with analyte-specific diffusion properties are employed. In addition individual sensor elements with generalized (i. e. nonselective) but differing analyte responses can often be combined into an array forming a highly selective sensor system 1.3.3 Reversibility Reversibility is the extent to which a sensor returns to its initial state after exposure to an analyte .2 We use the term reversible to specifically describe systems which automatically return to their pre-analyte state when placed in an analyte-free environment. In some cases irreversible sensors can be restored to their initial state by application of an outside influence such as a rinse chemical. We refer to this latter type of sensor as resettable Only sensors which can be fully restored to their pre-analyte condition can be

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4 reused effectively Devices which are neither reversible or resettable are limited to a single trial. 1.3.4 Cost Effectiveness Optical waveguide devices are notorious for having high packaging costs This single aspect, perhaps more than any other has hindered widespread commercialization of integrated-optic devices Not surprisingly, disposable waveguide sensors that are discarded after only one use are not economically viable. Waveguide-based sensors must either be fully reversible (or resettable), thereby ensuring long operating lifetimes, or utilize inexpensive packaging schemes in order to be cost effective Thus, packaging issues will play an important part in the discussions of each of the sensors presented herein

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CHAPfER2 NUMERICAL MODELLING OF OPfICAL WAVEGUIDES 2.1 Review of Waveguide Theory We shall begin with a brief review of basic waveguide concepts and establish a few definitions and terminology in order to facilitate our later discussion of guided wave chemical sensors. When light travelling in a dielectric medium of refractive index n2 is incident on a second medium with index n1 < n2 as shown in figure 2.l(a), total internal reflection (TIR) occurs when the angle of incidence 8, exceeds the critical angle defined by (2 1) By bringing two such interfaces into close proximity, as shown in figure 2 1 (b ) light can be constrained to propagate only in the direction tangential to the interfaces This structure represents a one-dimensional waveguide The higher-index central region is referred to as the core, while the lower-index surrounding regions are known as the claddings. In many waveguide sensor applications, it is the top layer of index n 1 which is exposed to the analyte. This layer is called the superstrate or the sensing layer It is also useful to characterize the refractive index mismatch between the top and bottom claddings in terms of the normalized waveguide asymmetry parameter, defined as3 (2 2) For energy to propagate a significant distance in the waveguide two criteria must be satisfied : total internal reflection (at both interfaces) and constructive interference at all points along the ray path As a result of the latter requirement only a discrete set of rays, 5

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6 y L, (a) y d z (b) Figure 2.1 Light incident on dielectric media n2 > n 1 (a) single interface (b) two interfaces (waveguide) n1 (cladding) n2 (core) n3 ( cladding)

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7 corresponding to waveguide modes, can be guided. For the guided light, two polarization ...... states are possible When the electric field, E, is perpendicular to the plane defined by the direction of propagation and the interface normal vector, the field is said to be TE polarized. Conversely, when Eis parallel to this plane, the field is 1M-polarized.4 For the TE guided modes, transverse field distribution can be written as where Y3Y Ae y <0 E ( Y) = B /'Y + Ce -j Ky, 0 < y < d D -"f1 (y -d) d e 'y> K=k0Jn~-N2 'Y1 = koJN2 n~ 'Y3 = koJN2 n~ k0 = 21t/A0 (2.3) (2.4) where A, B, C, and D are constants, A.0 is the vacuum wavelength, and N is called the mode index. The total field associated with the ith TE waveguide mode is ...... -jk. z Ei (y, z) = xEi (y) e I (2 5) where ki = (21t/A.0)Ni is called the propagation constant. In lossy waveguides, ki becomes complex-valued, with an imaginary term which characterizes mode attenuation : k kreal 'kimag i -i J i (2.6) The mode indices and field distributions correspond to the eigenvalues and eigenfunctions of the waveguide structure and are obtained by solving the eigenvalue (characteristic) equation. 1M mode fields have forms analogous to those presented above for the TE case, with some minor modifications

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As we will see later the performance of a waveguide sensor depends critically on the amount of power propagating in the sensing region For the ith mode, the fractional power propagating in the region y1 < y < y2 is defined as5 (2.7) -oo and simplifies to 00 TE J 2 f 2 r i = E (y) dy / E (y) dy (2 8) oo for the TE polarization 2.2 Modelling Graded Index Waveguides: The Transfer Matrix We have used the well-known transfer matrix technique for solving the waveguide eigenvalue problem for each structure In this approach, a continuous refractive index profile n(y) is quantized into a series of slabs each with a constant index value ni, as shown in figure 2 2(a) Application of boundary conditions at the interfaces between adjacent slabs allows the mode indices and transverse field distributions of the structure to be determined Starting with the TE polarization with Ey = :Ez = H x = 0 it is evident from figure 2.2(b ) that the total electric field in the ith layer xi, is the sum of the components transmitted through the i-lth interface and reflected off the ith interface. Defining Et and E i respectively as the components travelling toward and away from the ith interface and the total depth d' i as n (2 9) i=2

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n(y) a) b) Figure 2.2 Detennination of waveguide mode indices and field distributions by the transfer matrix method a) discretized refractive index profile, n(yi_) b) multilayer stack fonnulation of the waveguiding problem

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10 where n is the total number of layers The total field confined in layer i may be expressed as E E+ -jk;yY E-jky;Y d' d' .= e + e 1 N) yield sinusoidal solutions while imaginary values of kyi (ni < N) result in exponentially growing or decaying fields We expect the field in the core to be characterized by an oscillatory solution while fields in the surrounding media decay To solve for the actual mode indices and fields, the tangential electric fields as well as their derivatives along the y-direction are forced to be equal at each interface Thus, E = E ( 1 ) I XI X I y = d \_ 1 (2 .11) and dE XI dy dEx(i-1) = dy (2.12) Noting that aEx/dY = k y Exi and inserting (2 10) into (2 11) and (2 12) yields E+ -jky d\ 1 + E-jky( i -1>d';1 i-Ie i-Ie + -jk .d' I -jk d' I E }'I -+ E }'I -= ie ie d' y i 1 (2 13) + J k d' J0k d k E y(i-1> -+k E-. y(i-1> < i -1> y(i-1 ) 1 -1e y(i -1) 11 e k + -jky;d'i1 k jky;d'i-11 = -.E e + E e y1 I YI I y =d' i l (2.14) Finally, through equations (2.13) and (2.14) the fields in the top layer can be expressed in

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terms of those of the bottom layer as where and m = I n i=2 11 (2.15) (2 16) (2.17) In (2 17) P is a constant which equals unity for the TE polarization. When radiation couples into one of the guided modes of a waveguide consisting of n layers, E1 and En+ must be zero Thus, determining the mode indices of the structure is equivalent to solving for the roots of the equation M11 (N) = 0 Inserting the values of mode index into equation (2 15) yields the corresponding field distribution Under this algorithm, the integrated intensity distribution of each mode has an arbitrary value : (2.18) where the subscript designates the mode order For convenience we define the normalized mode fields as e1. (y) = ~E. (y) K. I I (2 19)

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12 so that (2.20) The derivation of the transfer matrix for the TM polarization is analogous to the TE analysis presented above. In the TM case, the fields are expressed in terms of Hx, rather than Ex, and an identical transfer matrix is obtained, with the exception that in (2 17), P = ni 1 / ni. 2.3 Numerical Solution of the Transfer Matrix In order to solve for the roots of the transfer matrix, the Newton-Rahpson algorithm is employed6 7 In this method, the function M11(N) is expanded in a Taylor series about an initial point as8 Mll (N+o) = Mll (N) +M'11 (N)o+M"11 (N)o2/2+ ... (2.21) where o is the difference between the estimated and the actual values of the root. Setting M11(N+o) = 0 and retaining only the linear terms in (2 21) yields o = -M11 (N) /M' 11 (N) (2 22) This correction is then added to the current estimate of the root and the process is repeated iteratively until o becomes sufficiently small Starting values for each of the roots of M11(N) are found from a course sampling of the function, during which the structure is assumed to be lossless.9 When multiple roots exist, the algorithm generally converges to the one which is nearest to the initial estimate. The Newton-Raphson technique is attractive for root finding because it converges very quickly In fact, it can be shown that the magnitude of the error associated with the difference of the actual value of a root versus its value after a given iteration decreases quadratically.7 In practice however, stability issues, such as the presence of local minima in the function of interest, often make it necessary to allow the algorithm to update the

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u current estimated value of the root with only a fraction of the full Newton step Since the Newton-Rahpson technique is easily extended to solving multidimensional roots, it is useful for determining the roots of lossy structures, such as metal-clad waveguides. We have been able to duplicate theoretical results given by Harris and Wilkinson 10 in the modelling of a surface plasmon waveguide structure, thus confirming the accuracy of our technique 2.4 Summary A basic introduction of waveguide concepts and terminology has been presented We have introduced the concept of fractional power which will figure prominently in the remainder of this work. A numerical approach to solving the waveguide eigenvalue problem has also been described In the following chapters, the simulator will be used extensively for modelling and optimizing a number of waveguide-based chemical sensors

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CHAPTER3 SURFACE PLASMON WAVEGUIDE SENSORS 3.1 Overview of Surface Plasmon Sensors Optical surface plasmon resonance (SPR) devices provide a highly sensitive means for detecting environmental changes involving small perturbations in refractive index. Sensors of this type have been employed in many diverse fields ranging from pollution monitoring11 and humidity measurement1213 to immunoassay14 and molecular self-assembly studies 15 16 SPR sensors operate by measunng changes in the refractive index of their surrounding media .17 As such, they are inherently generic devices with regard to what is actually detected An SPR sensor can be made to detect a specific environmental characteristic by coating the device with a layer of material whose optical properties are changed by the occurrence of a particular event, such as exposure to a certain chemical. Such a medium is often referred to as a transducing layer In immunoassay studies for example, this is commonly done by taking advantage of the natural affinity of complementary protein-ligand pairs .15 An SPR device coated with one material from the pair becomes a sensor for the other Antibody-antigen complexation may be monitored in the same fashion. Most commonly used SPR sensors consist of a dielectric prism with a metal cladding of typically either gold or silver on one face .14 1 5 16 This arrangement is known as the Kretschmann configuration In this setup a collimated beam of light shines into the prism through one of the clear facets reflects off the metal film and exits through the remaining clear facet. ~8 The incident beam must be TM-polarized with respect to the 14

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D metal film. Surface plasmon excitation can be observed by monitoring the reflected power as a function of the angle the beam makes on the front of the prism At a particular angle of incidence, 8sp, the reflectivity drops to nearly zero 8sp is highly dependent on the refractive index of the surrounding media. However, these systems tend to be somewhat cumbersome and are better suited to laboratory settings than field conditions. A considerable improvement in the functionality of such sensors can be achieved by taking an integrated optic (IO) approach and using SPR waveguides. The IO format enables the realization of devices which are compact and lightweight and, furthermore, allows the possibility of additional signal processing, such as polarization and wavelength filtering, to be performed on-chip. In addition, multiple sensing elements and reference channels can be incorporated into a single device. Guided-wave SPR sensors have been explored previously using a number of geometries, including D-fiber, 17 tapered fiber, 19 side-polished fiber ,20 2 1 and ion-exchanged GRIN waveguides.11,13, 22 2 3 3.2 Theoretical Formulation of Surface Plasmon Resonance A surface plasmon is a lossy TM-polarized wave supported by a metal-dielectric interface.4 24 Physically, the plasmon wave is an optically excited electron plasma oscillation in the metal When the metal is a film of finite thickness, individual surface plasmon waves are supported on both metal-dielectric interfaces Furthermore if the metal film is sufficiently thin ( on the order of the penetration depth of the optical wave) these two waves will couple to form the so-called symmetric and antisymmetric bound and leaky surface plasmon modes 20 The simplest integrated optic SPR waveguide sensor consists of a dielectric single mode waveguide overlaid with a thin layer of metal. A dispersion plot of the uncoupled waveguide TM mode and the plasmon modes is shown schematically in figure 3 .1. When the propagation constants of the waveguide mode and an SP mode are equal the two couple to form a lossy normal mode whose attenuation is proportional to the fractional

PAGE 25

kSB kAB 16 symmetric bound plasmon dielectric metal dielectric antisymmetric bound plasmon wavelength Figure 3 .1. Qualitative dispersion of the individual (uncoupled) TM propa~ation constants of the waveguide, kWG,. ~mmetric bound SP mode, k B, and antisymmetric bound SP mode, ~. The normal TM mode of the composite ( coupled) structure is shown by the dashed lines Field distributions for the individual plasmon modes are shown in the inset.

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17 power propagating in the metal layer When this condition, also known as phase-matching is not satisfied, propagation loss is considerably lower SPR excitation in these devices is thus strongly wavelength dependent. Wavelengths less than A. SPR cause the TM waveguide mode to couple primarily to the symmetric bound plasmon mode while wavelengths greater than this mainly excite the antisymmetric bound plasmon mode TE polarized waveguide modes do not interact with the surface plasmons and experience a small relatively wavelength-independent loss due to the presence of the metal layer The response of the IO SPR waveguide is characterized in terms of the polarization extinction ratio (PER), defined at a particular wavelength as the ratio of the propagation losses of TM to TE-polarized normal modes, or PER= ATM _ATE (3.1) where ATM.TE are the propagation losses of the TE and TM modes in dB/cm The losses are related to the normal mode propagation constants as A TE,TM = IO log { exp ( -2k~E,TM z) } / z (3 2) where ki = (21t/A{J) Ni, A{) is the vacuum wavelength, z is the propagation length, and Ni is the imaginary part of the mode index Since plasmon resonance induces large losses in the TM mode, while leaving the TE one relatively unaffected PER generally takes on negative values For single-mode waveguides the PER varies linearly with device length. In multimode devices, it is necessary to account for interference between the normal modes when computing the extinction ratio .25 By exciting SPR waveguides with circularly polarized light the TE signal can be used as an internal reference for signal normalization since its loss is relatively independent of the excitation wavelength We define the peak resonance wavelength A. SPR, as the wavelength at which the magnitude of the polarization extinction ratio achieves its largest negative value Perturbations to the refractive index of the region immediately surrounding the sensor also known as the superstrate affect A. SPR by producing unequal changes in the propagation constants of the

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HS uncoupled SP and waveguide modes. To simplify further discussions, we will refer to the largest negative value of the polarization extinction ratio as PERmax_ -+---11~z Air/Water 0 :::::::i::~,;,,i1111::citi2rg1m1::::: r: -J:!lillllll.lfB:allll tverlay low index buffer layer waveguide input signal X substrate output signal Figure 3.2 Schematic representation of the basic surface plasmon waveguide sensor. 3.2.1 Modelling of Surface Plasmon Waveguides SPR waveguides have been modelled extensively, using the previously described numerical simulation The first device considered here is based on a K+-Na + ion exchanged planar waveguide in BK7 glass, as shown in figure 3 .2. In order to excite resonance, several dielectric films a thin metal layer are deposited on the top surface of the waveguide We refer to these films, including the metal one, as the plasmon overlay For reasons that will be discussed later in section 3.2 2, the plasmon overlay in this case is comprised of thin films of SiO2 (buffer) TiO2 (tuning) and gold, at thicknesses of 500 nm 55 nm and 30 nm respectively The K+-Na + ion-exchange in BK7 glass is well characterized and produces high quality waveguides with a graded refractive index (GRIN) profile given by26 (3.3)

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19 where dx is the waveguide depth, to is the total thickness of the plasmon overlay, and AnTE,TM are the respective surface index changes for the TE and TM polarizations respectively. In order to ensure single mode operation in the red to infrared portion of the spectrum, dx was chosen to be 3 m. Although the K+-Na+ ion-exchange process in BK7 glass is known to produce birefringent waveguides, with An TE = 0 008 and An TM= 0.0092, only the latter value was used for modelling purposes since the attenuation experienced by the TE mode is small(< 1 dB/cm) and relatively independent of An. Refractive index dispersion data for the materials comprising the structure in figure 3.2 are taken from references 27-30. In order to best illustrate the function of a basic SPR waveguide as a sensor we first treat the superstrate region as a thin film of index 1.415 adsorbed onto the surface of the device and calculate PER as a function of wavelength By thin,' we mean that the thickness of the adsorbed film is less than the penetration depth of the evanescent wave at the surface The medium surrounding the device is assumed to be air (n = 1 ) As shown in figure 3.3, prior to film adsorption, the calculated PER spectrum is essentially flat, with the exception of a small positive-valued peak in the green region This positive peak is not related to the surface plasmon effect but rather arises from coupling between leaky TE modes in the tuning layer and the guided TE mode Upon the adsorption of a thin film surface plasmon resonance occurs and is evident as a large, negative-valued dip in the PER. In this case the full-width half-maximum (FWHM) of the resonance is about 30 nm 'A. SPR increases as the adsorbed layer becomes thicker At the same time, the magnitude of PERma x increases since the optical waveguide mode becomes less tightly confined at the longer wavelengths and interacts more strongly with the metal film This effect continues until the thickness of the film exceeds the penetration depth of the evanescent wave at the surface at which point 'A. SPR becomes constant. Thus, film thickness can be measured simply by monitoring the extinction ratio at a wavelength in the vicinity of 'A. SPR_

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20 I I 0nm --200 nm 0 -40 ______ ..,__ ____ ....;........a...;....+--+-+o-----------! I C / 0 I .5 -60-----------------~----_;_' -------i : 300 nm \ I \ I -so-------------------1000 nm -100 ___,----,-....,.......,.....-....,.......,.........,..........--............. .......--,........,...._.--.-___________ 0.5 0.55 0.6 0.65 0.7 0.75 wavelength (m) Figure 3 3 Simulated device extinction ratio as a function of wavelength with a thin adsorbed film of refractive index 1 415 as the superstrate. Adsorbate thicknesses are indicated next to the respective curves

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L.1 Intensity distributions corresponding to the cases from figure 3 3 are shown for the TM and TE modes at 670 nm in figures 3.4(a) and (b), respectively. Plasmon excitation, evidenced as an enhancement of the TM field at the air/metal interface (x = 0 in figure 3 2) becomes stronger at 670 nm as the adsorbed film thickness approaches 1000 nm. As expected, the increase in the fractional power propagating in the metal layer results in higher loss for the TM mode In contrast, the TE mode shows no surface field enhancement. Extending this analysis to the case of an infinitely thick superstrate, theoretical plots of PER as a function of superstrate refractive index are presented in figure 3 .5, using several different excitation wavelengths For each wavelength, there exists a unique value of superstrate refractive index which maximizes the polarization extinction ratio This variation of PER with superstrate index can be used to measure the latter We define the minimum detectable change in the superstrate refractive index as An. = mm MxPER(n) a -a PER(11,,n) nc (3.4) where M is the signal-to-noise ratio (as a percentage) and nc is the nominal superstrate index If we assume a superstrate index of 1.42 and a signal-to-noise ratio of 20 dB (1% measurement precision) using a 670 nm excitation source yields Anmin = 7x10-5 As is usually the case, there is a trade-off between sensitivity and dynamic range. In regions where the derivative term in equation (3 .4) is large and the sensitivity is high refractive index can only be monitored over a small interval Conversely when the derivative term is small refractive index can be measured over a wider range of values but Anmin is larger. Measurement of the actual refractive index of the superstrate requires a knowledge of the extinction ratio at two or more wavelengths.

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0.50 0.45 -;-0.40 0.35 ::::s ,e 0.30 ,!!.-0.25 ~0.20 0 i 0.15 I I --A= 670 nm J I I -A.,..-.., ... '-J / '~ I '\. I .......... 0nm "-300 nm :g 0.10 0.05 0.00 -0.5 I\ I I 1000 nm 0.50 0.45 0.40 -0.35 c ::::, 0.30 -e ~0.25 .2:-~ 0.20 Q) "E 0.15 V / ,'.._ ~// ..,,,,, 0 0.5 1 I I ->--A= 670 nm I j I I -----I I I 1.5 2 2.5 posHlon (um) (a) r--... I I \~ I --------0nm 3 3.5 4 \.. " "-. 0 .10 0.05 0.00 I 300 nm ------1000 nm V __, I I I -0.5 0 0.5 1 1 5 2 2 5 3 3.5 position (um) (b) Figure 3.4. Intensity profiles of the SPR waveguide at 670 nm with various thickness superstrates of index 1.415 (a) TM mode fields (b) TE mode fields 4

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0 ... -20 . \ I 594 nm \ I ./. ..-. -40 611 nm E I u I ...... .. m 633 nm I "C -60 ....., 0 I -., ... I C -80 I 0 \ = I u I C I -670 nm x -100 ' I -120 \ I 705 nm -140 1.34 1.35 1.36 1.3 7 1.38 1.39 1.40 1.41 1.42 1.43 1.44 superstrate refractive Index Figure 3 5 Response of the surface plasmon sensor to changes in superstrate refractive index The superstrate is assumed to be infinitely thick Excitation wavelengths are shown next to their respective curves

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From a comparison of figures 3.3 and 3.5, it is apparent that in order to maximize the derivative term in equation (3.4), the spectral width of the plasmon resonance must be as narrow as possible. The resonance width is proportional to t;_/E2 r, where Er and Ej_ are the real and imaginary parts of the permittivity of the metal layer, and in general decreases at longer wavelengths .31 As we will see later, the resonance wavelength can be readily controlled by the tuning layer. Thus, operating in the infrared region would improve sensitivity. In addition, other metals, notably silver, 32 support narrower plasmon resonances than gold. Optimization of the sensor design parameters should allow &\min to be reduced to the order of I o-6 or less. 3.2.2 Integrated-Optic Surface Plasmon Waveguide Structures The key to designing an SPR waveguide is to build the device in such a way that resonance occurs at a convenient wavelength Additionally, it is necessary to control the PER max, so that the TM output signal is measurable. The structures we have investigated, shown in figure 3.2, address both of these issues. As noted earlier, surface plasmon resonance is excited by placing a thin metal film in close proximity to an optical waveguide, so that the TM-polarized waveguide and plasmon modes are coupled The thickness of the metal film needs to be on the order of the penetration depth of the optical wave into the metal, typically about 20 to 30 run, to ensure maximum sensitivity to refractive index perturbations in the surrounding environment. As seen in figure 3.6, changing the thickness of the gold layer used in the SP waveguide design has a significant impact on both A SPR and PER max_ As the metal thickness is increased, ASPR moves to longer wavelengths, while PERmax decreases At the same time, the FWHM of the resonance broadens, resulting in decreased refractive index sensing capability From these calculations, it is evident that 20 to 40 run of gold are required for a useful device Furthermore, it is evident that tight fabrication tolerances are required to build SP waveguide devices with the desired resonance characteristics

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,f.,J 0 . -20 \ / .. .-. -40 E .. 40 nm m ,::, \ ._,,, -60 0 -\ a, ... C -80 0 --u \ I C .100 -120 \ I \ / 20 nm -140 \J 0.58 0.6 0.62 0.64 0.66 0.68 0.7 0.72 wavelength (m) Figure 3 6 Extinction ratio as a function of wavelength for several different metal (gold) layer thicknesses The device is coated with a 1 m thick adsorbed layer of refractive index nc = 1.415

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In this configuration, propagation losses are controlled by the insertion of a low index buffer layer (figure 3.2), placed between the waveguide and the metal film, as demonstrated in figure 3.7 Since the ion-exchanged waveguide is slightly asymmetric, the evanescent wave in the silica buffer layer decays rather rapidly and as a result, only a relatively thin buffer layer of less than I m is required to achieve an acceptable PER. Decreasing the thickness of the buffer layer from 750 nm to 400 nm increases PERmax by an order of magnitude, from about -20 dB/cm to -200 dB/cm at 670 nm In the quest to minimize Linmin, losses need to be maintained at levels which are measurable Metal induced losses to the TE mode, which are nearly 2 dB/cm when the buffer thickness is 400 nm also need to be minimized An important consequence of this design is that A SPR is virtually unaffected by buffer thickness. The primary feature that sets these designs apart from others17 20 22 25 is the inclusion of a high index dielectric tuning layer deposited either directly above or below (figure 3 2) the metal layer, which provides a large degree of flexibility in choosing the resonance wavelength. The function of this layer can be explained by a simple coupled mode argument: by virtue of proximity, the propagation constants of the SP modes are strongly dependent on the thickness of the high index layer, while those of the waveguide modes are not. Thus, in accordance with figure 3.1, changing the thickness of the tuning layer shifts the resonant wavelength. In figure 3. 8, the variation in A SPR is shown against tuning layer thickness. The peak resonance wavelength can be easily tuned over a 130 nm range, while maintaining a reasonable level of TE loss. A SPR is very sensitive changing by over 2 nm per nanometer of tuning layer thickness, which again underscores the need for close tolerances during fabrication. Conventional SPR waveguides lack the tuning layer and require that A SPR and the PERma x must be set by simultaneously adjusting the metal and buffer layer thickness.17 20 22 25 Waveguide parameters such as maximum index change and depth which determine mode index, are also important in determining A SPR in conventional SPR

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...... E m "C ...,_,, 0 i ... C "I 50.....---.------.-----,-----,---------r---r-20 0 -50 18 --+-16 14 e m -----------+----------4-4-----........_-------12 :!?-400 nm \ .,, u, .2 -100 u 10 .2 C .2 -+-__._ __________________ 8 I : \ \ .5 400 nm / 6 C i a, 500 nm -150 ---I 750 nm -200 I -----r--g'--:----.L.!-J 2 .. . ........... ........... ........... 0 0.58 0.6 0.62 0.64 0.66 0.68 wavelength (m) Figure 3 7 Extinction ratio as a function of wavelength for several d i fferent buffer (SiO2 ) layer thicknesses. The device is coated with a 1 m thick adsorbed layer of refractive index nc = 1.415.

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710 690 ....... E 670 s .c g>650 c 'ii 630 3: 610 C ., C 590 c 1570 c D. 550 530 V, V I -/ I ..... / V I -..... V I / I V I / V I r-/ w V I r-,# / / .... / ... r-/ V --------,__ --I I I ' ' ' ' ' 0 10 20 30 40 50 60 70 80 tuning layer thickness (nm) 7.0 ,,,_. E 6.0 m 'i:, -5.0 a C c -4.0 i .:..:: ., c 3.0 a. ii .,, .,, 2.0 .2 C 0 t: c 1.0 .5 0.0 w ... Figure 3 8 Peak resonance wavelength and TE insertion loss at resonance as functions of tuning layer thickness The device is coated with a 1 m thick adsorbed layer of refractive index nc = 1.415

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devices In contrast, the tuning layer design allows PER max and A SPR to be set relatively independently of one another Furthennore, the large flexibility in choosing A SPR in tuning layer-equipped devices allows the waveguide characteristics and metal film to be chosen fairly independently An alternative approach to tuning the resonance wavelength is to use electro-optic (EO) materials, such as liquid crystal, in the design of the SPR device.33 In this case, an EO material is positioned next to the metal layer of the plasmon device. Applying a voltage across the EO material changes its refractive index which consequently alters A SPR Thus, incorporation of EO materials in SPR devices allows active control of the plasmon response Shifts in resonance wavelength of almost 200 nm have been achieved by applying 30 volts across a liquid crystal layer in a bulk SPR device .33 3.3 Experimental Investigation of Surface Plasmon Devices 3.3.1 Device Fabrication The SPR sensors were built on planar K+-Na + ion-exchanged waveguides. BK7 glass substrates were cleaned by sequential immersion in trichloroethane, acetone, and methanol for 10 minutes each The substrates were then rinsed with deionized water blown dry with a filtered nitrogen gun, and baked in an oven at 90 C for 1 hour. The ion exchange was perfonned by placing the substrates in a bath of molten potassium nitrate housed in an aluminum vessel at 375 C for 3 .5 hours, as shown in figure 3 9 This produces a birefringent waveguide with ~nTE = 0 008 and ~TM= 0.0092 and d x = 3 m For wavelengths above 600 nm the waveguide is single-moded A thennocouple and temperature controller (Omega CN8500) were used to maintain a constant bath temperature The samples were then removed from the melt and cooled slowly to avoid thermal stress-induced cracking and rinsed in deionized water Next, thin films of SiO2 Ti 02 and gold were deposited sequentially on top of the waveguide in an electron-beam

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Aluminum Container JV Thermocouple Temperature Controller Figure 3.9. Fabrication apparatus for constructing potassium-sodium ion-exchanged waveguides

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JI evaporator, at thicknesses of 500 run, 55 nm, and 30 nm, respectively. Lastly, the substrate was cut into 1 cm squares, and endfacets were polished to permit endfire excitation. In the polishing process, the sample was first waxed to a dummy piece of glass with a low temperature wax to prevent chipping of the edges The sample/dummy composite was then mounted vertically with wax in a vise-like holder, with the endfacet held parallel to the plane of polishing A paste consisting of polishing powder (SiC or A12O3 from Buehler) and water was prepared on a clean, flat sheet of glass and the sample was moved across this paste (by hand) in a figure-eight pattern for a few minutes It was polished sequentially with 400 grit until the edge was flat (typically less than 5 minutes), 20 m size for 5 minutes, and 5 m size for 10 minutes. Between steps, the sample was rinsed thoroughly with water. Next, the sample was placed on a mechanical polishing wheel and ground on a 3 m pad wetted with 1 m alumina paste for about 60 minu t es At this point the endfacet was generally free of major scratches. The sample received its final polish on a soft TEXMET pad (Buehler), wetted with MASTERPOLISH 2 (Buehler) a colloidal suspension of 0 06 m silica in a high pH solution MASTERPOLISH 2 provides a chemical/mechanical polishing action, for optimum results The final pol i sh typically took about an hour, after which the endfacet appeared completely free of scratches under a 20X microscope lens The sample was typically about 9 mm long after polishing. It was found that at first, the polishing process tended to cause extensive scratches to the exposed and highly fragile gold film To alleviate this problem, a 75 nm layer of SiO2 was evaporated over the gold prior to polishing The superior abrasion resistance of this top SiO2 layer referred to as the cap, allowed the devices to be polished with minimal damage to the gold layer. The modelling shows that the addition of the cap will shift the plasmon response to slightly longer wavelengths

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SL. 3.3.2 Experimental Measurement of Refractive Index Figure 3.10 shows the experimental arrangement for the device characterization Light from a monochromator (Digikrom 240, dispersion 3 .2 nm / mm) was collimated with a 25 mm focal length lens, passed through a chopper (Ro l fin-Sinar) and fed into a 100 m core multimode fiber through a 20X microscope objective The fiber was then butt coupled directly to the SPR waveguide Light leaving the waveguide was collected b y a combination of a 40X microscope objective and a cylindrical lens, passed through a high extinction (30 dB) Gian-Thompson polarizer to resolve the individual polarization components and ultimately detected with a silicon photodiode A slit aperture was placed on the front of the photodiode to serve as a spatial filter and block light launched into substrate radiation modes. A lock-in amplifier was used to improve the signal-to-noise ratio With the monochromator output slit at 2 mm corresponding to a spectral resolution of 6.4 nm, typical output powers collected from the waveguide were on the order of a few tens of nanowatts at wavelengths off resonance The polarization extinction ratio of the SPR waveguide was measured by coating the device with different index matching oils. 34 The stated accuracy of the refrac tive indices of these fluids is+/-0 0002 Transmission spectra for the TE and TM modes were measured separately by adjusting the output polarizer. In order to compute the PER, the 1 oss terms from equation (3 1 ) ATE and A, are expressed as dB per unit di stance as ATE, T M = (_!_)lO lo (P:E,TM /pTE,TM) L g m out (3. 5) where Lis the length of the waveguide and P i n and Pout are the power coupled into and out of the waveguide In this experimental configuration only the output power Pout can be readily measured In our launching scheme using incoherent light and a multimode fiber it is reasonable to assume that the fractional powers launched into t he TE and TM modes

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monochromator c ______ :::> lens (f= 25 mm) chopper c ______ ::::, 20X lens multimode fiber lock-in amplifier SPR waveguide c ______ ::::> 40X lens ~--~ cylindrical lens D polarizer slit (spatial fil t er) photodiode Figure 3 10 Experimental setup for characterizing the SPR wavegu i de sensor.

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are approximately equal This allows equation (3. I) to be rewritten as (1) TM TE PER = L 10 log (P out/P out) (3.6) In figures 3.11 (a) (d), the TE and TM transmission spectra and corresponding PER are shown, for superstrate refractive indices of 1 (air), 1.340, 1.380, and 1.410 respectively Plasmon resonance, evidenced by a superstrate-dependant decrease in the TM transmission over a narrow wavelength interval, is clearly observed. In accordance with theoretical predictions, the resonance shifts to longer wavelengths as the superstrate index increases. As expected, the TE transmission spectra are relatively unaffected by the superstrate refractive index The wavelength dependance of the polarization extinction ratio is summarized and compared against the theoretical model (updated to include the SiO2 cap) in figures 3 12 (a) and (b), with slightly better PER than shown in figure 3 11. This improvement was realized by using a narrower slit over the detector. Experimental PER values are about an order of magnitude less than those predicted by the model. In addition, the peak resonance wavelengths are about 30 nm greater than expected. The large discrepancy in PERmax between theory and experiment is most likely due to imperfect spatial filtering of light launched into substrate radiation modes. PER is computed from the loss of the guided modes, in particular, from the guided TM mode which excites plasmon resonance Radiation modes do not significantly interact with the surface plasmon modes supported by the gold layer, and as such, experience minimal polarization-dependant loss TM polarized radiation modes which reach the detector can easily overwhelm the strongly attenuated signal from the guided TM mode, resulting in the appearance of a larger TM signal at the detector and causing the TM loss term in equation (3. I) to seem considerably smaller. While this problem could in principle be circumvented to some extent by using a single-mode fiber to excite the waveguide, it was found to be extremely difficult to couple

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j) 60 15 ./ pTE pTM / PER 50 10 E \ ... -'in' \ m 'I 40 0 ii \. 6 l! -30 : I C A. 0 11 J20 C 0 -i -5 10 c5 A. 0 -10 500 650 600 650 700 750 800 wavelength (nm) (a) 80 16 70 10 E 60 ... -'in' m =; 60 6 .S! ii l! C ~40 0 I I! A..30 0 ti ... C lit 0 .. 20 i -6 .. c5 10 a.. 0 -10 600 660 600 660 700 750 800 wavelength (nm) (b) Figure 3 .11. Transmissio n and polarization e xtin ct ion ratio spectra for the capped SPR waveguide sensor for differing superstrate indices (a) nsup = 1 (air) (b) nsup = 1 340 (c) nsup = 1.380 (d) nsup = 1.410

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JO 60--------------.----,-----...----"""T""------r-15 pTE _404------+---------------------+-------t . -E! 304-----------r---t----+--+--------------t .!!,.. !! a, -.; 20-+-------+----~----f--3oo~-'.---t-----hjP=,!l~~-L ... 0----................. ----..-....... ---.--------------+-.--.---.---,--+-....----.-.---.---+---10 500 550 600 650 700 750 800 wavelength (nm) (c) 0-+-....----.-.--.--+--,-....,........----.-t---,--,---,-....,....-+-..--,.......,.-.--+-....,....-,-..-......-1--,---,-....,....-,-+--8 600 660 600 660 700 760 800 wavelength (nm) (d) Figure 3 .11. Transmission and polarization extinction ratio spectra for the capped SPR waveguide sensor for differing superstrate indices (a) n sup = 1 {air) (b) nsup = 1.340 (c) nsup = 1.380 (d) nsup = 1.410

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SI 40 20 E u -0 m :!!--------------0 -20 C 0 .. u i -40 C j -60 : --a.. -80 : 1.310 n : n 1.335 : --------1.380 : n= .. .. -100 500 550 600 650 700 750 800 wavelength (nm) (a) 15.00 -.------r-=---,,----"r"'-----r-------r-----, n = 1 .000 E 10.00 -t-----.....-~----11----+---1 ---n = 1.310 u 5.00 -t------+----+--1----+---1 n 1.335 n 1 .380 0 1! i 0.00 \ 11 c: -5.00 -+------+--------,,---------~--++---.'--+----..._ i ... &, -10.00 -+-----+------+-----+---"'lr----+"-r-,.-"----t-----15.00 500.00 550.00 600.00 650.00 700.00 750.00 800.00 wavelength (nm) (b) Figure 3 .12 Polarization extinction ratio for the capped SPR waveguide sensor (a) theoretical prediction (b) experimentally measured response

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a detectable amount of power into such a fiber, using the available equipment. In particular, a tunable laser would be highly desirable To a lesser extent, defects in the gold film may have also contributed to some degradation of the PER.21 From figure 3.6, it is clear that variations in the thickness of the gold film significantly affect resonance characteristics Metal films with thicknesses on the order of a few tens of nanometers tend to be somewhat porous and exhibit grain boundary-related roughness which contributes to light scattering and possibly polarization conversion. Some evidence of metal roughness is seen in the width of the plasmon resonances shown in figures 3 12(a) and (b). The measured resonance width is about 60 nm, roughly twice that predicted by the simulation. In addition, the refractive index of metal films varies with both the film thickness and the deposition technique. Refractive index data used in modelling were taken for a 50 nm thick gold film, 24 compared to the 30 nm thickness in our experiment. This may account for the difference in resonance wavelengths between theory and experiment. High temperature annealing could have been employed to improve the quality of the gold layer, but would have altered the waveguide characteristics, complicating the overall design of the device From figures 3.6 and 3.8, it is also apparent that errors in the thickness of the gold layer as well as that of the TI02 tuning layer would result in a shift in 11,SPR_ To calibrate the SP waveguide for refractive index measurements the polarization extinction ratio was measured at 658 nm and 708 nm as a function of superstrate index. This data is shown in figure 3 13. This device is clearly useful in measuring refract iv e index over a wide range depending on the choice of excitation wavelength. Using equation (3.4), a sensitivity, or minimum detectable index change of -7xl0-5 had been predicted for this device, at a signal-to-noise ratio of 13 dB. However, as noted earlier experimentally measured PER values were found to be about an order of magnitude less than predicted Consequently the minimum index change which was experimentally -4 resolvable was only about 5xl0

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2 0 .-. E u ...... -2 m ,:, .._,, 0 -4 i 0::: C 0 -6 =e .5 iH -8 C D 0 D I i -10 \ I II 0 a. -12 -14 -+--.....-......... --+----........ i----,.----.----+-........ ,.........,.--+---.--,--.....--+---,----,--,,---1 1.3 1.32 1.34 1.36 1.38 Superstrate Refractive Index 658 nm, measured 0 708 nm, measured 658 nm, theoretical, +10 -708 nm, theoretical, + 10 1.4 1.42 Figure 3. 13. Theoretically predicted (lines) and experimentally measured (symbols) polarization extinction ratio against superstrate refractive index for excitation wavelengths of 658 and 708 nm Theoretical curves have been scaled to be a factor of 10 smaller for comparison purposes

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4U 3.3.3 Humidity Measurement Many materials, in particular organic polymers, exhibit a humidity-dependant refractive index By coating a thin layer of such a material onto the surface of an SP waveguide, it is possible to create a device in which the PER at a given wavelength varies with atmospheric moisture content. In this manner, the SP waveguide can be used as a humidity sensor It was observed that Nation fluoropolymer exhibits a tendency to swell to the point of cracking when immersed in water As such, this material was deemed to be appropriate for use as a humidity transducing layer. 35 Nation fluoropolymer was obtained from Aldrich as a 5% polymer solution in a mixture of lower aliphatic alcohols and water and diluted to I% by the addition of methanol. A few drops of the dilute solution were deposited onto the SP waveguide and allowed to dry at room temperature. This produced a layer of Nation on the order of 5 m thick, which covered the entire surface of the dev i ce Variations in the thickness of the Nation layer do not significantly affect device performance, since the film thickness is always greater than the penetration depth of the evanescent tail of the plasmon into the polymer The humidity-induced variation in the PER of the Nation-coated SP waveguide is shown in figure 3 14 over a range of 20% to 50% relative humidity (RH). Over this range the sensor exhibits a reasonably linear response changing by 0.030 dB/ RHcm for 658 nm excitation and -0 073 dB/0/oRHcm for 708 nm excitation. Humidity-dependent changes in the index of Nation occur rather rapidly on the order of tens of seconds, and appear to be fully reversible No attempt was made to control the temperature, which varied from 22 C to 26 C over the course of these measurements Comparing figures 3.13 and 3 .14, we find that the refractive index of Nation changes from 1.3460 0005 at 20% relative humidity to 1.358 002 at 50% relative humidity For comparison Fan and Harrison36 have measured the refractive index ofNafion as 1.32 .03 at 632 8 nm, using ellipsometry. At first glance it appears counterintuitive that the refractive index of a material like Nation which swells in water should increase for higher humidities In the following

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-3.0 -3.5 .-.. E u -4.0 ..... m ~-4.5 0 :::: -5.0 C .2 -5.5 u C -6.0 w C .2 -6.5 I as -7.0 0 a. -7.5 ----8.0 20 ...... 11.... .._ --.......... r-.... ., 25 41 T ~-----...... ...... ,.-------..... - H :1-4 -....... ,..__ I -......... I I -............. ..._ ...J 30 35 40 Relative Humidity (%) = 658 run D =708nm -.......:; I 1.., ..... ....... .H 45 50 Figure 3 14 Humidity response of the capped surface plasmon waveguide when coated with a thin film of Nafion fluoropolymer

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we analyze this result theoretically. From the Lorentz-Lorenz relation, the refractive index of a material may be related to its density, p, as37 2 (n -1) = Kp (n2 + 2) (3.7) where K is a constant. At zero humidity, the density of the film is (3.8) where mF and VF are the mass and volume of the dry film, respectively Thus, in principle, humidity-induced swelling should reduce the density of the Nafion film, resulting in a consequent decrease in index. However, in addition to swelling, Nafion also absorbs large quantities of water from the surrounding atmosphere, resulting in a significant mass increase Sadaoka et al.38 find that the water-content of Nation films can be as high as -llO mg/g at 80% relative humidity, depending on the film processing conditions. To examine the simultaneous effects of swelling and water gain, we take the derivative of (3.7), yielding 2nAn[-l n2 -1 ] = K[AmF mFAVF] n2 + 2 (n2 + 2) 2 VF v; (3. 9) where AmF and AVF are the humidity-induced changes in film mass and volume respectively, and An is the consequent change in refractive index Rearranging terms in (3 9) to solve for the index change gives An= (n2-l){n2+2)[AmF_AVF] 6n mF VF (3. 10) Clearly, water absorption and swelling compete in the overall change in refractive index When (AmF/mF) < (AVFNF) swelling dominates, and refractive index decreases with increasing humidity Conversely and evidently in the case for Nafion in the humidity range studied, mass increase due to moisture uptake dominates volume increase, and refractive index increases with higher humidity, when (AmF/mF) > (AVFNF)-

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4J The refractive index of polyimide, another material with a well-known moisture dependant refractive index, behaves similarly in that the index increases with humidity. Moisture uptake in polyimide is on the order of 2%. Franke et al.39 report that the refractive index of the polyimide SIXEF 33 changes from 1.5512 at 52% RH to 1.5525 at 96% RH, which is about an order of magnitude less than for Nation. 3.4 Proposed Surface Plasmon Structures With Improved Performance The SPR sensor discussed to this point offers an excellent means for detecting extremely small index changes arising from changes in the environment. This high sensitivity, in conjunction with appropriate transducing layers, makes these devices appropriate for use in a wide range of applications. However, from a "real world" viewpoint, large volume manufacture of these devices would probably not be possible The unfortunate drawback of integrated optics is that in general, packaging issues, namely endfacet polishing and fiber coupling, may account for as much as 90% of the final device cost. Thus, even though the ion-exchanged SPR waveguides are simple in design, it is unlikely that a low-cost commercial product will be realizable through this route Because of this, we have also explored designs for SPR sensors based on Si02/Si waveguides. Silicon is perhaps the most mature processing technology, due to the large competition in the semiconductor sector. Building devices on silicon wafers offers the inherent advantage of batch processing, allowing the potential for high volume manufacture. Silicon has a high quality natural oxide, Si 02 which can be grown either by high temperature oxidation or a number of other means The single-crystal nature of silicon allows waveguide endfacets to be prepared simply by cleaving the substrate along the appropriate crystal plane Furthermore, through anisotropic etching with solutions of either potassium hydroxide/isopropanol/water40 or tetramethylammonium hydroxide/ water,41 V-groove structures to facilitate passive fiber coupling can be built into the

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44 substrate. Thus, packaging of silicon-based waveguides is considerably easier and less expensive than that of glass ones. Waveguides are deposited on silicon substrates by chemical vapor deposition (CVD), flame hydrolysis, or plasma enhanced CVD (PECVD) These techniques allow layers of silica to be deposited at micron thicknesses Unlike thermal oxidation, these techniques allow the addition of dopants to the oxide during deposition. Typically, the oxide is doped with boron and/or phosphorous to reduce stress and the resultant g l ass is known as a boron-phosphorous silicate (BPSG) Silicon oxynitride, SiOxN1_x, can also be deposited Both SiOxN1_x and BPSG have higher refractive indices than silica and through tailoring of the exact composition substantial refractive index variations are possible.42 43 As an example, a typical SiO2/Si waveguide could consist of an SiO2 cladding sandwiched between a BPSG core and a silicon substrate. The quality of CVD deposited oxides is not as high as thermally grown ones As such, the starting point for SiO2/Si waveguides is generally a thermally oxidized silicon wafer We have designed and modelled an SPR waveguide based on the S i O2/Si technology Shown in figure 3 15(a) the structure uses an oxidized silicon wafer for the substrate. The waveguide core either SiOxN1_x or BPSG, is deposited next and has a step-index profile. A second layer of silica, serving as the buffer layer is deposited on top of the core, followed by a 35 nm layer of silver and an 86 nm SiO2 tuning layer The core and buffer layers are 1 .5 m and 2 .5 m thick respectively The refractive index of the waveguide core is chosen to be 0.01 higher than that of pure silica which is compatible with existing deposition processes The dispersion of the core is assumed to be equal t o that of pure silica. In principle, the waveguide core and buffer layers can be made during a single run, simply by changing the gas chemistry Figure 3 15(b) shows the proposed SPR device on a substrate with an integrated V-groove structure for fiber coupling In this case a cut from a wafer saw would be used to prepare a flat waveguide endfacet at the fiber pigtail rather than cleaving.

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4:, air/water input signal .. doped Si02 core (BPSG, SiOxN1_:x) output signal .. : : If mi ~f~Bl~~Bli!elir~ !I silicon substrate (a) End View Side View fiber waveguide \ endfacet waveguide ~= fiber light ... ,------, -1------'I V-groove V-groove silicon substrate silicon substrate (b) Figure 3 .15. The Si02/Si surface plasmon waveguide structure in the protected metal configuration. (a) device structure (b) device with integrated V-grooves for fiber coupling

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40 Note that several modifications have been made to the original SPR waveguide design (figure 3 2) Silver has been used instead of gold as the metal layer, which, as will be seen shortly, improves sensitivity. Since the waveguide is symmetric, a much thicker buffer layer is required to achieve a reasonable PER. An advantage of using the thicker buffer layer however, is that metal-induced TE propagation losses are reduced to less than 0.01 dB/cm, a considerable improvement over the previous GRIN waveguide designs which had TE losses on the order of 1 dB/cm! In addition, the tuning layer has been repositioned to be above the metal, which both protects the metal layer and increases the range over which A SPR can be tuned Furthermore, the accessibility of the tuning layer makes possible post-deposition trimming to correct errors in the resonance wavelength which may arise from fabrication tolerances in the thickness and index of the various constituent layers in the plasmon overlay This design offers a degree of durability which is generally lacking in SPR devices and we aptly refer to it as the "protected metal configuration." As noted earlier, the advantage of substituting silver for gold in the SPR sensor lies in the width of the spectral response of the PER. In figure 3 16, PER for the SiO2/Si SPR waveguide is plotted against wavelength for various adsorbed films with thicknesses ranging from Oto 300 nm and an index of 1.415 The medium above the adsorbed film is water (n = 1.33) The observed plasmon resonance has a FWHM of about 11 nm, roughly one-third of that of the gold-based GRIN SPR device Additionally, the shift in A SPR produced by adsorption of the thin films is much larger than in the previous device Adsorption of a 100 nm thick film changes A SPR by 90 nm An interesting consequence of positioning the tuning layer above the metal film is that the positive-valued PER peak at shorter wavelengths seen in figure 3 .3 vanishes In fact, the plasmon resonance dip is observed even prior to film adsorption. Extending this analysis to the case of infinitely thick superstrates, we next present the PER as a function of superstrate index in figure 3 .17 Excitation wavelengths of 633

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0 -.. I I e -5o ,, u ...._ 0nm m ,:::, '-" 0 -100 -m ... C .2 -150 ,:; C x CD C -200 0 -m i -250 -0 c.. -300 600 .. 650 47 -. ----------------:>c ----' v\ I \ I \ I \ I I \ I I i \ I ,, 100 nm 200 nm I I I I I I I I I I I 700 750 800 850 wavelength (nm) Figure 3 16 Spectral response of the SiO2/Si silver-based SPR waveguide (protected metal configuration) during the adsorption of thin films of varied thickness The adsorbed material has a refractive index of 1.415

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0 -10 ..-. E u -20 m 'tJ .._.. .S! -30 C .S! -40 u C 'Sc -50 C 0 ii lll -60 11 -0 c._70 --------.. I \ I \ I 633 nm \ I -4lS \ \ I I \ I \ I \ \ I I I I \ I \ I \ I \ I I I I 670 nm \ I \ I I ,, -80 1.31 1.32 1.33 1.34 1.35 1.36 superstrate Index -----/ I I I 1.37 1.38 Figure 3.17. Refractive index dependance of the polarization extinction ratio for the SiO2 / Si silver-based SPR waveguide (protected metal configuration). The superstrate is infinitely thick. The excitation wavelength is 633 nm.

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run and 670 run are used. Again, the range of index values over which resonance is excited is much smaller than the previous device Using equation (3.4), at a superstrate index of 1.334, An = 4x10-5 assuming a signal-to-noise ratio of 20 dB. This represents a 40% mm improvement in sensitivity over the gold-based sensor 3.5 Application of Surface Plasmon Resonance to Monolayer Detection An important area of interest for surface plasmon sensors is immunoassay, in which extremely thin films must be detected.14 15 16 In such applications, the SPR device is coated with a protein or antibody monolayer film, on the order of 1 to 2 nanometers thick. This film serves as a transducing layer to catalyze various types of chemical interactions, such as complexation and desorption, with analytes in the surrounding envirorunent. These reactions are monitored through changes in the plasmon resonance characteristics.22 For instance a common immunoassay study is molecular self-assembly, which involves the accumulation of several monolayers of different materials During the process, the addition of each monolayer produces a discrete shift in the plasmon resonance Morgan et at.15 have used this technique to monitor the sequential deposition of monolayer films of biotin avidin and bisbiotin onto a gold surface The highly selective nature of protein-ligand and antibody-antigen binding offers the potential for realization of analyte-specific sensors In order to examine the applicability of the SiO2/Si SPR sensor to immunological studies, we have calculated PER as a function of adsorbed film thickness using a fixed excitation wavelength of 633 run. The adsorption of films of index 1.40 and 1.45 is analyzed and the medium surrounding the device is water (n = 1.33). As shown in figure 3.18 the device is quite sensitive and is in fact able to detect films thinner than 1 nm! Adsorption of a 2 nm film causes the PER to change from -40 dB/cm init i ally to -33 dB/cm and -27 dB/cm for films of index 1.40 and 1.45 respectively The plasmon response is inherently nonlinear and the sensitivity defined as the derivative of PER with

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0 -5 .-. E u -10 m ,, --15 0 ii ... -20 C .2 ,:; C -25 C -30 .2 ii -35 ., 0 a.-40 -45 ::,u I I 633 nm I nc = 1.45 ---...---/ i...-----------/ .,,,,,,,..,,,,,,,.--/;" I /, nc = 1.40 // I / / ~" V / / / / /,, / lt/ 0 2 4 6 8 10 12 14 superstrate thickness (nm) Figure 3 18. Response of the SiO2/Si silver-based SPR waveguide (protected metal configuration) to ultra thin adsorbed films. The excitation wavelength is 633 nm The tuning layer thickness is chosen so that A SPR initially (prior to film adsorption) coincides with the excitation wavelength

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::,1 respect to adsorbed film thickness, decreases as the film becomes thicker Nevertheless, films of up to about 10 to 15 nm can easily be measured with this device, which is adequate for detecting monolayers. Sensitivity is also clearly larger for high index films The improvement in the sensitivity of this device as compared to the design discussed previously results from narrowing the spectral width of the plasmon resonance. 3.6 Conclusion Surface plasmon resonance provides a highly sensitive means for detecting small perturbations in the refractive index of the surrounding environment. Integrated-optic SPR sensors, consisting of waveguides coated with thin metal and dielectric layers have been modelled extensively. A dielectric tuning layer which simplifies the design process was added to the basic SPR waveguide structure Designs based on both GRIN and SiO2/Si waveguides have been examined. A GRIN SPR sensor have been fabricated by depositing a plasmon overlay with a thin layer of gold onto an ion-exchanged waveguide Measurements of the polarization extinction ratio of the GRIN SPR waveguide show reasonable agreement with theoretical predications and a humidity sensor was produced by coating the device with a thin film of the moisture-sorbing polymer Nation Simulations of an SiO2/Si SPR waveguide which uses silver instead of gold in the design show the potential for a 40% improvement in refractive index sensing capability over the GRIN device. Furthermore, calculations show the SiO2/Si SPR sensor to be capable of monitoring the adsorption of films thinner than 1 nm, making this device attractive in immunological studies such as molecule self-assembly and protein-ligand binding.

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CHAPfER4 FABRICATION AND CHARACTERIZATION OF POLYMER WAVEGUIDES 4.1 Advantages of Polymer Waveguides Waveguiding in optically transparent polymers has been studied since the early 1970s. Today, polymers with a wide range of optical, chemical and mechanical properties are available, offering numerous possibilities for novel optical devices Extremely low propagation losses have been achieved, using polymethyl methacrylate (0. 12 dB/cm at 633 nm),44 polycarbonate {0. 19 dB/cm at 830 nm),45 optical grade epoxy (0.3 dB / cm at 633 nm),46 deuterated fluoromethacrylate (0 1 dB/cm at 1300 nm),47 and others A number of companies, including DuPont, Amoco, Allied Signal, and others are presently developing materials with even better performance From an economic viewpoint polymer waveguide fabrication requires little specialized equipment and, within the constraint of reliability issues, has the potential to be considerably less expensive than other guided-wave technologies such as Ti: LiNbO3 and SiO2/Si. The application which perhaps receives the largest benefit from the diverse nature of polymers is the sensor field. Material properties, such as preferential adsorption of specific chemicals and environmentally-induced swelling, can be exploited in polymer waveguide sensors allowing unparalleled levels of performance and versatility A significant advantage which polymer waveguides have over conventional integrated-optic technologies such as lithium niobate and ion-exchanged glass is the relative ease with which a chemically-sensitive dopant can be incorporated into the polymer matrix.4 6 ,48, 49 This offers possibilities not readily achievable otherwise and allows the detection of a wide range of analytes Several integrated-optic chemical sensors based on waveguides 52

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JJ fabricated from a material known as polyimide will be presented in chapters 5 through 7 However, in order to facilitate a better understanding of those devices, we will first describe the polyimide waveguide fabrication process 4.2 Fabrication of Polyimide Waveguides Polyimides are a broad class of polymers which possess excellent chemical, mechanical, and thermal stability They are used extensively in the semiconductor industry as dielectric materials in integrated circuits50 and are also used in planarization51 and micromachining52 applications. Additionally, many polyimides may be photolithographically patterned through commonly used photoresist processing techniques Recently, a number of companies, including DuPont, Amoco, OCG Microelectronics Materials, Hitachi, and Hoest-Celanese have developed optically transparent polyimides, laying the foundation for the use of polyimide in waveguide applications. Propagation losses in planar polyimide waveguides as low as 0.2 dB / cm at 800 nm53 and 0 3 dB/cm at 1300 nm54 have been reported. Unlike many other types of polymers waveguide materials, the glass transition temperature of these materials is very high, often well in excess of 300 C 55 56 57 Thus, polyimide waveguides can survive elevated temperature environments In contrast, the glass transition temperature of polymethyl methacrylate is only 85 C .44 We have experimented with the Probimide 400 series of photosensitive polyimide from OCG Microelectronics Materials. This material is obtained as a solution of fully imidized benzophenone tetracarboxylic dianhyride-alkylated diamine (BTDA) polyimide, dissolved in y-butyrolactone (GBL) solvent and is slightly amber in color. Probimide 400 may be deposited on various substrates by spin coating and behaves as a negative resist for photolithographic purposes. Three products are available in this series : Probimide 408 OCG Microelectronics Materials, Inc 200 Massasoit Ave East Providence, RI 02914

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(8 5% solids, 580 cs viscosity), which can be deposited from a thickness of 0.5 m to 4.0 m, Probimide 412 (12.5% solids, 3500 cs viscosity), which can be deposited from 3.0 m to 12.0 m, and Probimide 410 (8 5% solids, 8200 cs viscosity), which can be deposited from 5 0 m to 20.0 m.58 The glass transition temperature for this material is 356 C The procedure employed to produce polyimide waveguides is shown in figure 4 .1. In the following, we describe each step. 4.2.1 Substrate Preparation Thermally oxidized silicon wafers are attractive as substrates for spin-cast polymer waveguide, both because of the high optical quality of the Si02 layer and the simple fact that spin-deposition processes work best on round substrates As will be seen shortly, the refractive index difference between polyimide and Si02 is rather large Therefore, as seen from equation 2.4, y3 is large and the field decays rapidly into the Si02 layer As such, only relatively thin buffer layers of the oxide are required to reduce the absorption due to the underlying silicon. Oxidation is performed by placing clean, 2" diameter, <100> oriented silicon wafers into a tube furnace at 1050 C. Steam is pumped into one end of the tube, creating a "wet oxygen atmosphere", which greatly enhances the rate of oxidation. Under these conditions, the growth of a 2 m thick layer of Si02 takes 10 hours .59 4.2.2 Wafer Priming Although Probimide adheres reasonably well to glass and silicon, it is generally necessary to treat substrates with a silane-based adhesion promoter for optimum results, particularly with respect to endfacet preparation (section 4.2 7). The adhesion promoter, available from OCG as QZ 3289, is diluted at a ratio of 1 to 9 with a solution of 90/10 v / v ethanol/water (QZ3290). The diluted adhesion promoter solution is stirred thoroughly and allowed to sit for at least one hour prior to use to ensure proper mixing. During this time wafers are baked at 120 C to remove moisture from the surface of the oxide layer After

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1. Oxidize 2" silicon wafer 10 hours at 1050 C under wet 02 2. Apply -0.8 mL 1 : 9 diluted silane adhesion promoter, spin 4000 rpm/20 sec, bake on hotplate at 100 C/20 sec 3 Dispense 0 6 mL P412 (large diameter syringe) 4 Spin wafer: 400 rpm/20 sec, 1000 rpm/10 sec, 2000 rpm/30 sec Wait 3-5 minutes, I silane I Si02 ''111\ii'I polyimide _.::::i:rJrttt ftlf Immritt=== soft-bake on hotplate at 100 C/15 min 5 Photolithographic patterning : UV exposure Development 6 High temperature cure 7. Dope with organic dye (optional) polyimide photomask -----cross-linked region (Same structure) dye-diffused polyimide Figure 4.1 Schematic representation of the polyimide waveguide fabrication process.

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56 the dehydration bake, the wafer to be primed is placed on the spinner and the dilute silane adhesion promoter is applied in sufficient quantity to cover the entire wafer For a 2" wafer, typically about 0 8 mL of solution is required The sample is then spun at 4000 rpm for 20 seconds. This leaves a thin layer of silane on the wafer surface. The presence of moisture on the wafer can interfere with uniform dispersal of the silane film and should be avoided 4.2.3 Polyimide Deposition Deposition of a uniform polyimide film is the key to realizing high quality waveguides However, spin-coating of polyimide is somewhat more complicated than that of conventional photoresists. Specifically, in order to prevent the inclusion of air bubbles in polyimide films during deposition, a syringe with a large diameter tip needs to be used to transfer polyimide solution onto the wafers. This is accomplished by cutting off the end of a 3 mL plastic Luer-Lok tipped syringe (Becton-Dickinson), producing a tube with a diameter of 8 mm. Butyrolactone was used to wipe off the ink markings on the bottom of the syringe, so as not to contaminate the stock polyimide solution. The syringe plunger is pushed down until there is no air trapped in the tube. Approximately 0 6 mL of Probimide 412 (12 5% solids, viscosity 3500 cps) is drawn into this modified syringe and held vertically to maintain vacuum The polyimide is then dispensed onto the center of the wafer into a puddle approximately 2 cm in diameter. Any air bubbles which are visible at this time must be removed, either by piercing them with a sharp, clean object, or simply by letting them rise to the surface and break of their own accord A three-step spin process is then employed to distribute the polyimide solution. The spinner is set for 60 seconds at 400 rpm and activated. After 20 seconds, while the wafer is in motion, the spin speed is increased to 1000 rpm. This rate is held for 10 seconds and then increased to 2000 rpm for the remaining 30 seconds. This procedure is well-suited to uniform dispersal of the high viscosity polyimide across the wafer. The wet film is left to sit for 3 minutes to settle and allow any air bubbles trapped at the polyimide/SiO2 interface to migrate out of the film.

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57 The polyimide-coated wafer is then placed on a hotplate at 100 C and baked for 15 minutes. During this soft-bake, the silane forms a strong chemical bond with the polyimide film, ensuring excellent adhesion to the oxidized silicon substrate. It is important that the soft-bake step be performed on a hotplate, rather than a conventional box-type oven When polyimide films are soft-baked in an oven, the air exposed surface of the film dries first, forming a skin which impedes solvent removal. As the solvent evaporates, this outer skin tends to crack, leading to large surface roughness. At the same time, microvoids are formed inside the film. Both effects cause strong scattering and increase propagation loss by about an order of magnitude. When dried on a hot-plate however, polyimide dries at the substrate interface first and the solvent is efficiently removed without formation of defects 4.2.4 Photolithography 4.2.4.1 Planar Waveguides While planar waveguides do not require photopatterning, a moderate UV dose (365 nm) is nevertheless required to cross-link the polyimide chains and make the films insoluble in organic solvents The required UV dose varies with thickness. Typically, for the process described above, a dose of about 0 8 J/cm2 is necessary Moreover, cross-link density can also be used to control the diffusion of an organic dye into the polyimide matrix48 (section 4 2.6), and in some cases, UV doses of 2 to 3 J/cm2 may be required to optimize concentrations This topic will be dealt with more in chapter 6 In general, the polyimide takes on a darker shade of amber when exposed to UV radiation, due to increased absorption at shorter wavelengths. 4.2.4.2 Channel Waveguides Photomasks with various channel waveguide patterns are placed between the polyimide-coated silicon wafer and the UV light source so that only the exposed regions are cross-linked, much like a negative photoresist. The masks were designed with the

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58 MAGIC and CADENCE software packages and fabricated on a model GCA MANN 3600 pattern generator A UV dose of 0 6 to 0 8 J/cm2 is used. The film is then placed in a developer solution (50/50 wt. butyrolactone/xylene) housed in an ultrasonic bath for 6 minutes During this time, unexposed regions are dissolved Ultrasonic agitation during the developing stage improves pattern contrast.SI After developing, the sample is placed sequentially in two identical solutions (50/50 wt. developer solution/xylene), referred to as crossover baths, for 20 seconds each. Lastly, the sample is rinsed in xylene for 30 seconds and blown dry with nitrogen. The crossover solutions are necessary to avoid precipitation of the polyimide when going from the highly polar butyrolactone-based developer to the non-polar xylene rinse Precipitation causes the polyimide to tum opaque white, rendering it useless for waveguiding purposes 4.2.5 Curing of Polyimide Films A high temperature cure is required to achieve good mechanical properties in the polyimide This needs to be performed under a nitrogen atmosphere, as the presence of oxygen during curing degrades the mechanical performance and increases coloration in these materials Samples are placed in a vacuum oven with a nitrogen purge and ramped up to 280 cc at a rate of 5 cc/minute. Peak temperature is held for I hour, after which the samples are cooled back to room temperature at about I cc/minute After the cure cycle planar polyimide films and wide(> 50 m) channel waveguides are approximately 5 m thick. Narrow channel waveguides tend to be up to I m thicker, depending on the channel width .so 4.2.6 Doping (Optional) A wide variety of dyes can be used to dope the polyimide, using a simple diffusion process. Cross-linked and partially cured polyimide waveguides are soaked for 10 to 15 minutes in I xl0-4 M ( or less) butyrolactone solutions of the desired dye Butyrolactone causes cross-linked polyimide to swell allowing dye molecules to easily penetrate the

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59 matrix After the desired diffusion time samples are quickly rinsed with methanol to remove residual dye from the surface, immersed in water, blown dry with nitrogen, and baked on hotplate at 100 C for 30 minutes to remove residual solvent. After solvent evaporation, the polyimide matrix contracts, effectively trapping the dye and preventing clustering and migration The peak dye concentration introduced into the polyimide films by this process varies inversely with the degree of UV-induced cross-linking 48 The uniformity of the doping mimics the quality of the polyimide waveguide The laser dyes cresyl violet 670, oxazine 720 nile blue 690 oxazine 725 oxazine 750, LOS 698 and LOS 751 has been successfully introduced into the polyimide matrix using this technique Dyes can also be introduced into polyimide from methanol solutions but this requires longer diffusion times and results in lower peak dye concentrations than when butyrolactone i s used It appears that the effectiveness of various solvents in transporting dye into the polyimide matrix is related to the amount of swelling induced. 4.2. 7 Endfacet Preparation One of the main advantages of depositing polymer waveguides on oxidized silicon wafers is that endfacets can be prepared by cleaving along the crystal planes of silicon provided the polymer exhibits sufficient adhesion In this case the <100> oriented wafers are cleaved along <11 O> directions producing rectangular samples A small scratch is made on the wafer in the desired cleave direction. Applying a small pressure to the wafer so that it bends along the direction of the scratch will cause the wafer to cleave along that plane. With practice this process takes only a matter of seconds and produces qual i ty endfacets In contrast the preparation of good endfacets on glass ion-exchanged waveguides by the polishing process described in chapter 3 takes several hours.

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oU 4.3 Characterization of Polyimide Films The nominal value of the refractive index of Probimide 400, measured by prism coupling, is found to be about 1.626 at 633 nm, for the TE polarization Process parameters, primarily the peak bake temperature and the UV dosage affect this value to some extent. By measuring the index over the wavelength range of 594 nm to 780 nm, we have fitted the TE refractive index to the second-order Sellmeier equation, 2 2 1.141320..2 0 2113021).. nTE = l+-----+-----(4.1) ')} + 0.3670977 ,..20.1570585 where A is in micrometers. TE refractive indices are shown m figure 4.2. The birefringence, defined as (4 2) was found to be very small {3x10-4) in Probimide 400 films This is consistent with the amorphous nature of the BTDA polyimide, which is comprised of short, flexible polymer chains .60 As such this material lends itself to use in devices such as splitters and switches where polarization-insensitivity is desirable In contrast, the polyimides SIXEF 33 (Hoest Celanese) and Ultradel 9000 (Amoco), which are based on longer stiffer polymeric chains, are more crystalline in nature and have birefringences of 2xl0-3 and 3.3xl0-2 respectively 39 6 1 The propagation losses of polyimide waveguides have been measured in the visible and near infrared spectrum, using a tunable Helium-Neon laser (PMS Electro Optics LSTP-0010) and semiconductor diode lasers operating at 677 nm and 780 nm. Light is coupled into a guided mode using the prism coupler and an optical fiber is scanned along the length of the guided streak to collect scattered light, as shown in figure 4 3 The scattered light is proportional to the power propagating in the mode and decays exponentially along the length of the waveguide as given by imag I (z) = K.I. (z) = K.I. (0) exp (-2k. z) S, I I I I I I {4.3)

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X 1.630 Q) "C C 61 -~ 1.625 -+--------------------t------t 0 -Q) 1.620 -t------+---+-------+'~--+------t 1. 61 0 -t--r--,--,--.,..-+-"'T"""""l---r--+-r--,--,--.,..-+-"'T"'"'ir---T"'-+-T""""T--r--1 550 600 650 700 750 800 wavelength (nm) Figure 4 2 TE refractive index of the Probimide 412 polyimide, fitted to a second-order Sellmeier equation

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input beam 62 lock-in amplifier photodetector t pressure coupling pnsm scan multimode optical fiber ... ... z Figure 4.3. Experimental set-up for measuring propagation loss in waveguides

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63 where Ki, Ii(O), and ktag are a constant, the initial power launched into the ith mode, and the field attenuation coefficient of the ith mode, respectively. To a rough approximation, Ki is proportional to the intensity of the mode field at the air/polyimide interface A typical measurement of scattered intensity as a function of position along the guided streak is shown in figure 4.4, along with the values of the loss coefficients obtained by fitting data to equation (4 3). Unfortunately, the technique tends to overestimate losses when used to evaluate polymer waveguides The pressure applied to the substrate to hold the waveguide against the prism tends to physically deform the polymer film, causing light to be launched simultaneously into several modes at once, instead of just one. Thus, the detected scatter signal is actually the sum of the individual scatter signals from each excited mode. Higher order modes scatter more strongly than lower order ones (a consequence of having a larger intensity at the air/polyimide interface), and tend to be more lossy. Thus, excitation of higher order modes while attempting to measure the attenuation of a low order mode can make the loss appear to be larger. Signal-to-noise constraints generally restrict the use of this method to modes with losses of greater than 1 dB/cm. Propagation losses are shown in figure 4 .5 for the TE0 mode of a planar waveguide. At 780 nm, the loss is less than 1 dB/cm At shorter wavelengths, losses are considerably higher due to increased absorption. In fact, a weak orange fluorescence is observed in the material when 543.5 nm radiation is launched into the films Loss was not observed to depend significantly on the film thickness, which indicates that scattering losses arise primarily from roughness at the air/polyimide interface rather than from scattering sites distributed within the bulk of the polyimide film itself. Propagation losses can be improved by optimizing the cure process. During curing the polyimide becomes darker in color and exhibits increased absorption. This effect is more pronounced at higher cure temperatures. so The use of a lower peak cure temperature sacrifices some of the polyimide s chemical and mechanical resistance but

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. -e II ._,. en C a 'E 1i C a, en 64 1------------------------------, 0 0 sample 1, 677 nm sample 2,677 nm ......................................... ............................................................................................................... . ........ ...... sample 1: 2.2 dB/ cm loss, R = 71% sample 2: 2.7 dB/ cm loss, R = 93% 0.5 -----........... .......+............ ---.,......,......------........... --......,_.---0 1000 2000 3000 4000 5000 6000 7000 8000 posHlon (um) Figure 4.4 Variation in scattered power along the length of the waveguide for the TEo mode at 670 nm R is the correlation coefficient, which describes the degree to which experimental data fall along the exponential fitting curves.

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65 ,.... E u '20....-l----f------+-----+----+-------t m ,, \wl en 15 C 0 i10-+-----++----+-----+----+----t C) 11 a. e 5--------------1,-------1-----------a. 0 w ~o_.._ _..... __ .._ _...... __ 540 590 640 690 740 790 wavelength (nm) Figure 4.5 Propagation losses for the TEo mode of a 3 6 m thick polyimide waveguide on a soda-lime glass substrate

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66 appears to be necessary to attain good optical properties .62 Slowing the heating rate during the cure cycle to -1 C/min also reduces loss .56 Using a modified cure cycle, we have been able to reduce propagation losses considerably Under the new cure schedule, samples are heated to 150 C, at a rate of 5 C/min. After 15 minutes at 150 C, the temperature is ramped to a final temperature of 250 C at the same heating rate. Peak temperature is maintained for 30 minutes, after which samples are cooled slowly to room temperature over several hours Using this modification, we have achieved propagation losses of 3.1 dB/cm at 633 nm and 1.5 dB/cm 670 nm in the TMo mode of a 5 m thick planar polyimide waveguide. Finally, it should be noted that the coloration of polyimide solutions becomes more pronounced with age. The quoted shelf-life of the Probimide 400 series is 1 year. However, this specification is based largely on changes in photospeed, rather than absorption. Most of the waveguides in chapters 5 through 7 were deposited from solutions which were 2 to 3 years old and still showed good performance 4.4 Summary Polymer waveguide fabrication is less expensive than other guided-wave technologies, such as Ti:LiNbO3 and SiO2/Si, but requires careful attention to process conditions in order to produce devices which are competitive with respect to loss Using the photosensitive polyimide Probimide 400, we have obtained propagation losses on the order of 1 to 2 dB/cm at 670 nm in multimode waveguides deposited on oxidized silicon substrates This material is easily doped with a variety of organic dyes using a simp l e diffusion process, making it particularly attractive for sensor applications. The refractive index has been measured over a wide wavelength range and fitted to a second order C. T. Sullivan, Sandia National Laboratories, private communication C T. Sullivan, Sandia National Laboratories, private communication D Roza, OCG Microelectronics Materials Inc private communication

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67 Sellmeier equation. We shall now present several chemical sensor designs which u t ilize polyimide waveguides

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CHAPfER5 EVANESCENT WAVE SENSING WITH POLYMER WAVEGUIDES 5.1 The Evanescent-Wave Absorption Sensor The most common types of optical sensors are those which employ changes in the optical absorption characteristics of an indicator material, such as a pH sensitive dye as a means of analyte detection In sensors based on optical waveguides, the indicator is immobilized in an analyte-permeable host material and used as either the core or the cladding (defined in figure 2 .1). The relative ease with which organic polymers can be doped with a variety of analyte-sensitive dopants, makes this class of materials very attractive in this application We refer to the portion of the waveguide which houses the indicator as the sensing region When the sensing region is a cladding layer as is commonly the case due to fabrication requirements only the evanescent tails of guided modes interact with the indicator and the device is accordingly termed an evanescent wave absorption (EWA) sensor .63 64 Table 5 1 lists a few examples of absorption-monitoring waveguide sensors Most absorption-monitoring integrated optic sensors (and in fact all of the examples given in table 5 1) are based on multimode waveguides. Therefore a brief review of the formulation of wave propagation in multimode structures is in order. We shall consider a simple two-dimensional multimode waveguide, consisting of a high index media of index n2 bounded between two media with indices of n 1 and n3 respectively as shown in figure 5 .1. For the sake of generality we will also assume the each of the materials forming the waveguide has a bulk absorption coefficient Ki 68

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Table 5.1 Examples of Absorption-Monitoring Waveguide Sensors Ref. core cladding indicator 65 K+-Na+ IE-PWG Si02 lutetium biphtalocyanine 66 multimode fiber silicone analyte 67 Ag+-Na+IE-CWG silicone analyte (multimode) 68 Ag+-Na+IE-CWG sol-gel bromcresol purple (multimode) 69 glass rod poly vinyl sodium picrate alcohol 70 AgClxBr1_x fiber none(*) fiber core 71 sol-gel film on none(*) bromophenol blue glass IE CWG: glass ion-exchanged channel waveguide IE PWG: glass ion-exchanged planar waveguide (*) : analyte-sensitive material located in waveguide core analyte Chlorine hydrocarbons trichloroethylene NH40H sodium cyanide SF6 pH A coherent light source is to be endfire-coupled into the waveguide At z = 0, the power launched into the ith order mode is 2 pi = po J e i (x) einput (x) dx X (5.1) where Po is the input power eiCx) and einput(x) are the respective transverse field distributions of the ith order mode and the excitation source at z = 0 Thus input power is launched unequally into the various guided modes. Inside the waveguide the power propagating in a given mode can be written as (5.2)

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... Light Source ... 70 lens Input field distribution, Einput Figure 5 .1 Launching light from a free-space beam into the guided modes of a waveguide ( endfire excitation)

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71 where k:mag is imaginary part of ki, the propagation constant of the ith mode. The total power in the waveguide is thus (5.3) We define the transmission of a waveguide of length L as P(L )/P(0). For a given mode, k:mag depends on the fractional power, defined in equation (2.7), travelling in each of the three regions shown in figure and the bulk absorption coefficient of that region: (5.4) In an evanescent wave absorption sensor, losses occurring in the sensing layer (region 1) must dominate all other forms of loss and thus, r1iK1r2iK2+r3iK3 Under this assumption, inserting (5.4) into (5. 3) yields (5. 5) In the evanescent wave sensor, the absorption coefficient of region 1, K1 is expected to change in the presence of analyte. Therefore, in order to maximize device sensitivity (i. e analyte-induced change in the transmission of the waveguide), we would like to have as much power travelling in the sensing layer as possible. We have used the waveguide simulator developed in chapter 2 to analyze the mode fields of a simple three layer waveguide. In this model, we have chosen n2 = 1 625 (polyimide), n3 = 1.45 (SiO2), thickness d = 5 m, and used a wavelength of 633 nm. In figure 5 2(a), r1i the fractional power carried by the ith mode in region 1, is shown for a few values of n1 As expected, higher order modes are less tightly confined than lower order ones and propagate a larger fractional power in region 1. This is particularly true when (n2 -n1 ) is small. From figure 5 2(a) and equation (5. 5), we see that the launching conditions at the input of the waveguide (z = 0) play a key role in determining the overall propagation loss, and hence, sensitivity of a multimode waveguide absorption sensor For

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72 0.18 ----------------.--------------.----. "1=1. 62 0.16 -------'----------------"1 = 1 55 .,... 0 .14 ___ ___,,,....._ ________________ ....__ ___ C 0 s, 0.12 -----------------------! "1 = 1.61 .5 0.10 ... i 0.08 &. 0 .06 -t-,P----+--+---1----+----+---n""---+---+--..__-,--_--::::;i,411 ii g 0.04 ~-,,__--r-----t-----::;;;_r------r--;----=::;::;;:-...-=:;__---:t:::::::::;:::;;:aaotp i 0.02 c 0.00 0 1 2 3 4 5 mode order (a) 6 "1 = 1.35 7 8 10 --------------------..,,.. --0.80 I 9 -0.60 8 -t~--------+----+------+---------.....--...-1-r-t:: -9 .,... '"'--...: .. 0.40 S 7 ', ..--~--o en ,.... 0.20 ,:; 6 :-J! C \ -a -0 .00 '!lo,, 5 -1'. : 0 '-," ; 4 -1-----f----4---"'-'-~'-~A~Jl--ll~rl-/_\J---41.......i-l____.:..: _--0.20 1 o :--,...JV -:'ii : a. 3 .... ,.... --0.40 ::' ii ,.... ., S 2 t::::::=::::::-="1-------=--i---r---;-.....=.......';:----;----:------t-0 .60 ', 1; 1--------------------'", --=----0.80 C: ,. 0 -+--,o__, __________________ ........... _1.00 1.35 1.40 1.45 1.50 1.55 1.60 1 .65 n1 (b) Figure 5 2 Power distribution in a three-layer waveguide n2 = 1.625 n3 = 1.45 and the thickness of region 2 is 5 m The excitation wavelength is 633 nm. (a) Fractional power carried in region 1 by each guided mode (b) Total fractional power (summed over all guided modes) and waveguide asymmetry as a function of sensing region index, n 1

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n example, an EWA sensor with power launched predominately into lower order modes will be less sensitive to analyte-induced changes in cladding absorption than an identical sensor with more power launched into higher order modes Continuing the numerical example, we next calculate the total fractional power propagating in region 1, defined as Ilirli (5.6) as a function of n1 These results are presented in figure 5.2(b) along with the corresponding waveguide asymmetry factor aE, defined by equation (2 2) In these calculations, each mode is assumed to carry equal power for convenience. As the core/ cladding index difference (n2 -n1 ) decreases and the decay term in equation (2.4) becomes smaller, the evanescent tails of the guided modes become longer and the total power in region 1 increases Sharp oscillations in the total power in region 1 occur as individual modes go to cut-off. Decreasing waveguide asymmetry also results in more power in region 1. Up to 10% of the light launched into the waveguide can propagate in region 1 when (n2 -n1 ) is less than 0.1. In contrast to the EWA sensor many optical detection systems measure the transmission of a free-space beam passing through a bulk absorption cell. In order t o establish a comparison between the EWA sensor and a bulk device we define the effective interaction length oflight in an absorption sensor (either bulk or EWA) as Leff= r1L (5. 7) In bulk absorption measurements where a probe beam is passed through a dye cuvet thin film etc ., r1 = 1. However as seen from figure 5 2 r1 is typically only a few percent in EWA senso r s and is also dependant on the launching conditions at the waveguide input.

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I'+ Examples of bulk and EWA sensors will be presented in the following sections 5.2.4 and 5 2.9, respectively. Note that this analysis of absorption-based sensors is equally applicable to the surface plasmon devices examined previously. Having established the basic operating principles of evanescent wave absorption sensors, we shall now present a specific example of an EWA device designed to monitor the concentration of ammonia in water.72 5.2 Detection of Aqueous Ammonia Industrial pollution of rivers and lakes poses a serious hazard to wildlife in affected areas Increased algae blooms and the consequent red tides are common side-effects of fertilizer run-off into swamplands and coastal waters. Pesticide and fertilizer run-off into groundwater result in elevated ammonia and nitrite levels which can be hazardous to wildlife at even very low concentrations For example, total dissolved ammonia concentrations (NH3 + NH4 +) on the order of a few parts-per-million are harmful to fish,73 74 on a time scale of hours .75 76 Whereas the ammonium ion is relatively innocuous, the non-ionized form of ammonia is highly toxic to aquatic life and must not exceed 40 to 400 ppb, depending on species, temperature, water chemistry, and pH.73 75 77 In order to monitor industrial pollution effectively, highly sensitive devices with short response times are required Ideally, in order to minimize cost issues, sensors also need to be reusable In this chapter, we focus on the development of an optical waveguide based sensor for the detection of aqueous ammonia which satisfies these criteria. 5.2.1 Choice of Sensing Layer Materials Much of the efforts to date in the development of ammonia sensors have dealt with the detection of vapor phase ammonia. Guiliani et al.78 were able to rapidly and reversibly detect ammonia vapor concentrations of 60 ppm by monitoring the transmission of a quartz rod coated with the pH sensitive dye oxazine 170. A similar system utilizing a

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.,, sensing layer comprised of ninhydrin immobilized in films of poly (vinyl alcohol) and poly (vinyl pyrrolidone) exhibited a detection limit as low as 60 ppb, but required nearly an hour to achieve full response and was not reusable .79 Oxazine 750-doped silicone and bromcresol purple doped porous sol-gel films have been used as claddings on optical fibers80 and ion-exchanged waveguides68 respectively, producing ammonia vapor sensors with detection limits of less than I ppm with rapid, reversible responses. In most cases, humidity has a significant effect on the performance of sensors based on pH indicators. Unfortunately, the aqueous environment generally places more severe restrictions on the materials used in EWA sensors Structural degradation prohibits use of water soluble materials in the sensor design Immobilization of the indicator in the sensing layer is of paramount importance to avoid problems such as dye migration and clustering. Aqueous-based ammonia sensors with rapid, reversible responses have been demonstrated using bromophenol blue71 and oxazine 17081 entrapped in inorganic sol-gel matrices but some degree of leaching (loss of indicator) was observed Oxazine 170-doped poly (methylmethacrylate) films deposited on glass have also been investigated for ammonia sensing, but were found to respond very slowly and exhibit poor substrate adhesion when immersed in water for long periods of time 81 Thus, realization of a useful sensor for an aqueous environment requires a careful optimization of the mechanical as well as chemical characteristics of the constituent materials We have studied the performance of pH sensitive dyes in two polymers, polymethylmethacrylate and Nafion In the following, we describe their performance. 5.2.1.1 Polymethylmethacrylate Polymethylmethacrylate (PMMA} is an extremely transparent polymer often used in the fabrication of plastic optical fibers Previous studies have explored the use of PMMA as a host material for various laser dyes, such as rhodamine 6G,82 rhodamine B,83 and DCM.84 It was assumed that the immobilization of a pH sensitive dye in a PMMA

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76 matrix would produce a sensor useful for the detection of ammonia. Following the procedure outlined by Chemyak et at.81 a chloroform solution of PMMA (5% wt. solids) was doped to lxl0-4M with oxazine 170. The dip-coating technique was used to prepare a 1 m thick film of PMMA/oxazine 170 on a soda-lime microscope slide Unfortunately, this material failed to exhibit a significant change in absorption when exposed to ammonia vapor In aqueous environments, the PMMA film separated from the substrate (though remained largely intact). These results were not sufficiently encouraging to merit further study of this material. 5.2.1.2 Nation We have investigated the use of DuPont's Nation fluoropolymer, a copolymer of polytetrafluoroethylene (PTFE) and an acid (S03 -) -terminated perfluorovinyl ether, 85 as a cladding material for evanescent wave sensing of aqueous ammonia The chemical structure of Nation (1100 equivalent weight) is shown in figure 5.3. This material has been Figure 5 3 Chemical structure of Nation fluoropolymer

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77 investigated for this purpose previously by Churchill et al., 86 who produced a fast and highly sensitive, albeit irreversible, ammonia vapor sensor based on Nation films containing the dyes oxazine 720, Nile blue 690, and bromothymol blue Nation has also been used previously by Zen and Patonay87 for pH measurement and by Ballantine et al.88 for acid vapor detection. Nation has several properties which make it attractive for ammonia sensing. It is expected that sulphonic acid groups of Nation will react strongly to the presence of ammonia, producing a large change in the pH of the polymer. Annealed Nation films exhibit ion-selective transport properties,89 which favor the diffusion of positively charged species, such as NH/, through the polymer network over that of negatively charged ones like er .90 In addition, Nation swells considerably in water, allowing rapid penetration of the analyte into the polymer matrix. We have successfully demonstrated an EWA sensor for the detection of aqueous ammonia using Nation doped with pH-sensitive indicator dyes from the oxazine family as a sensing layer 5.2.2 Fabrication of Oxazine-Doped Nafion Nation was obtained as a 5% solution of 1100 equivalent weight polymer in lower aliphatic alcohols and water (Aldrich) Alcohol solutions of the oxazine dyes cresyl violet 670, oxazine 720, Nile blue 690, oxazine 725, and oxazine 750 were mixed with the Nation solution, producing mixtures that contained 1 % polymer (by weight) and -0. 1 mmol dye. The chemical structures of these dyes are shown in figure 5.4 .91, Dye concentration was kept reasonably low to avoid dimerization After thorough mixing, the dye-doped Nation solutions were deposited on clean glass microscope slides using a coverage of 30 L/cm2 and allowed to dry at room temperature (24 C, 45% relative humidity). The resultant films were reasonably uniform in color and thickness in the Exciton, private communication

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/0 H5C2-.a NoN"C,H5 / 0 H5C2 C 2Hs Oxazine 725 0 Oxazine 750 NN H5C2-.~ I / 0 H5C2 Nile Blue 690 Figure 5.4 Chemical structure of several oxazine dyes.

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79 center of the samples, but were considerably thicker at the outer edges of the slide. The films were on the order of 1 to 2 m thick in the center of the slides During the course of this study, it was found that careful attention had to be paid to subtle processing details to produce a high-quality film. Specifically, the choice of solvent used for the dye, which forms 80% of the total solvent in the dye-doped polymer solution, is critical to determining stability of the final Nation film in water .92 93 Films deposited from methanol-based dye solutions showed poor mechanical properties, crumbling into pieces and separating from the glass substrate when immersed in water Baking had been proposed as a means to improving the mechanical integrity of Nation films by inducing some degree of crystallinity in the polymer matrix However, baking alone was found to be insufficient to improve the resistance of methanol-deposited films to water The water stability problem was finally solved by switching the dye solvent to isopropanol, which produced films which were pliable and cohesive. When immersed in water, isopropanol deposited films exhibited excessive swelling and eventual substrate separation but remained largely intact. More importantly and unlike the case in methanol-deposited films, it was observed that baking the isopropanol-deposited films at 120 C for 60 minutes was sufficient to completely suppress these undesirable effects, resulting in the realization ofNafion-coated glass slides which were stable in water. It was also discovered that the dye-doped Nation solutions had shelf-lives of less than one day in liquid form Films deposited from "old" solutions showed different absorption characteristics than ones deposited from fresh mixtures. Therefore, the dye and Nation solutions were mixed only immediately prior to film deposition 5.2.3 Characterization of Oxazine-Doped Nation Oxazine dyes like those shown in figure 5.4 respond to local pH variations by reversible deprotonation of an amino endgroup (auxochrome) .9194 In dye-doped Nation films deposited by the above method, the polymer matrix is highly acidic in nature and the dyes are fully protonated and generally lightly colored However, when exposed to

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~u ammonia, these films change color rapidly, corresponding to the deprotonated (basic) state of the dye. Furthermore, the deprotonated state is retained after the ammonia source is removed We propose that in the presence of ammonia, hydrated ammonium ions rapidly penetrate the Nation network and bind the polymer's acid groups by the reactions NH3 +H20 HNH4+ + Oir S03 + NH4 + NH4S03 (5.8) (5.9) Based on the irreversible nature of the reaction of Nation to ammonia, we may conclude that the resultant ammonium salt is very stable at room temperature With the side groups in the salt form, the pH of the Nation matrix is significantly higher than in its acid form, causing deprotonation of the indicator Sadaoka et al.95 have suggested a similar reaction for poly (acrylic acid). Based on this model, Nation-based sensors can measure only ammonium ion concentration. However, from this information, the non-ionized ammonia concentration in a solution can be calculated, given a knowledge of the temperature, pH, and other factors. 96 97 Immersion of oxazine-doped Nation films in pure water was also observed to cause a color change corresponding to dye deprotonation, but only while the samples were wet. When removed from water and dried these films returned to the initial acidic state whereas those exposed to ammonia did not. As might be expected, films in acidic state are sensitive to atmospheric moisture and a study of the humidity dependency of the absorption characteristics of oxazine-doped Nation has been performed separately. Bulk absorption spectra were measured in the range of 500 nm to 800 nm by passing collimated light from a monochromator (Digikrom 240) through the dye-doped Nation-coated glass slides at normal incidence and monitoring transmission with a silicon photodetector. The output slit on the monochromator was set to I mm, corresponding to a G A. Stewart, "Humidity dependant transmission characteristics of Nile blue doped Nation," Independant study project, University of Florida, 1996

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lS 1 spectral resolution of 3 2 run. Light was also passed through an uncoated glass slide as a reference The absorption was determined as (5.10) where Tc(A) and Tr(A) are the transmissions of the Nation-coated and reference slides respectively Films were measured in the as-deposited state exposed to fumes from an ammonium hydroxide solution and remeasured We denote the films to be in the acidic state prior to ammonia exposure and in the basic state after ammonia exposure. Figures 5 5 through 5.9 show the absorption spectra of the oxazine dyes cresyl violet 670 oxazine 720 Nile blue 690, oxazine 725 and oxazine 750 respectively, in Nation films before and after exposure to ammonia vapor. In all cases the presence of ammonia causes a radical change in the absorption spectrum The primary concern here is the wavelength dependance rather than the peak absorption value for each system since dye concentration and film thickness vary in each case Interestingly despite the relative similarities in the structures of these dyes two distinctly different types of absorption response are observed Cresyl violet 670 oxazine 720 and Nile blue 690 all exhibit weak absorptions initially but form strong and well-defined absorption bands in the visible region when exposed to ammonia Conversely oxazine 725 and oxazine 750 show strong initial absorption in the near-infrared and exhibit a 30 to 40 run blue-shift when exposed to ammonia. Nile blue was chosen as the most convenient i ndicator dye for the ammonia sensing since it showed a large ammonia-dependant change in absorption over the red region of the spectrum where laser diodes and HeNe lasers are readily available 5.2.4 Bulk Ammonia Sensor Response Nile blue-doped Nation solutions were prepared by mixing 1 part Nation solution to 4 parts of a 0 3 mmol mixture of Nile blue in isopropanol. Films were then made b y depositing 30 L / cm2 of solution onto clean soda-lime glass microscope slides The

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0.800 ---------------------Nanon / Cresyl Vlolet 670 i 0.500 -C 0 e0.400 0 flt i 0.300 ___ ,__ _______ -+--,,,'I ___________ I I 0. 000 -+-r-"T""'T"',r-+T""T""T""r+-,,..,.....,...,....+-,""'T""T""T"+-r-T""'T'""T"+-T""M-r+.,..,_r""'T"'"t"'T""T""T'"M'""T'"'T'"T"M 400 450 500 550 600 650 700 750 800 850 wavelength (nm) Figure 5.5 Absorption spectra of cresyl violet 670/Nafion (methanol solution) in the acid and base forms The film is yellow in the acid form and purple in the base state.

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0.450 0.400 0.350 0.300 i 0.250 0 i 0 0.200 .,, .a II 0.150 0.100 0.050 0.000 I j I I I / I I j I I ""-_/ '\ !, I \ \.1 I \ Naff on / Oxazlne 720 I base form / / ';I" acid form / \/ V "'"\ IJ / / \ I '---\ ~\ 400 450 500 550 600 650 700 750 800 850 wavelength (nm) Figure 5 6 Absorption spectra of oxazine 720/Nafion (methanol solution) in the acid and base forms The film is green in the acid form and blue in the base state

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0.40 0.35 0.30 io.25 .._.. C 0 :;:: 0.20 e0 0 i 0.15 0.10 0.05 0.00 I Nafion / Nile blue 690 I r I I/. I \I\ J I I \ 'r--J '_j I ~; ./. (\ V' base form / V / I> acid form I \ I V 1; ), ct \ r_~ ,----, ~r &v...,,' 400 450 500 550 600 650 700 750 800 850 wavelength (nm) Figure 5.7 Absorption spectra of Nile blue 690/Nafion (isopropanol solution) in the acid and base forms The film is yellow in the acid form and blue in the base state

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1.000 0.900 0.800 0.700 i 0.600 -C 0 = 0.500 e0 J 0.400 ., 0.300 0.200 0.100 0.000 I Nanon / Oxazlne 725 r\ r -' acid form I I v-\ I I --~ I I base form ,~ I I \ I I .......... / / I j I I I I'\./ I I I\ r----. I \) \/ I \ I \ \ \ I \ I I \ I I \ I I \ \ \ \ \ \ \. 400 450 500 550 600 650 700 750 800 850 wavelength (nm) Figure 5 8 Absorption spectra of oxazine 725/Nafion (methanol solution) in the acid and base forms The film is black in the acid form and deep blue in the base state

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00 0.900 --------------------Nanon / Oxazlne 750 \ 0.600 ____ _.._.. _____ -+------t-~,------t t I \ c 0.500 ---------+------++----+------..--+----t .2 I \ f 0 .400 _________ __.__------11---+--t--+------t ,,, .a ., 0.300 ---------.-,-t-,--+---++---+----++-----1 0 .000 --f-,p,,o,...,....,.~~+-,-,i,........,..+-,-,-~~.,...,...+........,......,...+..,...,...,~+-,-,,........+-"l"""'f""..,.... 400 450 500 550 600 650 700 750 800 850 wavelength (nm) Figure 5 9 Absorption spectra of o x azine 750/Nafion (methanol solut i on) in the acid and base forms The film is aqua in the acid form and blue in the base state

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87 solution was allowed to dry at room temperature and was then baked at 120 C on a hotplate for 60 minutes. The final film was approximately 1.4 m thick and yellow in color. The indicator-doped Nation films prepared in this manner were found to be very stable in aqueous environments In fact, a film immersed in water for a period of 5 weeks exhibited negligible leaching of the indicator and only minor swelling at the edges of the substrate Measurements of the change in the bulk transmission of Nile blue-doped Nation films with ammonia concentration were performed at 632.8 nm using a HeNe laser. The coated slides were repeatedly immersed in ammonia solutions in order of increasing concentration, up to about 3 ppm, for 1 minute, blown dry with nitrogen, and baked on a hotplate at 50 to 70 C for 5 minutes In this manner, the reversible reaction of Nation to water could be differentiated from the irreversible reaction to ammonia. Transmission was measured after each immersion/drying cycle The response of the bulk device 1s shown in figure 5 10. For ammorua concentrations below 1 ppm, the Nation film is in the acidic state (yellow) and the Nile blue indicator absorbs only weakly In the range of 1 to 2 ppm, localized blue spots begin to form on the film which persist after drying With each subsequent immersion/drying cycle, these spots grow in size until the entire film is blue When one of these spots intersects the HeNe beam, in this case at 2 ppm, transmission drops sharply Unfortunately, the step-like response observed here results in a sensor with an exceedingly small dynamic range Given the film thickness and taking into account the transmission of the glass substrate, we have calculated the absorption loss of the Nile blue-doped Nation to be 16 000 200 dB/cm at 633 nm when the polymer is in the base state In the acid state the absorption is too low to be measured reliably by this technique. 5.2.5 Demonstration of Reversibility In order to allow Nile blue-doped Nafion films to function as reusable ammonia sensors, a rinse technique was developed which allows ammonia-exposed films to be reset

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i -C 0 100-----------------------, 90 " " " I " 0 " f " " " I " " " 1 " " " I" " " 80 0 0 0 0 I -0 0 0 -0 0 0 : 70 E --,--... i ---.. --,--.. -.. i -----.-en C !! 160 50 ----_,_ .. --l ----_,_ ---.. J ------.. --.. -.......................................................................... I I I I 40-----.--.--.-+---------.--.------....--.----.,......,.....,.......,...--___,--.--o 500 1000 1500 2000 2500 3000 ammonia concentration (ppb) Figure 5 10 Bulk transmission of a Nile blue-doped Nation film on a microscope slide to various aqueous ammonia concentrations The excitation wavelength is 632 8 nm The film is approximately 1.4 m thick.

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back to the original acidic (yellow) state. In this process, base-state (blue) sensors are immersed in 1 :20 solutions of acetic acid/deionized water for 30 seconds, blown dry with nitrogen, and dried on a hotplate at 90 C. After the rinse, the films are again yellow in color. To demonstrate the viability of this technique, we have monitored the transmission of a bulk sample alternately exposed to a high concentration ammonium hydroxide vapor followed by the dilute acetic acid process. As shown in figure 5.11, the rinse process is extremely effective in restoring the acidic form of Nafion, cycle after cycle Thus, this technology has the potential to be economically viable, as sensors can be reused many times. We would like to emphasize that during the course of these experiments, no problems with substrate adhesion were encountered. 5.2.6 Selectivity of the Nafion Response As may be inferred from figure 5.3, the transmission-based Nafion sensor exhibits little selectivity amongst strong bases For example, exposure to aqueous NaOH converts Nafion's acid groups into ionic salts in the same manner as NH4OH and produces the same color change in the Nile blue indicator. The fact that Nafion responds irreversibly to bases (prior to the acid rinse) but reversibly to water offers some level of discrimination. In a sensor based instead on the diffusion rates of neutral species through a polymer matrix, Nafion's permaselective nature would provide some measure of selectivity. However, in order to obtain a selective response to ammonia, the Nafion sensor would have to be used in conjunction with several other sensors in a multielement array. Ideally, each sensor element would exhibit different analyte response characteristics The composite array response could then be treated as a vector in a multidimensional pattern space and analyzed by a neural network algorithm, providing both selectivity and multianalyte detection capability

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90 85 80 ~75 -C 0 : 70 E .,. C 65 t60 55 50 acid ------ammonia I I 0 1 90 acid acid acid ammonia ammonia ammonia I I I I I 2 3 4 number of rinse cycles Figure 5 .11 Bulk transmission of a Nile blue-doped Nation film on a microscope slide at 632.8 nm after sequential exposure to aqueous solutions of 5% acetic acid and concentrated ammonium hydroxide vapor. The film is approximately 1 m thick

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91 5.2. 7 Waveguide Issues in Evanescent Wave Sensor Design An EWA sensor for the detection of aqueous ammonia has been developed by coating a polyimide channel waveguide clad with a thin layer of Nile blue-doped Nafion An oxidized silicon wafer is used as the substrate. The entire device is shown in figure 5. 12. An advantage of the choice of the materials in this structure is that refractive indices output light input light Figure 5 .12 Device structure of the evanescent wave absorption sensor. of the SiO2 substrate (n = 1.45) and the Nation cladding (n = 1.35 undoped35 36 ) are fairly similar This low degree of waveguide asymmetry increases the penetration depth of the evanescent wave associated with each of the guided modes into the Nafion cladding thereby enhancing sensitivity (see figure 5. 2(b)) Rectangular cross-sectioned ridge waveguides were used instead of planar ones in order to further increase the fractional modal power propagating in the Nafion cladding. Through numerical simulations the total fractional power travelling in the Nation cladding of this structure is estimated to be

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92 approximately 2.5% when all modes are excited equally. In contrast, as the channel width approaches infinity and the structure becomes a planar waveguide, only 1.6% of the total power travels in the sensing region 5.2.8 Evanescent Wave Sensor Fabrication Using the process outlined in chapter 4, polyimide ridge waveguides were photolithographically patterned on oxidized silicon substrates and partially cured at 280 C The waveguides were 7 m thick, 5 to 100 m wide, and 10 to 15 mm long Nafion was obtained as a 5% solution of polymer in lower aliphatic alcohols and water (Aldrich) and mixed with a 0.3 mmol isopropanol solution of the pH-sensitive oxazine dye Nile blue (Exciton) in a ratio of 1 part Nafion solution to 5 parts dye solution. After thorough mixing, approximately 30 L of the Nafion/Nile blue solution was pipetted onto the polyimide ridge waveguides. The solution dispersed along the entire length of the waveguides via capillary action, but did not run over the endfacets The solution was allowed to dry at room temperature and then baked on a hotplate at 120 C for 1 hour The resultant Nafion cladding was reasonably uniform in both thickness and color along the length of the waveguides We refer to these devices as the type I (acid-baked) sensor 5.2.9 Performance of Evanescent Wave Absorption Ammonia Sensors The response of EWA ammonia sensors was assessed by monitoring the transmission of a 2 mW, 632 .8 nm HeNe laser through the waveguide. The laser was endfire coupled into the waveguides through a 40X (0 65 NA) microscope objective. A precision XYZ translational stage, equipped with sub-micron resolution differential micrometers was used to align the waveguide to the laser. Output light was collected by a 20X microscope objective passed through a pinhole, and detected by a silicon photodiode EWA ammonia sensors were exposed to ammonia in a manner identical to that used for the bulk sensors The devices were immersed in ammonia solutions in order of

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93 increasing concentration for 1 minute. After each immersion/drying cycle the sample was positioned on the XYZ stage and aligned for maximum transmission of the HeNe laser Once the response of the EWA sensor was saturated (i.e all dye molecules in the Nation film are deprotonated) the device was subjected to the acid rinse and retested Initially the transmission of ammonia-exposed device was observed to recover with time. This was attribtuted to photochemical decomposition (bleaching) of the Nile blue indicator by the high optical field propagating in the cladding In order to avoid bleaching neutral density optical filters were used to reduce the power of the probe beam to 270 W. Several transmission measurements were taken at each concentration and the results were averaged We have observed that although the exact launching conditions (i. e fractional power coupled into each mode at the input facet) are not reproducible for a given level of ammonia, transmission measurements on the EWA sensor can be repeated with less than a 4% deviation Figure 5.13 shows the response of a 6 m wide, 13 mm long device The transmission is observed to drop as ammonia concentration increases ultimately levelling out at 60% of its initial value when the ammonia level exceeds -1 ppm The lower detection limit of the EWA sensor is less than 300 ppb In the range of300 ppb to 800 ppb the change in transmission is almost directly proportional to the ammonia concentration giving the device a linear dynamic range of 500 ppb. Both of these values are considerably better than in t he bulk sensor Beyond about 900 ppb the response of the sensor becomes ambiguous as several ammonia concentrations are seen to produce the same response After several rinses the behavior of the sensor was found to be largely unchanged. As was the case previously ammonia e x posure causes blue spots to form in localized regions on the Nation film These spots grow in size as ammonia concentration increases until the entire cladding becomes blue The improvement in dynamic range can be explained by noting that as the blue spots grow, the length of the waveguide in contact with the absorbing reg i on increases In effect the interaction length changes with ammonia

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94 1.1 -----------------------, i 0.9 ..._, ... CD a.0.8 '5 a. '5 0 ,, 0.7 CD a, E g 0.6 x fresh 0 1rst rinse I I ---------I I 2nd rinse ......................... D 3rd rinse I I .. .. .. .. 1"" .. .. .. ""1 .. "" .. "" i .. .. .. 1"" .. .. .. .. I.. .. .. .. ""1 .. .. '"' .. l .. .. .. .. ---:_ --_: ------* ----1 I I I 0.5 ......... ~..,..........,.....,....,.......,..,.....,..~---T"""T"""T---t-T"""T"_.....,.....,.....,__,..""'l""""'....,.....,....,.--r-T"""'1 0 200 400 600 800 1000 1200 1400 1600 ammonia concentration (ppb) Figure 5 .13 Response of the type I (acid baked) integrated-optic sensor to aqueous ammonia at various concentrations using 632.8 nm excitation. The waveguide is 13 mm long 6 m wide and 7 m thick.

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95 exposure. The higher sensitivity of the EWA sensor appears to stem from the fact that the Nafion film used here is thinner than the one used in the bulk device. During the course of these experiments, it was observed that the Nafion films would only react to the highest ammonia concentration to which they have been exposed For example, a film exposed sequentially to 200 ppb, 500 ppb, and 800 ppb aqueous ammonia solutions behaves identically to a film exposed to a single 800 ppb solution Thus, there is no need to rinse the sensor, as long as the ammonia concentration to be measured is expected to be larger than the previous one. Increasing the immersion time of the sensor in the ammonia solutions to 2 minutes did not significantly affect sensor performance, thus indicating a response time of less than 1 minute One interesting feature of figure 5 .13 is the behavior of the sensor in the initial testing cycle prior to the first acid rinse Contrary to expectations, transmission was observed to increase as ammonia concentration increased from O ppb to about 300 ppb, although no blue spots are observed on the Nation film Beyond this point, transmission characteristics are identical to the tests after rinsing. This behavior is inconsistent with deprotonation of the indicator and more likely points to a change in the interface between the two polymers (Nafion/polyimide) used in construction of the EWA. We have observed that coating the polyimide waveguides with Nafion reduces propagation losses by diminishing optical scattering at the air-exposed polyimide surface to some extent. However, polyimide is a relatively high pH material, which may cause the acidic Nation to form microscopic cracks and blisters, resulting in a poor optical interface between the two materials 89 As Nafion is exposed to ammonia and becomes more basic, these blisters may shrink, improving the interface and reducing the scattering After exposure to ammonia though, the Nafion matrix becomes stiffer, 85 and, when converted back into acid form via the rinse process may be unable to reform blisters. The major problem encountered with the EWA sensor lay with the rinse process After each rinse cycle small cracks running parallel to the waveguide, were observed to

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form in the Nation film Although the Nation film continued to change color from yellow to blue upon exposure to ammonia, after 3-4 rinses, the consequent absorption loss experienced by the waveguide suddenly decreased sharply. This indicates separation of Nafion and polyimide at the waveguide core/cladding interface Weaker rinse solutions of 1 part acetic acid to 20 parts deionized water were found to be equally effective in returning ammonia-exposed Nafion films to the yellow state, but did not alleviate the cracking problem to a significant extent. Thus, the type I (acid-baked) EWA sensor could only be reused a limited number of times. Use of a waveguide core material with a pH closer to that of Nation would most likely improve the quality of the core/cladding interface Contrary to expectations, the peak ammonia-induced propagation loss in the type I (acid-baked) EWA sensor was observed to be fairly small, only about 1.7 dB/cm. In fact, comparing figures 5 10 and 5 13, we see that the absolute transmission changes exhibited by the bulk and EWA sensors are very similar This is rather surprising given that the light travels 13 mm in the waveguide device but only -1 m through the bulk film. Through numerical simulations, we have estimated that when all waveguide modes are excited equally, about 2 5% of the total power launched into the EWA sensor propagates in the Nafion film. Using the effective length concept from equation (5 7), the EWA sensor is thus equivalent to a 325 m thick bulk film and show exhibit a peak absorption change of 325 m x 16,000 dB/cm = 520 dB/cm. Clearly, this is not the case A number of factors are believed to contribute to the lower-than-expected absorption change observed in the EWA sensor Part of the problem can be attributed to the measurement technique, which relies on maximizing transmission after each ammonia exposure to ensure reproducibility By reoptimizing the input coupling conditions between each test, we in effect preferentially excite modes which have the smallest evanescent waves propagating in the strongly absorbing cladding. Thus, the fractional power travelling in the Nation film is certain to be much less than the 2 5% predicted perhaps by

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':JI as much as a factor of 5 or more We will show shortly that with fixed launching conditions, as would be the case with a fiber-pigtailed device, the situation improves considerably. The nature of the interface between Nation and the polyimide also plays a major role in determining the overall change in absorption. If the interface is of poor optical quality, and a large number of air pockets exist between the two polymers, then the guided modes will experience less analyte-dependant absorption loss and more analyte independant scattering loss Some evidence of such a problem is provided by the cracking phenomena which accompanied each acid rinse. In addition, it is highly likely that during deposition of the cladding layer, the alcohol solvent causes some of the Nile blue to diffuse out of the Nation and into the polyimide Since only Nile blue molecules immobilized in the Nation film exhibit ammonia-induced changes in absorption, this out diffusion would also serve to reduce the peak transmission change of the EWA sensor Lastly, photobleaching of the Nile blue molecules by the high optical field at the polyimide/Nafion interface also was suspected to be a problem. However since data obtained in the various test cycles fall along the same curve it is evident that the dye absorption characteristics are unchanged, and thus photobleaching can be ruled out. In view of the above, the fabrication process was modified slightly. The new process was similar to that for producing the type I (acid-baked) sensor with the exception that the Nation films were exposed to a high concentration of ammonium hydroxide vapor prior to the 120 C bake. We find this modifies the mechanical properties of Nation, particularly its adhesion to the polyimide waveguides, and improves the thermal stability of the indicator dye during the bake We refer to this variant as the type II (base-baked) EWA sensor After the bake, the sensors were rinsed in a 1 :20 solution of acetic acid/ deionized water blown dry with nitrogen and dried at 90 C for 5 minutes The transmission characteristics of the type II (base-baked) sensor, shown in figure 5 14 bear a strong resemblance to those of the type I (acid-baked) device, prior to the first

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1.6 -------------------1.4 -----,--------., ------.-. .,, :t= C ::::, 1.2 ------. -e ., i 1.0 --------Q. '5 ~0.8 .. --. --------0 ,:, CD .r::. 0.6 ---, -----I --I -i -----, -I ----ii 0 C 0.4 ------------0 200 400 600 800 1000 1200 1400 ammonia concentration (ppb) Figure 5 14 Response of the type II (base baked) integrated-optic sensor to aqueous ammonia at various concentrations, using 632.8 nm excitation The waveguide is 12 mm long, 6 m wide, and 7 m thick.

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99 rinse. Initially, transmission increases with increasing ammonia concentration, up to a level of about 400 ppb. Beyond this point, transmission decreases sharply, ultimate l y saturating at about 30% of its initial value (corresponding to a 3 8 dB/cm increase in loss) for ammonia concentrations above 1000 ppb However, unlike the previous (type I) EWA sensor, acid rinsing does not remove the non-monotonic response of the type Il (base baked) device. In order to function effectively, the type Il EWA sensor needs to be exposed to a 400 ppb ammonia solution prior to use This sacrifices the minimum level of detection, but produces a device with a monotonic response in the range of 400 ppb to 1000 ppb and a linear dynamic range of about 300 ppb In order to assess the performance of a fiber-pigtailed device, the ammorua response of a type Il EWA sensor under fixed launching conditions has also been studied This is achieved by maximizing the coupling of the HeNe laser into the waveguide and then monitoring the change in transmission as the device is exposed to vapor from a stock 30% ammonium hydroxide solution In this way, the source/waveguide alignment remains fixed while the analyte interacts cladding When the sensor is exposed to the vapor, output power is observed to drop by a factor of 10 in less than 20 seconds! This is considerably larger than the change experienced under the previous measurement technique, in which the source/waveguide alignment was reoptimized after each ammonia exposure The advantage of the type Il (base-baked) EWA sensor lies in its reliability. After numerous rinses in 1 : 20 acetic acid/water solutions, no significant degree of cracking was observed in the Nation film. As such, the type Il (base-baked) EWA sensor can be reused many times. This improvement appears to be a result of depositing a higher pH form of Nation on the polyimide waveguides, which forms a better interface and avoids cracking and blistering problems. Some evidence of the improvement of the optical interface between Nation and polyimide is also seen from a comparison of figures 5 .13 and 5.14, noting that the ammonia-induced absorption is about twice as large in the type IT (base-

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100 baked) device Unfortunately, the price for this improvement in operating lifetime larger output response is a two-fold increase in the lower detection limit of ammonia 5.3 Summary The operating principles of the evanescent wave absorption sensor have been developed from basic waveguide concepts A device based on these concepts has been designed to detect ammonia in aqueous environments, using Nile blue-doped Nation-clad polyimide ridge waveguides The Nation chemistry provides the sensor with a rapid response to the analyte and exhibits negligible leaching of the indicator, even when deployed in aqueous environments for several weeks. Ammonia concentrations as low as 200 to 300 ppb are currently detectable An acid rinse technique has been developed which allows the sensor to be reused several times with minimal performance degradation Although Nation provides a nonspecific response to ammonia and other strong bases, the simple sensor demonstrated herein could easily be incorporated into a multi-element sensor aNileray to provide both specificity and multianalyte detection capability.

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CHAPTER6 FLUORESCENCE-EXCITED EVANESCENT WAVE ABSORPfION SENSORS 6.1 Introduction Endfire-excited waveguide-based chemical sensors, such as the surface plasmon and evanescent wave absorption devices described in chapters 3 and 5 respectively, are easily demonstrated in laboratory environments using prisms,71 gratings,98 or lenses for input and output light coupling However, sensors designed for field operation require optical fibers for light delivery. Unfortunately, the high cost of assembling fiber waveguide pigtails, as well as the tight alignment tolerances associated with endfire excitation, have hindered wide-spread commercialization of these conventional EWA sensors Silicon V-groove supports for fiber coupling, like those discussed in section 3.4, improve the situation somewhat, but are still relatively expensive To surmount the shortcomings associated with conventional fiber-pigtailed EWA sensors, we have explored an alternate excitation technique, wherein a free-space pump beam is used to generate fluorescence inside a doped waveguide. 99 Some of this fluorescence is captured by the guided modes of the waveguide and is used for EWA measurements in much the same manner as a conventional endfire-coupled probe beam as described in chapter 5. We refer to this device as the fluorescence-excited evanescent wave absorption (FEEWA) sensor This approach provides a more robust and considerably less expensive means of launching light into waveguide sensors In addition, we show theoretically as well as experimentally, that the FEEWA sensors offer a higher level of sensitivity than conventional endfire-excited EWA devices. 101

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6.2 Principle of Operation The FEEWA sensor, shown schematically in figure 6.1, consists of a waveguide whose core is doped with a fluorescent material, such as a laser dye or a phosphor. IOO A section of the waveguide is coated with a cladding whose absorption characteristics are envirornp.entally-dependant. A transverse pumping arrangement is used to excite the fluorophore. A fraction of the fluorescence generated in the waveguide is captured by the various guided modes and propagates along the structure, while the remainder is radiated away In the clad part of the structure, each mode experiences an attenuation proportional to the amount of power travelling in the cladding (see equation 5.4). Thus, the fluorescence leaving the waveguide is determined by the cladding absorption characteristics The transverse excitation geometry of the FEEWA, shown in figure 6.1, is clearly attractive in terms of alignment tolerances Whereas endfire coupling of the excitation source, either via lens or fiber butt-coupling, often incurs sub-micron alignment requirements, transverse pumping simply requires that the excitation source irradiate part of the top surface of the waveguide. In effect, the alignment tolerance of the transverse geometry is on the order of the diameter of the pump beam. 6.3 Theoretical Formulation of Fluorescence Capture by Guided Modes Marcuse101 and Egalon et. al .102 103 have developed relations to describe fluorescence capture in optical fibers. In their analysis, fluorescent sources are located within the fiber core or cladding and are assumed to be uniformly distributed between radii r1 and r2 The amount of fluorescence launched into the guided mode (v) by these sources 1s given as r2 21t L P~~ided oc J J J dSleu (r, q,) l2rdrdq,dz (6.1)

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X 103 transverse pump beam ___ ...,~ L ...... __ radiated fluorescence z ... -----~ guided fluorescence analyte-sensitive absorbing cladding output signal ... Figure 6.1 Geometry of the fluorescence-excited evanescent wave absorption sensor (FEEWA), using transverse excitation.

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1U4 where e'O(r, ) is the nonnalized fiber mode field at the fluorescence wavelength, Sis the source strength (when sources are distributed uniformly), Lis the fiber length containing the fluorescent sources, k = 2rr/Ao, and Ao is the vacuum wavelength As seen from equation (6.1), P!:ided depends on the spatial overlap between the guided modes and the fluorescent sources. It has been shown theoretically102 103 and experimentally104 that fibers with sources in the core capture up to two orders of magnitude larger fluorescence than ones with fluorescent claddings We shall now modify equation (6 1) to reflect the planar waveguide geometry of the FEEWA shown in figure 6.1. Since the distribution of power amongst the guided modes of an EWA has a profound impact on the sensitivity of the device,63 we need to first consider the spatial distribution of the fluorescent sources in the waveguide core under the transverse excitation scheme. To a first-order approximation, the light emitted by a fluorescent source is linearly proportional to the absorbed pump power. Therefore, the spatial distribution of S follows that of the pump beam inside the waveguide core, which varies with distance as (6 2) where <,, is the pump absorption coefficient and P0 is the incident pump power We assume the waveguide doping to be unifonn, so that <,, is independent of position The pump power absorbed in the interval dx at a position xis Inserting dS (x) oc dPabs (x), into (6 1) gives d L Pfuided oc J J apexp (-apx) lei (x) l2dxdz 0 0 (6.3) (6 4) in two-dimensional Cartesian coordinates where Lis the length of the irradiated region of

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105 the waveguide (equivalent to the spot size of the pump beam) and dis the waveguide core thickness. We have ignored the transverse variation of the intensity of the pump beam. In order to examine the effect of transverse pumping, the integral in equation (6.4) is plotted against the mode order, i in figure 6.2(a), for several values of c,,. The waveguide used in this model consists of a 5 m thick polyimide film (n = 1.625) on a silica substrate (n = 1.45) The superstrate is air (n = 1). The fluorescence wavelength is chosen as 670 nm and the mode field distributions are found using the numerical simulator developed in chapter 2 In figure 6 2(a), the amount of fluorescence coupled into each mode is observed to be nearly equal for the lower pump absorption levels (200 to 400 dB/ cm). This result is expected as the pump power does not appreciably vary through the waveguide depth However, as Up becomes larger and pump power decays sharply below the surface, fluorescence is captured preferentially by higher order modes. This effect is particularly attractive for sensing applications, as it enhances the fractional power propagating in the cladding, resulting in stronger evanescent wave interaction and higher sensitivity. In fact, at first glance, it appears that the "trick" to optimizing the performance of the FEEWA sensor is to have pump absorption as strong as possible, either by heavily doping the waveguide, or by matching the pump wavelength to the absorption maximum of the fluorophore However, if we look at the total amount of guided fluorescence, summed over all the guided modes, as shown in figure 6 2(b ), there exists a level of pump absorption which maximizes the total amount of captured fluorescence In addition, and more importantly, doping needs to be kept low enough to minimize reabsorption of the guided fluorescence Thus, there is a trade-off between evanescent wave enhancement and fluorescence signal power leaving the endfacet.

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106 1.0 -- -. 2000 dB/ cm 0.8 (/) +-' c :::::, -e ~0.6 (I) u 400 dB/ cm C: Q) u (/) (I) L.. g 0.4 lj::: u (I) L.. -.a ---a. 200 dB/ cm m 0 0.2 8000 dB/ cm 4000 dB/ cm 0.0 0 1 2 3 4 5 6 7 8 9 mode order (a) Figure 6.2 Theoretical predictions of fluorescence captured by each of the guided modes of a waveguide with a fluorescent core, assuming a transverse pumping geometry (a) Relative fluorescence coupled into each guided mode. Pump absorption coefficients are shown by each curve (b) Total guided fluorescence as a function of pump absorption coefficient.

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1 0 0.9 -0.8 Cl) +J C 07 .c I... ro 0 6 C CD u 0 5 I... 0 :::J 0.4 CD I... :::J a.0.3 ro u ro 0 0.2 +J 0 1 0.0 0 107 r-,..__ -............. I I I I I I I I I O I 0 I I I I I o o ' 2500 5000 7500 10000 12500 15000 17500 20000 pump absorption loss (dB/ cm) (b) Figure 6 2 Theoretical predictions of fluorescence captured by each of the guided modes of a waveguide with a fluorescent core assuming a transverse pumping geometry (a) Relative fluorescence coupled into each guided mode Pump absorption coefficients are shown by each curve (b) Total guided fluorescence as a function of pump absorption coefficient.

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108 6.4 Sensor Fabrication 6.4.1 Materials 6.4.1.1 Choice of Cladding Materials An ammonia-sensitive cladding of Nation fluoropolymer, doped with the pH sensitive dye Nile blue, was used in the FEEWA sensor. As shown in figure 5 7, this material exhibits a large change in absorption between 550 nm and 660 nm when exposed to ammonia Furthermore, as seen from figures 5 .13 and 5 14, Nile-blue doped Nation is sensitive to ammonia at sub-part-per-million levels. 6.4.1.2 Choice of Core Materials The polyimide Probimide 412 (OCG Microelectronics Materials) was chosen as the waveguide core material because of its low optical loss and the relative ease with which it could be doped with various dyes using the diffusion process described in chapter 4 Polyimide films were doped with a number offluorophores AHeCd laser (442 nm) and a tunable HeNe laser (543 .5, 594 .1, 611.9, 632.8 nm) were used as excitation sources Fluorescence was measured with an optical multichannel analyzer (OMA) The most promising candidates were found in the oxazine and pyridine family DCM, a highly efficient laser dye was tried but showed poor solubility in the butyrolactone solvent. Several other dyes including methylene blue and Styryl 7 (available under the name LDS751 from Exciton), were also tested but found to be only weakly fluorescent in the polyimide matrix In choosing a fluorophore for this application, some care needs to be excercised For the FEEWA design to be effective the wavelength of the fluorescence captured in the waveguide core needs to match the absorption spectrum of the cladding However the peak fluorescence wavelengths quoted by laser dye manufacturers generally pertain to the radiated emission shown in figure 6 I.105 For most dyes the fluorescence and absorption

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109 bands overlap to some extent. This phenomenon, known as self-absorption, causes the fluorophore to reabsorb some of the fluorescence emitted at shorter wavelengths The radiated fluorescence has only a short interaction length with the fluorophore (of the order of the waveguide thickness) and experiences negligible self-absorption However, guided fluorescence travels a substantial distance inside the doped waveguide, where self absorption effectively removes the shorter wavelengths. Thus, the spectra of the guided and radiated fluorescence are significantly different. To illustrate the self-absorption effect, we present absorption and fluorescence spectra of polyimide films doped with the laser dye LDS698 in figures 6.3 and 6.4, respectively. The sample used for the absorption measurement is on a glass substrate, while the one used for fluorescence is on an oxidized silicon substrate The well-defined oscillations observed in the radiated fluorescence spectra arise from constructive/ destructive interference from reflections off the silicon substrate.106 The maximum absorption and radiated fluorescence wavelengths occur at 500 nm and 510 nm respectively, while the guide fluorescence peaks at 655 nm. In this case, self absorption produces nearly a 150 nm shift in the peak wavelengths of the guided and radiated fluorescence spectra! Figures 6.5 through 6 8 show the radiated and guided fluorescence spectra for the dyes cresyl violet 670, oxazine 720, oxazine 725, oxazine 750 in polyimide films deposited on oxidized silicon substrates. With all of the oxazine dyes, guided fluorescence peaked about 30 nm higher than the radiated fluorescence Of the dyes tested, LDS698 was deemed to be the best choice because of the close match of its guided fluorescence to the absorption characteristics of Nile blue-doped Nafion, high energy conversion efficiency, 107 and resistance to photobleaching .108 In addition, the absorption spectrum of LDS698 in polyimide peaks at about 500 nm, which is far removed from the guided fluorescence.

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110 'in' :t:: 0.15 --------------..--T----T----T---------t -e .!, C 0 i O .1 -+---t-+----+----+-------1t------+----+--~t------+----t---t 0 .,. .a II 0 -t-r-"'l"""l"'"r-t-,o..,....,..,-,+-,...,...,..-.-+-,r-T""T'..,.--t-m"T""t-~""T"'T'"'l"""l"'"l""'T""'t'..,....,..,..-r-1...,...,.."'l"""l"'"t-"t--l'~ 400 425 450 475 500 525 550 575 600 625 650 wavelength (nm) Figure 6.3 Absorption spectra ofLDS698 (pyridine 1) in polyimide

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1.0 0.9 == 0.8 C :::, ,e 0.7 ., ~0.6 u, 111 LDS698 ,' ,. ' . . . . . . . I . . I .. . I .. ,. .. ... .. . .. I . . ,: t I .. . I '. . . .... I fi 0.5 . I c ; 0.4 u C 0.3 u, a 0.2 :::, C 0.1 pump ,ii .. ........ .,, .. ,. .. ... I . ,. . . . 0.0 400 450 500 .. . I I I . ~ .. ... I ...... \/ I ,., 550 600 wavelength (nm) radiated guided I \. \ I \ I \ I \ I \ \ \ '\ \ 650 700 Figure 6 4 Radiated and guided fluorescence spectra ofLDS698 (pyridine 1) in a polyimide waveguide on an oxidized silicon substrate. The excitation wavelength is 442 nm. The spectra are scaled individually by different normalizing constants 750

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1.0 0.9 ~0.8 C ::::, ,e 0.7 -!!.. CD 0.6 u C CD 112 Cresyl Violet 670 .. .. . . I I . -. I I . I . I I ',J . ,. . . I /JiM I \ .. \ \ I \ \ 7\ \ . pump I . :;: 0.5 f g 0.4 -== ,:::, 0.3 "iii E 0.2 0 C 0.1 1-----0.0 550 . 575 . . . I I . . I . . . I I . I . .. . I . I . f INN"n.,.,v ,., ., 600 625 650 675 wavelength (nm) radiated guided . \ V 'J " \" "' 'I\ . . .. .. . .. -. 700 725 750 Figure 6 5 Radiated and guided fluorescence spectra of cresyl violet 670 in a polyimide waveguide on an oxidized silicon substrate. The excitation wavelength is 594 1 nm The spectra are scaled individually by different normalizing constants

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1.0 0.9 -;;-0.8 :t:: C 0.7 -e -!!.0.6 .,, 5; 0.5 :c: CD 0.4 u C CD 0.3 CD ... 0 ::s 0.2 -== 0.1 0.0 550 -575 113 /\ .. . . . \ . . I . . . I . I I I I I I I I . I I I I I .r I I ., I pump . .. I I I .. . I I I I I I I I I I . I . . . I I I . I , . I 'JI . . -... -600 625 650 675 wavelength (nm) radiated guided \ \ \ \ \ \ \ \ \ i\ . . . . . ... 700 725 Figure 6 6 Radiated and guided fluorescence spectra of oxazine 720 in a polyimide waveguide on an oxidized silicon substrate The excitation wavelength is 611.9 nm. The spectra are scaled individually by different normalizing constants. 750

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1.0 0.9 ,;-0.8 ::I:: C 0.7 -e -!!.. 0 6 ~-.,. i 0.5 c 0.4 u C CD 0.3 g 0.2 s:: 0.1 pump J~ .. . . .. . . . I . . . . . 114 .. .. . I . . . . I . . . . . ..... ,. I . . . . I .. I -, I I I I ,~, I \ I \ I I \ I \ I \ I \ \ \ \ \ i- \ \ I \ I \ . \ . \ . .. \,,. .. . .. I i,. ...... I'-'"' ... --.. -. ">-'\.:,,; ,, 0.0 600 625 650 675 700 725 wavelength (nm) guided radiated 750 775 Figure 6.7 Radiated and guided fluorescence spectra of oxazine 725 in a polyimide waveguide on an oxidized silicon substrate The excitation wavelength is 632 8 nm The spectra are scaled individually by different normalizing constants 800

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1.0 0.9 -0.8 .,, ::I::: C ::s 0.7 -e -!!..o.6 .,, fi 0.5 c CD 0.4 u C CD 0.3 CD ... 0 .e 0.2 0.1 0.0 600 115 Oxazine 750 ,. r\ .. .. [ \ . pump .4 I. . . . . . . -. -625 650 I I I I j I . I I I I . I I I I I I I I I I I I . I I I I I ... I I I .. I . I I I : / I I . .. .. ., I i I I I I I . I : .,. I I I I I I .... -, I I ,.J 675 700 725 wavelength (nm) guided radiated \ \ \ \ \ \, \. \ \ \ "1 I\. '\. "', I . . .. ... --.. .. ______ 750 775 800 Figure 6 8 Radiated and guided fluorescence spectra of oxazine 750 in a polyimide waveguide on an oxidized silicon substrate The excitation wavelength is 632.8 nm. The spectra are scaled individually by different normalizing constants

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116 6.4.2 Waveguide Fabrication Five micrometer thick polyimide films were spin-coated onto oxidized silicon substrates and cross-linked by a 2.1 J/cm2 UV dose. Using the modified bake cycle described in section 4.3, the films were partially cured at 250 C. LDS698 was diffused into the polyimide films by immersing the samples in a 5x10-5M butyrolactone solution of the dye for 10 minutes. After doping, the films were rinsed with deionized water, blown dry with nitrogen, and baked at 100 C for 30 minutes to remove any residual solvent. Lastly, the samples were cleaved into -20 mm by 8 mm planar waveguides A small drop of a solution containing 1 part (5% Nafion solution) : 5 parts (lxl0-4 M Nile blue in isopropanol) was deposited at the edge of each sample and allowed to dry at room temperature. The resultant film was yellow in color and extended -5 mm from the output facet. The samples were then baked at 100 C for 30 minutes and cooled slowly. The doped-Nation cladding was measured to be about 0.4 m thick using a Dektak surface profiler. Bulk transmission measurements of doped Nafion films prepared from the same solution on glass microscope slides showed the films had an absorption loss of 12,800 dB/cm at 632.8 nm after exposure to ammonia 6.5 Sensor Characterization 6.5.1 Laser Pumping Initially, a 15 mW CW HeCd laser operating at 442 nm was used to pump the FEEWA sensor (figure 6. 1) by directing the beam at normal incidence to the polyimide film at a spot about 6 mm from the output endfacet. The beam diameter was about 2 mm. The pump produced a green/yellow radiated fluorescence over the irradiated region of the film. Strong red guided fluorescence was clearly visible to the naked eye along all of the edges of the waveguide. Since the refractive index of Probimide 412 (n = 1.62, undoped) is much larger than that of Nafion (n = 1 35, undoped) and the cladding layer is very thin

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117 negligible radiation loss and mode conversion occurs as the guided fluorescence travels from the unclad part of the FEEWA sensor into the clad region The solid curve in figure 6.9 shows the guided fluorescence measured by the OMA with the Nafion cladding in the yellow state. The guided fluorescence spectrum was rather broad, ranging from 600 nm to 720 nm and peaking at 650 nm When a 100 L droplet of ammonium hydroxide (30% NH3 by wt.) was placed near the FEEWA device the doped Nafion cladding immediately turned blue, and as shown by the dotted curve in figure 6 9 the guided fluorescence signal at 632 8 nm dropped by a factor of 55 This corresponds to a 17.4 dB increase in the propagation loss of the guided fluorescence due to evanescent wave absorption by the doped-Nation cladding. As expected, the blue (absorbing) color of the Nile blue-doped Nafion cladding was retained after ammonia exposure The acid rinse process described in section 5 2 5 was employed to return the cladding to the original yellow (transparent) state Using this recycling procedure we reused the sensor several times and obtained near identical results To ensure reproducibility, the sensor was always exposed to enough ammonia vapor to saturate the absorption response of the doped Nafion cladding 6.5.2 LED Pumping While the FEEWA performance obtained using the HeCd laser as a pump were encouraging two of the key requirements for a sensor designed for use outside of the laboratory are price and portability The complete sensor system including excitation source detector and power supply needs to be both compact and lightweight for operation in the field. The fact that the HeCd pump laser used in the preceding section is essentially limited to laboratory environments due to its high cost bulky size and poor electrical effic i ency ( 0 1 % 109 ) poses a severe limitation on the FEEWA sensor However in recent years bright blue and blue-green light emitting diodes with output powers on the order of a few milliwatts and electrical efficiencies of up to 9%, 1 10 have become commercially available from Ni chi a Toyota-Gusai Panasonic and others These LEDs cost about $10

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1 0.9 .. 0.8 ::::, -e as 0.7 ._,,, f 0.6 C CD :E 0.5 CD u C CD 0.4 u flt CD o 0.3 ::::, C: 1 0.2 "O s m 0.1 0 inHial device"I .... ..... .. ,,, .. ,, .,,,.,,,., ... 118 r \ I \ \ J I~/ \ I \ I \ / after ammonia \ I exposure (X 5) \ ,,' .. ._ \ .. ,,. 1 ,., .... n ,, .... ,.,~, .. ., ..... ,, .. .. ,. ,., ... ,,, 525 550 575 600 625 650 675 700 725 wavelength (nm) Figure 6 9 Response of fluorescence-excited evanescent wave absorption sensor to ammonia vapor The ammonia-sensitive cladding layer is 5 mm long and the excitation source is a HeCd laser, operating at 442 nm

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119 to $20, which, while expensive for an LED, is inconsequential compared to the price of the HeCd laser. Using an LED to pump the FEEWA sensor would thus represent a substantial improvement in the cost, portability, and power consumption. To investigate the feasibility of using an LED to pump the FEEWA, the HeCd laser was replaced with a bright blue-green LED (NSPE590S-B, Nichia). The peak wavelength and spectral width (FWHM) of the LED were measured to be 490 nm and 35 nm, respectively. The LED produced 2.4 mW of power for a current of 30 mA. A FEEWA sensor with a 2 mm long cladding was used in this testing. Approximately 0.7 mW of the LED output was focused into a 2 mm diameter spot on the FEEWA sensor, at a point 4 mm from the endfacet. As expected, the LED-pumped FEEWA sensor performed much like the laser pumped one, although the guided fluorescence signal was weaker by a factor of about 20 ( corresponding to the difference in the pump power). The spectra of the guided fluorescence from the LED-pumped sensor, before and after exposure to ammonia, are shown in figure 6.10. Both spectra are virtually identical to the results obtained previously under HeCd excitation. In this case, ammonia exposure produces a 9 dB increase in propagation loss at 632.8 nm. Based on these results, LED pumping of FEEWA sensors appears feasible. Ifwe compare the LED-pumped FEEWA to the laser-pumped one and account for the difference in the length of the claddings (2 mm vs 5 mm), we find that the LED pumped device shows stronger evanescent wave absorption, 35 dB/cm for the laser pumped device and 45 dB/cm for the LED-pumped one. This stems directly from the fact that the pump absorption is higher at the LED emission wavelength, as seen in figure 6.3. As a result, the LED excites more fluorescence into the higher order modes (see figure 6 2(a)) which are absorbed more strongly in the cladding. Thus, the sensitivity of the FEEWA sensor is strongly influenced by the excitation wavelength.

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120 1 -----------------------0.9 --------------++-"----+---1---1-----4 'in' :t:: 0.8 Initial .c ti 0.7-------11--------+--+--+----+-------------1 ...__. ,,, 0.6-------------------------4 C .5 0.5 ---------------------- u after ammonia 5; 0.4---------exposure 5 0.3 ::, IC "O 0.2 -+----+----+---+---l---~-.-~-,. ...... - -r1---~111c---l ,,1, '. "O _..,.., ,,. ''' 5, 0 .1 ----~'------.. ---. to---'"'..a..'--t---+---#--+<. r=-'.r"9'n""....&.'li .,., .. .. ..,, ,,, .. ,. -.. 0 ~~""""""""l-+-~~.:;.:....:~ ........ ....,...., ........ 1-...... ........................................................ ....... .......4 525 550 575 600 625 650 675 700 725 wavelength (nm) Figure 6.10 Response of fluorescence excited evanescent wave absorption sensor to ammonia vapor The ammonia-sensitive cladding layer is 2 mm long and the excitation source is an LED operating at 490 run

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121 6.5.3 Conventional Evanescent Wave Measurement The FEEWA device from section 6 5 1 was also operated as a conventional evanescent wave sensor for comparison purposes. A 632 8 nm HeNe laser was endfire coupled into the waveguide using a lOX microscope objective (NA = 0.25) This wavelength corresponds to the maximum change in the absorption of the Nile blue-doped Nation cladding and hence, peak sensitivity In this case, when the waveguide was exposed to ammonia vapor the output power dropped by 13.4 dB The sensor was then acid rinsed and retested, this time using a 40X objective (NA= 0.65) for coupling With the higher NA lens ammonia exposure reduced waveguide output by 15.5 dB. This indicates that as the numerical aperture of the lens increases, evanescent wave absorption becomes stronger This is the direct result of launching more power into higher order modes when a higher NA lens is used Nevertheless, the fluorescence excitation technique clearly outperforms endfire excitation We find that the sensitivity of the laser-pumped FEEWA is 150% greater than the EWA when the 40X lens is used for coupling and 250% greater when the 1 OX coupling lens is used As noted in chapter 5, the primary limitation of an endfire-excited multimode planar waveguide sensor is that most effective way to reproducibly measure evanescent wave absorption is to maximize the transmission of the source through the waveguide prior to analyte exposure Since mode loss usually increases with order this approach favors the launching of power into lower order modes which are less sensitive to changes in cladding absorption In addition, even when the source and waveguide are aligned for maximum transmission, the exact launching conditions are difficult to reproduce from one test to the next. Both of these problems are solved using the excitation arrangement afforded by the transversely-pumped FEEWA design

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122 6.6 Limitations of Fluorescence-Excited Waveguide Sensors Fluorescence-excited waveguide sensors are highly cost-effective in that the majority of the expense is shifted from coupling oflight into the waveguide to detection of light at the output. Clearly, the FEEWA sensor requires a more sensitive photodetector than a conventional EWA device However, since the FEEWA uses free-space excitation and does not need to be fiber pigtailed, the sensor element would be sufficiently inexpensive to allow it to be disposable Thus, the issue of reversibility is not as important as in conventional endfire-excited EWA sensors. The obvious disadvantage of the FEEWA approach is that prolonged operation leads to photobleaching of the fluorophore For instance, under 15 mW of 442 nm CW pumping, the intensity of the guided fluorescence decreased by 85% after only 1 hour, as shown in figure 6.10 Continuous pumping with the LED, matched to the peak absorption of LDS698, degraded the dye even faster. One means to avoiding this rapid fluorophore degradation is to use pulsed rather than CW excitation For example, when the LED used to pump the FEEWA was pulsed at 10 Hz by 100 mA current pulses of 10 msec duration (10% duty cycle), the guided fluorescence decreased by only -7% after 8 hours. The peak LED power incident on the FEEWA during the pulsed measurements was 2.1 mW. Nevertheless, fluorophore degradation poses a serious problem for fluorescence-excited sensors, as it effectively alters the calibration of the device. In order to function effectively, FEEWA sensors require an internal reference to monitor fluorophore degradation. This last point will be explored further in chapter 8 where we suggest novel schemes to overcome this problem. 6.7 Summary A novel evanescent wave absorption sensor known as the fluorescence-excited evanescent wave absorption sensor, has been demonstrated. This device uses guided

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123 1----------------------........---- laser .;,-= 0.8 -+---------------o LED c ::::::, -e 0.7 -+-----+------Yo.+-----+----+-----+--------1 e f 0.6---------....-----------c l; 0.5 --------------1------------1 u C 0.4 In CD ... g 0.3-+-----+-----+-----+-----+-.......,,=-----t-----t s:: 'tJ _g 0.2 -t-----t-----t-----t-----,,.--t----....,.,,.-.;::----; "5 tn 0.1 -+-----+-----+------i---=-4,,1----1--------1 0 -+-.,.......,,......,.......,....-+-.,.......,,......,..--,--+-.,.......,,......,..--,--+-.,.......,,......,..--,--+---,........,.--.--+--.--,........,.---,-~ 0 10 20 30 time (min) 40 50 60 F igure 6.11 D egr ada t ion o f edge fluorescence in an LDS698 dop e d poly i mide planar wave gu ide under CW e x citation from a 15 mW 442 nm H eCd laser (solid l i ne) and 2 mW 490 nm LED ( dashed line ).

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124 fluorescence generated inside a doped waveguide by transversely-directed free-space pump beam to probe the absorption characteristics of an analyte-sensitive cladding The FEEWA sensor demonstrated here exhibits an evanescent wave absorption which is up to 250% larger than that obtained with a conventional endfire-coupled EWA device. This performance improvement stems from the fact that transverse pumping preferentially excites fluorescence into higher order guided modes. As a result, the fractional power propagating in the analyte-sensitive cladding of the FEEWA device is much higher than in conventional endfire-excited EWA sensors. In addition, the simple excitation geometry of the FEEWA sensor makes the source-waveguide alignment trivial. The improved sensitivity of the FEEWA device, combined with the ease of alignment represents a substantial improvement in evanescent wave sensing

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CHAPTER 7 OPfICAL GAIN IN DYE-DOPED POLYMER WAVEGUIDES 7.1 Introduction In recent years, considerable efforts have been directed toward the development of fiber-based optical amplifiers and lasers, primarily for use in end-to-end optical telecommunications networks and medical applications Erbium-doped silica fiber amplifiers (EDFA), currently provide gains of on 30 dB 111 and more in the 1.55 m communications window, allowing data transmission at rates in excess of 5 gigabits/ sec.112113 At the same time, extensive research has also been conducted on waveguide based amplifiers and lasing action has been demonstrated in a number of media A few examples of waveguide lasers and amplifiers are given in table 7.1. Most of the waveguide work has been based on organic polymers doped with various fluorescent dyes Porous sol-gel glasses have also shown to be suitable hosts However, since most dyes fluoresce at visible to near-infrared wavelengths, polymer waveguide amplifiers have been largely excluded from applications in optical communications. In Chapter 6, it was shown that a polymer waveguide-based sensor, which used an optical pumping scheme to generate fluorescence inside a waveguide, functioned quite well as a chemical sensor. The output signal from that device, known as FEEWA, was the spontaneously-emitted guided fluorescence captured in the guided modes In light of the numerous reports of gain in dye-doped polymers though it seems possible to extend the FEEWA concept and build a chemical sensor from either a waveguide amplifier or laser. In such a chemically-sensitive optical amplifier, the presence of a particular analyte would alter the optical gain of the doped medium This approach is exactly the opposite of the 125

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126 evanescent wave absorption sensors in Chapter 5, which measure analyte-dependant propagation loss. Table 7.1 Active Polymer Waveguide Systems Ref. host media dopant lasing//gain comments 82 PMMA rhodamine 6G lasing at 570 nm distributed Bragg reflector mirrors 83 PMMAfiber rhodamine B 27 dB gain at fiber is 50 cm long 591 nm 114 modified pyrromethane lasing at 571 nm, 33% degradation PMMA 570 83% slope efficiency after 20,000 shots 115 modified oxazine 17 lasing, 8% efficiency degrades after PMMA 1000 shots 116 polyurethane rhodamine B 12.5 cm-1 gain at transverse pump 632nm 117 polyurethane rhodamine 6G 0 87 dB/cm gain at active media in 595 nm cover region only 117 polyurethane rhodamine B 0 26 dB/cm gain active media in cover region only 118 ORMOSIL rhodamine B 54 dB/cm gain transverse pump 119 ORMOSIL rhodamine 6G lasing at 568 nm 20% degradation after 3500 shots 119 ORMOSIL coumarin 153 48 cm-1 gain at 554 nm 20% degradation after 6000 shots 120 sol-gel sulfolasing at 621 nm, 1 J threshold with rhodamine 640 20% slope efficiency 532 nm pump 121 xerogel perylene red lasing at 620 nm, 50% degradation 19% slope efficiency after-20,000 shots 122 polycom perylene lasing at 578 nm, 50%, degradation glass orange 29% sloped efficiency after 2300 shots We have demonstrated gain in a dye-doped polyimide waveguide Before detailing the performance of these devices though, it will be helpful to first review the basic

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127 principles of optical gain. After this, we shall explore the extension of our waveguide amplifier into a chemical sensor 7.2 Optical Amplification An optical amplifier consists of a medium which supports optical transitions (i e fluorescence) over a range of wavelengths. The power of a signal beam whose wavelength falls within this region, can be increased as it passes through this medium. To accomplish this, an optical pump beam at a shorter wavelength than the signal, is used to excite atoms in the amplifying medium into higher energy states After a characteristic time, these atoms return to the ground state by emitting photons and/or phonons whose energy corresponds to the difference between the excited and ground states Photons can be produced either spontaneously or by stimulated emission Whereas spontaneous photons simply result in randomly fluctuating noise at the output, stimulated emission coherently amplifies the signal beam When all of the atoms contributing to the stimulated emission see identical environments, the gain medium is referred to as homogeneously broadened The intensity of a signal beam passing through an amplifying medium with homogeneously broadened gain grows as124 1 di 'Yo =-----1 dz 1 + g (A) I/Is (7 1) where Yo is the small signal optical gain coefficient, A is the wavelength, lg is a parameter known as the saturation intensity and g(A) is the normalized lineshape function which describes the emission spectrum The intensity of the signal beam at a distance z inside the amplifying medium is found by integrating equation (7 1) This yields I(z) g(A) In--+ --[I (z) -I (0)] = y0z I (0) Is (7 2) where 1(0) is the signal intensity at z = 0. The gain G = I ( z) / 1 ( 0) (7.3)

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12M can be calculated by solving equation (7.2) numerically In the small signal regime, where I (z) ,I (0) I8 equation (7 2) reduces to the familiar exponential growth law 'YoZ I(z) = I(0)e (7.4) which allows the expression for amplifier gain to be simplified to G = I (z) /I (0) = exp (y0z) (7 5) However, outside of the small signal regime, equation (7 5) is not valid To illustrate this, the exact solution of equation (7.3) is plotted in figure 7.1. In these calculations, we have chosen I8 = 10 W/cm2 and y0z = 2 cm-1 For signal intensities much smaller than the saturation intensity, the gain is observed to remain constant. However, as the signal intensity approaches the saturation intensity, the amplifier gain begins to drop. This phenomena, known as gain saturation, occurs when photon density of the signal beam becomes comparable to the density of atoms in the excited state 7.3 Characterization of Optical Gain in Dye-Doped Polyimide Waveguides We have measured optical amplification in a multimode polyimide waveguide doped with the laser dye cresyl violet 670 .123 To the best of our knowledge, this is the first demonstration of gain in this material. Unlike many of the other organic polymers previously studied in gain measurements, such as PMMA 82 and polyurethane, 117 the polyimide host exhibits excellent thermal and mechanical properties. Extensive measurements of optical gain as functions of input signal and pump energy have been performed. In addition, measurements of gain for each of the guided modes have been made.

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9 8 7 6 3 2 1 0 0.001 0.01 129 \ \ \ \ i\ \ \__ 0.1 1 10 100 1(0) / 15 Figure 7 1 Signal intensity gain as a function of position inside an optical amplifier In the small signal regime input light grows exponentially with distance When the signal intensity becomes comparable to the saturation intensity the gain becomes nonlinear and begins to saturate In this model y0z = 2

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lJU 7.3.1 Active Waveguide Fabrication Five micrometer thick polyimide films were spin-coated onto oxidized silicon wafers and cross-linked with a UV dose of 1.8 J/cm2 The samples were partially cured at 280 C and then cleaved into 20 to 25 mm long planar waveguides The undoped films had propagation losses of 1.5 to 2 dB/cm at 670 nm and supported 10 guided modes. Cresyl violet 670 was diffused into the polyimide films by immersing the samples in 1 xl0-4 M butyrolactone solutions of the dye for 10 minutes Samples were then rinsed with deionized water, blown dry with nitrogen, and baked at 120 C for 30 minutes to remove residual solvent. 7 .3.2 Experimental Set-up The gain measurements were performed at the Department of Physics in the Federal University of Pemambuco, Recife (Brazil) in the laboratory of Professor Cid B. de Araujo. The experimental arrangement is shown in figure 7 2 An LDS698 dye laser (oscillator cell only) operating at 670 nm provided the signal beam while a rhodamine 6G dye laser (oscillator and amplifier cells) operating at 590 nm served as the pump source. A frequency doubled Nd:YAG laser producing 10 ns pulses of 532 nm radiation at a repetition rate of 5 Hz was used to synchronously excite both dye lasers The 670 nm signal was coupled into the desired mode of the polyimide waveguide through a flint glass prism. A second prism mounted on the waveguide, 21 mm away from the first, was used to couple the 670 nm signal out of the waveguide The output signal was passed through several neutral density filters and a 0.25 m monochromator and detected with a photomultiplier tube (PMT) A 400 MHz digital oscilloscope was used to observe the voltage signal from the PMT. Optical gain was achieved by employing a transverse pumping geometry, in which the 590 nm pump beam was focused into a line by a cylindrical lens and directed to the sample surface at normal incidence such that it

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532nm LDS698 dye laser ( signal source) input pnsm 131 Nd:YAG laser 1064 nm second harmonic generator 532nm rhodamine 6G dye laser (pump source) focusing optics photomultiplier tube monochromator amplified signal beam + fluorescence pnsm Figure 7 2 Experimental setup for measurement of optical gain in cresyl violet 670-doped polyimide waveguides.

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U2 overlapped the guided streak of the signal beam between the prisms A bright orange fluorescence was observed on the waveguide where the pump beam was incident. 7 .3.3 Measurement of Optical Gain Gain was determined from the ratio of the magnitude of the voltages at the oscilloscope associated with the signal beam when the pump was present and absent termed Ys+p and Vs, respectively The spontaneous emission (fluorescence) contribution V p was measured by pumping the waveguide while blocking the signal beam and subtracted from V s+p Thus, (7.6) Gain was measured independently for several of the lower order modes, using pump and signal energies of 190 J and 20 nJ, respectively. The pump beam was focused into a 21 mm long line, giving a peak power of 9 kW/cm. As shown in figure 7 3, higher order modes were observed to have significantly larger gains The sixth order mode was amplified by a factor of 14.4 dB while the fundamental mode experienced a gain of slightly less than 1 dB! This result is consistent with the use of transverse pumping, since as discussed in chapter 6 higher order modes have a better spatial overlap with the pumped region This effect is further enhanced by the nature of the dye-diffusion process which produces a Gaussian concentration profile ,4 8 4 9 resulting in larger absorption of the pump at the surface As a result, higher order modes which propagate a larger fractional power near the surface of the waveguide experience larger gains than lower order modes which are more tightly confined. The gain saturation characteristics were examined next by measuring the gain of the fourth order mode against signal energy Signal energy was varied from 10 nJ to 23 J, corresponding to peak powers of approximately 2 W and 2 3 kW, respectively Pump energy was held fixed at 190 J. The measured data are presented in figure 7.4 and compared against equation (7 2) using fitting parameters of y0 = 1.37 cm-1 and Is = 8 J.

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133 16 14 ___ .... .o 12 10 CD "'C ..__, C: cu C) 8 6 4 2 !~ 0 0 / 1 /' I )I / 2 3 mode order I I 4 5 I Figure 7.3 Gain at 670 nm as a function of mode order Pump and signal energies are 190 J and 20 nJ, respectively The curve is only a guide 6

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134 14----------.------...--------r-------, measured theoretlcal 8-------------------------.------------. m i::, -C 6-----------+--------+---------+----- 4----------1--------,..-------,..,-----------1 2----------1--------t--------,..-------1 0 -+---,---,--,--,-....,..."T+---r----.--T""'T""'T'T"r"'TT'"---.--"""T""""'T"""T"'T"T'TTT"-"""'T'9--r"...,.-l""'T""1~ 0.01 0.1 1 10 100 slgnal energy (J) Figure 7.4 Gain of the fourth order mode at 670 nm as a function of signal energy Pump energy is 190 J. Gain saturates at about 4 8 dB for signal energies in excess of 7 J

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135 (the intensity tenns in equation (7 2) have been replaced by energies, since the pump is pulsed). For a signal energy of 20 nJ and less, a gain of 12.5 dB was obtained. Gain was observed to decrease as the signal became larger, ultimately saturating at 4.8 dB when the signal energy exceeded 7 J. This level of amplification is maintained for signal energies as large as 23 J, which was the upper limit of the laser used to generate the signal beam Although the peak gains exhibited by the cresyl violet-doped polyimide waveguide are rather modest compared to those reported elsewhere, 116118 it must be noted that the peak signal powers used here are rather high Using the assumption of homogeneous broadening, equation (7 .2) accurately describes gain saturation over most of the range of signal energies tested. However, the measured gain is considerably higher than that expected under this model when the input signal energy exceeds 20 J. Although this may simply be related to experimental uncertainties in the measurement, it is also possible that the underlying assumption of homogeneous broadening is not strictly correct. Although the polar nature of the polyimide matrix favors the monomeric fonn of cresyl violet, dimers and higher order olgimers of the dye might also exist inside the waveguide. This would lead to inhomogeneous broadening, since several distinct types of molecules would be contributing to the gain. In order to predict gain for large input energies, it may be necessary to modify equation (7.2) to reflect the inhomogeneous contribution To further illustrate the potential of the cresyl violet-doped polyimide waveguide, the gain of the fourth order mode was measured against the pump energy, while holding signal energy fixed at 20 nJ. Gain (in dB) was found to increase with pump power in a near linear fashion as shown in figure 7 5 No evidence of gain saturation was seen for pump energies as large as 190 J. From this data, it is clear that larger gains could be achieved through use of more intense pumping. For instance, Chang et al.116 use a peak pump power of 15 kW /cm to obtain 54 dB/cm of gain at 633 nm in a rhodamine B-doped Richard Steppel, Exciton, private communication

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14 12 10 C 'iij 6 C) 4 2 0 uo / v' / V. V / V V V / ~y V / 0 20 40 60 80 100 120 140 160 180 200 pump energy (J) Figure 7 5 Gain of the fourth order mode at 670 nm as a function of pump energy. Signal energy is 20 nJ. The line is only a guide

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137 polyurethane film Extrapolation of the data in figure 7.5 to a pump level of 15 kW/cm (equivalent to 316 Jin our system), indicates that the cresyl violet-doped polyimide films should be capable of producing gains of 20 to 25 dB At high levels of pumping, gain was observed to decrease slightly with time. This was attributed to increased relaxation rate of excited levels caused by heating of the sample. Full gain could be recovered by blocking the pump beam for a short time and allowing the sample to cool. Over the course of these experiments, the cresyl violet-doped polyimide waveguide exhibited a high degree of resistance to photobleaching Gain was observed to decrease linearly with the number of shots from the pump laser, but still retained 80% of its initial value after 3500 shots This is comparable to the performance of rhodamine doped ORMOSILs 119 and considerably better than that obtained in PMMA.115 In addition, no evidence of dye precipitation or clustering was observed over a period of weeks Thus, we may surmise that the polyimide matrix provides a stable environment for cresyl violet. 7.4 Optical Amplifiers as Chemical Sensors For an optical amplifier operating in the small signal regime, variations in gain produce exponential changes in the output signal power ( equation (7.5)). If the materials comprising the amplifier were carefully chosen such that exposure to a particular analyte affected the gain, then the polymer waveguide amplifier could be used as a highly sensitive chemical sensor Since the gain is related to the fluorescence, it was felt that a suitable fluorophore/analyte combination could be found through bulk fluorescence measurements. The emission spectra of a number of dyes in polyimide hosts were monitored in the presence of ammonia, xylene, and octane, using a SPEX fluorimeter However, in all cases, analyte exposure produced minimal changes in the fluorescence We attribute this to the rather polar nature and high pH of the polyimide host which apparently dominates the response of the fluorophore While the concept of building a

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138 chemical sensor based on an optical amplifier appears promising, it appears that a different host material will be required. 7 .5 Conclusion In conclusion, we have studied optical gain in a cresyl violet-doped polyimide multimode waveguide An amplification of 14.4 dB has been achieved at a wavelength of 670 nm, a wavelength at which red laser diodes are commercially available The photostability of the dye-doped polyimide is comparable to that of dye-doped ORMOSILs, with gain decreasing by only 20% after 3 500 shots Larger gains can be achieved by more intense pumping and also by optimizing the doping process Active cooling of the sample would also improve gain by reducing the relaxation rate of the excited levels A chemical sensor based on analyte-induced perturbations in the gain of an amplifier was proposed but remains unproven.

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CHAPfER8 CONCLUSIONS AND FUTURE WORK 8.1 Summary The development of new dyes and polymers with specifically-tailored chemistries will undoubtedly figure prominently in the future of optical waveguide-based chemical sensors, particularly with regard to selectivity, reversibility, and operating lifetime. We have demonstrated though that the choice of waveguide structure (i.e SPR, EWA, FEEWA, etc.) plays an important role in determining the overall sensitivity of these devices A number of sensor configurations have been explored, through numerical simulation and experimental characterization In the course of this study, emphasis has been placed on obtaining high analyte sensitivity through optimization of the waveguide structure, rather than the sensing chemistry. Several types of optical waveguide-based chemical sensors have been developed using surface plasmon resonance, and evanescent wave absorption. An optical gain perturbation technique has also been proposed. Surface plasmon resonance waveguides have been designed which are sensitive enough to detect refractive index variations on the order of 10-5 Although not demonstrated experimentally, it has been shown that the adsorption of monolayer films (-1 nm thick) can also be monitored with a silver-based SPR device, making it applicable to immunoassay and molecular self-assembly studies The inclusion of tuning layer into the conventional SPR waveguide structure has been shown to add flexibility to the design process and allow refractive index sensing over a wide range using only a limited number of materials for device fabrication A humidity sensor was created by coating an SPR 139

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140 waveguide with a thin layer of Nation a moisture-sensitive fluoropolymer At the same time, the humidity-dependant refractive index of Nation was characterized Several ammonia sensors which utilize Nation sensing layers have also been developed Conventional endfire-excited evanescent wave absorption sensors, comprised of polyimide ridge waveguides clad with thin films of Nation doped with a pH sensitive dye were studied. These devices were shown to be capable of detecting aqueous ammonia at levels as low as a few 100 ppb. Through improved processing techniques many of the problems previously encountered, particularly in regard to aqueous stability, substrate adhesion, and reversibility of Nation-based sensors, were solved A novel variant of the EWA sensor which uses fluorescence excited inside a waveguide by a free-space beam to probe cladding absorption was also developed This latter device, known as FEEWA provides a reduction in alignment tolerances (ultimately translating to lower packaging costs) and an improvement in sensitivity due to preferential excitation of higher order modes Lastly, a new sensing technique based on measuring analyte-induced perturbations in the optical gain of dye-doped polymer waveguides was proposed In this approach, the presence of a particular chemical species is expected to cause an exponential change in the transmission of a probe beam Waveguide amplifiers were produced by diffusing various laser dyes into polyimide films In a polyimide waveguide doped with cresyl violet 670, a gain of 14.4 dB at 670 nm was achieved in the 6th order mode However dyes were found to exhibit minimal analyte sensitivity in the polyimide host and as a result a chemically sensitive waveguide amplifier has yet to be demonstrated. 8.2 Future Work Clearly the field of optical chemical sensing will continue to benefit from continued research on the development of novel waveguide structures Some extensions of the sensors developed herein are now presented

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141 8.2.1 Improved Fluorescence-Excited Evanescent Waveguide Absorption Sensors 8.2.1.1 Compact Detection System Through use of LED excitation, the FEEWA design discussed in Chapter 6 combines both high sensitivity and low electrical power requirements in an extremely compact and inexpensive package. Low repetition rate pulsed excitation should allow relatively long sensor operating lifetimes. However, operation of the FEEWA sensor remains restricted to laboratory environments because of the OMA detector system used for fluorescence detection. In order for the FEEWA sensor to be applicable to field operation, the dependance on the OMA needs to be removed Since the fluorescence output of the FEEWA sensor tends to either be on" or "off' (see figures 6 9 and 6.10) the total fluorescent output power is of more interest than the actual shape of the spectrum Therefore one alternative would be to replace the OMA with either an amplified photodiode or miniature photomultiplier tube, such as the Hamamatsu HC135-02 As long as the background is kept to a minimum, integration over a set time (such as the pulse width of the excitation source) should produce a sufficiently measurable output signal. 8.2.1.2 The "U-waveguide" FEEWA Structure As noted earlier in chapter 6, FEEWA sensors require some type of internal reference to monitor photodegradation of the fluorophore One way of constructing a FEEWA sensor with an internal reference source is to use a channel waveguide curved into the shape of a "U", as shown in figure 8 .1. At the apex of the "U", the waveguide is locally doped with a fluorophore. Local doping is performed after waveguide fabrication by photolithographically defining a polyimide ring around the area to be doped. A tiny drop of dye solution is placed inside the ring As long as the amount of solution transferred to the substrate is small and the sample is held flat, the liquid is contained within the ring The HC135-02 photomultiplier tube operates on a 5 volt supply and draws a current of 3 5 mA.

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142 guided fluorescence fluorophore-doped region reference arm polyimide ridge waveguide spectrometer sensmg arm Figure 8 1 The U-waveguide structure showing an internal reference arm and a sensing arm Analyte concentration is determined by comparing fluorescence spectra leaving each arm in a spectrometer

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143 by surface tension After an appropriate diffusion time, the dye solution is rinsed away and the device is dried A free-space pump beam is directed at the tip of the "U'', where the fluorophore is located The resulting guided fluorescence travels into either arm of the "U'' in relatively equal amounts. One arm serves as an internal reference, while the other is coated with an analyte-sensitive cladding and acts as an evanescent wave absorption sensor Analyte concentration can be ascertained from the ratio of the fluorescence spectra leaving the two arms. Since dye degradation affects the fluorescence entering both arms equally, the overall effect of reduced fluorescence signal on sensor calibration is minimal. A FEEWA sensor based on the CT-waveguide structure was fabricated using a polyimide ridge waveguide that was 5 m thick and 50 m wide LDS698 was used as the fluorophore. Four and five millimeter radii of curvature were used for the "U'' junction Unfortunately, despite the tight lateral mode confinement arising from the high index difference at the polyimide/air boundary, these bends were found to be too sharp and lossy. An insignificant amount of guided fluorescence was detected at the output of either arm It appears that this structure could be more easily realized using either a plastic fiber with a larger bending radius or a larger more gradually curved ridge waveguide 8.2.1.3 FEEWA Sensor Arrays The FEEWA sensor concept discussed in chapter 6 represents a significant advance in the sensitivity and cost effectiveness of evanescent wave sensing. However the basic FEEWA structure shown in figure 6 1 actually utilizes fluorescence rather inefficiently, since only guided fluorescence travelling in the +z direction is collected. In fact, as shown in figure 8 2, the rectangular geometry of the planar FEEWA allows guided fluorescence to be collected from the four edges of the waveguide ( corresponding to the <110> cleavage planes of the silicon substrate). By placing cladding materials with different analyte-dependant absorption spectra at each edge, one can produce a small sensor array comprised of four FEEWA devices on the same chip The composite output

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pump beam guided fluorescence signal I ""~ guided fluorescence signal 4 doped polyimide silicon 144 radiated fluorescence (reference signal) / guided fluorescence signal 2 // guided fluorescence signal 3 Figure 8 2 The FEEWA concept extended to three dimensions Each of the cladding materials has different analyte-dependant absorption characteristics. Fluorescence generated in the pump region which is radiated out of the film can be used as an internal reference to correct for fluorescence degradation

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145 signal, formed from the fluorescence collected at each endfacet, would then give the array some measure of selectivity as well as multianalyte detection capability Finally the radiated fluorescence can be collected and used as an internal reference to correct for fluorescence degradation Since the device is driven by a single pump beam each of the individual FEEWA sensors is automatically calibrated with respect to the others In this configuration, virtually all of the fluorescence is used in one way or another. 8.2.2 Active Waveguides for Chemical Sensing 8.2.2.1 Chemically-Sensitive Polymer Waveguide Amplifiers In order to realize a waveguide amplifier with an environmentally dependent gain some research into new material systems will be required Candidate host materials for this application must be suitable for optical waveguide fabrication and sufficiently permeable to the analyte of interest. With the intense level of research in the development of novel polymers this goal appears to be within reach 8.2.2.2 Chemically-Sensitive Polymer Waveguide Lasers Chemically sensitive lasers are the logical e x tension of the amp l ifier based sensor proposed in chapter 7. Such devices have already been demonstrated by Wu and coworkers at the University of Florida using specially designed GaAs laser diodes.125 In these lasers the evanescent tail of the mode field is accessible at the surface of the laser chip over a 100 m long region Sensors for ammonia antibodies and polymer monolayers have been demonstrated by coating the laser with various analyte-sens i ti ve materials In these lasers the presence of an analyte changes the cavity loss thereb y affecting the threshold current, slope efficiency and spectral characteristics This sensor is known as the surface sens i tive diode laser (SSDL) The main drawback of the SSDL is that the depth of penetration of the optical field into the sensing region is quite small typically only a few tens of nanometers due to high refractive inde x of GaAs (n = 3 5) as compared with tha t of the sensing layer (n = 1.5) Although SSDL is in fact quite sensitive

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146 a laser sensor fabricated from a lower index material, such as a doped polymer, would probably work even better The polyimide waveguide amplifier studied in chapter 7 can be made into a laser by placing a mirror at each end of the structure, forming an optical cavity This could be accomplished by holographically writing distributed Bragg reflector (DBR) gratings either directly in the polyimide or in a photoresist cladding Alternatively, multilayer dielectric mirrors could be deposited on the endfacets of the waveguide Note that in the latter case, the mirrors are automatically aligned parallel, since the waveguide endfacets are defined by the <110> cleavage planes of the silicon substrate Part of the structure would be coated with an analyte-sensitive cladding and the laser would be optically pumped. With the exception of the mirrors, this is actually equivalent to the FEEWA sensor discussed in chapter 6. Of course, as compared to the current injection used to excite the SSDL, optical excitation is considerably less convenient for field operation. Nevertheless, the polymer laser-based chemical sensor has two distinct advantages over the SSDL First, assuming the waveguide loss is low enough, the laser cavity could be several millimeters long if not more. This permits use of a much longer cladding for stronger evanescent wave interaction than is possible in the SSDL. In addition, the refractive index of polyimide is -1.62, which gives a penetration depth of a few tenths of a micron into polymer claddings. Thus, evanescent wave interaction for the polymer laser should be much stronger than for the SSDL. Based on the performance of the FEEWA, it seems reasonable to believe that a polymer waveguide laser-based chemical sensor would offer an extremely high level of sensitivity The primary limitation of the polymer laser-based chemical sensor is photobleaching. In order to have a reasonable lifetime, the polymer waveguide laser would have to pulsed at a low repetition rate, perhaps on the order of 1 Hz. However, significant advances have been made in the development of robust fluorophores, and dyes with operating lifetimes in excess of 20,000 shots are presently available.114 122

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147 8.2.3 Polyimide Waveguides as Selective Chemical Recognition Elements In the course of the studies reported in chapters 4 and 5, it was observed that under endfire-excitation, the transmission characteristics of bare (i. e no dopants or cladding layers) waveguides fabricated from partially-cured polyimide were affected by the presence of solvents A small drop (-50 L) of an organic solvent such as acetone or methanol placed near the waveguide was observed to cause the transmission to vary in time as the solvent evaporated. This effect stems from the lack of chemical resistance of the partially-cured films as compared to the fully-cured material. When exposed to solvents partially-cured polyimide films tend to swell and possibly crack. This distorts the input facet and degrades the launching efficiency. In addition, scattering losses arising from both surface and bulk defects may also change while solvent is present in sufficient quantities In most cases, the transmission of the waveguide returned to its initial value some time after the solvent had completely evaporated In most applications, the lack of solvent resistance exhibited by partially-cured polyimide films would be undesirable. However, individual solvents were observed to produce unique temporal transmission signatures Thus, by monitoring the time response of the transmission, it appears that partially-cured polyimide waveguides can be used to differentiate between various chemical species We hypothesize that the selectivity of the transmission arises from the transport properties of each analyte through the polyimide matrix. Diffusion properties, including adsorption and desorption rates vary by analyte In addition, it is likely that the nature of the polymer matrix plays an important role in determining the overall level of selectivity offered by this approach. Thus, different materials may be better suited for this purpose. Clearly, more effort is required to fully realize an optical sensor based on selective analyte transport

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APPENDIX LIST OF ACRONYMS BPSG ........ . ... .............. ........... ... .......... ....... ....... ... ... Boron phosphate silicate glass CVD ... ..... ..... ............... .............................................. Chemical vapor deposition CWG ........ ... .... ....... .......... ..... ..... ....... ....... . ....... ........ Channel waveguide EO ... .... ... .... ...... .............. ... . ...... ... ........................ .... Electro-optic EWA .. ...... ... .............. .... ......... .... .... . ... .............. ....... Evanescent wave absorption sensor FEEWA . ...... . ....... ... ....... .... ....... . ... ............ ....... ..... Fluorescence-excited evanescent wave absorption sensor FWHM .............. ... ... ......... . . ..................... .......... .... Full-width half-maximum GRIN .......... ........... ... ..... ......... ....... ... . ...................... Graded refractive index IE . .... ... ................. ... ..... ............................................. .Ion-exchanged IO ... ........... .................. ...... ............ .... .......... . .... ...... Integrated-optic LDL ... ... ................... ... ....................... ....... ... ............ .Lower detection limit LED ......... ... ...... ......... .................... .......... .... ..... ... .Light emitting diode OMA .......................... .............. ........ ........ .... .......... Optical multichannel analyzer ORMOSIL ............... ...... . ... ........ ............................ . Organically modified silicate PECVD ... . ........ ... ...... .... ... ........... . ....................... .... Plasma enhanced chemical vapor deposition PER ..... .... . ... ... ..... ..... ... .... ..... ................................. Polarization extinction ratio PM}d.A ....... . ................. .... ... ..... . ....... ... ...... ... .... ... ... Polymethylmethacrylate PMT ... ........ ....... ............ ................... .... ........ ... ......... Photomultiplier tube PTFE .... ... .... ................................ .... ....... . ..... .... . .... Polytetrafluoroethylene PWG ... ... ... ...... ... ..................... ................. ... ... ........ .... Planar waveguide 148

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RII ................ ......... ........... ................. .................... .... Relative humidity SP ...... ......... ..................... . ....................... ................ Surface plasmon SPR ............ ................................... ...... ....................... Surf ace plasm on resonance TE ...... ................................................. ...................... Transverse electric TIR. ............................................................................. Total internal reflection TM ....... ............................................... ....................... Transverse magnetic

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1. 2 3 4 5 6 7 8 REFERENCES S M. Sze, Semiconductor Sensors (John Wiley & Sons, Inc. : New York, 1994). 0 S. Wolfbeis, in Mo]ecu]ar Luminescence Spectroscopy Methods and Applications-Part II, edited by S. G Schulman (John Wiley & Sons, Inc.: New York, 1988) p 131 H. Kogelnik and R V Ramaswamy, "Scaling rules for thin film optical waveguides," Applied Optics, vol. 13, 1974 pp. 1857-1864. A. Yariv and P. Yeh, Optical Waves in Crystals (Wiley : New York, 1984), pp. 489-495 G Keiser, Optical Fiber Communications (McGraw-Hill : New York, 1988). J. N. Polky and G L Mitchell, "Metal-clad planar dielectric waveguide for integrated optics ," Journal of the Optical Society of America B vol. 64, 1974 pp. 274-279 C A. Hulse, and A. Knoesen, "Iterative calculation of complex propagation constants of modes in multilayer planar waveguides," IEEE Journal of Quantum Electronics, vol. 28, 1992, pp. 2682-2684 W H. Press, S A. Teukolsky, W T. Vetterling, B. P Flannery, Numerical Recipes in CThe Art of Scientific Computing, 2nd edition (Cambridge University Press: New York, 1992) 9. K. H Schlereth and M. Tacke, "The complex propagation constant of multilayer waveguides: an algorithm for a personal computer," IEEE Journal of Quantum Electronics vol. 26, 1990, pp 627-630. 10 R. D Harris and J. S Wilkinson Waveguide surface plasmon resonance sensors ," Sensors and Actuators B vol. 29, 1995, pp. 261-267 11. C R. Lavers and J. S Wilkinson "A waveguide-coupled surface plasmon sensor for an aqueous environment," Sensors and Actuators B vol. 22, 1994, pp. 75-81. 150

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151 12 J. Homola, "Optical fiber sensor based on surface plasmon excitation presented at Europtrode, 1994 Firenze 13. M. N Weiss R. Srivastava and H. Groger, Experimental investigation of a surface plasmon-based integrated-optic humidity sensor Electronics Letters vol. 32, 1996 pp. 843-842 14 Th. Wagner and S Roth, Surface-plasmon resonance investigations ofLangmuirBlodgett films of donor-acceptor substituted polyenes : linear optical and electro optic properties ," Synthetic Metals vol. 54, 1993, pp. 307-314 15 H Morgan, D M Taylor and C D Silva "Surface plasmon resonance studies of chemisorbed biotin-strepavidn multilayers ," Thin Solid Films vol. 209 1992 pp. 122-126 16 L. J. Noe, M Tomoaia-Cotise, M Casstevens, and P N Prasad Characterization of Langmuir-Blodgett films of 3 4-didecyloxy-2 5-di( 4-nitrophenylazomethine )thiophene in a stearic acid matrix," Thin Solid Films vol. 208 1992 pp 274-279 17 M A. Sletten and S R. Seshadri Experimental investigation of a thin-film surface polariton polarizer ," Journal of Applied Physics vol. 70, 1991 pp. 57 4 579 18 A. S Barker Jr., "Optical measurements of surface plasmons in gold," Physical Review B vol. 8 1974 pp 5418-5426. 19 A. J C Tubb F. p Payne R. Millington and C R. Lowe, "Singlemode optical fibre surface plasma wave chemical sensor ," Electronics Letters vol. 31 1995 pp 1770 1771. 20. W Johnstone G Stewart, T. Hart, and B. Cutshaw Surface Plasmon Polaritons in Thin Metal Films and Their Role in Fiber Optic Polarizing Devices," IEEE Journal ofLightwave Technology vol. 8 1990 pp 538-543 21. J. Homola and R. Slavik, Fibre-optic sensor based on surface plasmon resonance ," Electronics Letters vol. 32 1996 pp. 480-482 22. C. R. Lavers R. D Harris S Hao, J. S Wilkinson K O 'Dwyer, M Brust, and D J. Schiffrin "Electrochemically-controlled waveguide coupled surface plasmon sensing ," Journal ofElectroanalytical Chemistry vol. 387 1995 pp. 11-22

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152 23. M N Weiss, R. Srivastava, H. Groger, and P. Lo, "A theoretical investigation of environmental monitoring using surface plasmon resonance waveguide sensors," Sensors and Actuators A, vol. SNA051/2-3, 1996 pp. 211-217 24 H Rathaer, Surface Plasmons on Smooth and Rough Surfaces and C'TTatings, (Springer-Verlag : New York, 1988). 25 T. Nakano, K. Baba, and m Miyagi Insertion loss and extinction ratio of a surface plasmon-polariton polarizer : theoretical analysis," Journal of the Optical Society of America B vol. 11, 1994, pp 2030-2035 26 A. Miliou H Zhenguang H. C Cheng R. Srivastava and R. V Ramaswamy Fiber-compatible K+-Na+ ion-exchanged channel waveguides: fabrication and characterization," IEEE Journal of Quantum Electronics vol. 25 1989 pp. 18891897 27 M W Ribarsky 1985 in Handbook of Optical Constants of Solids edited by E D Palik (Academic Press : New York 1985) pp. 798-801. 28 D W Lynch and W R. Hunter, 1985 in Handbook of Optical Constants of Solids edited by E D Palik (Academic Press : New York, 1985) pp. 289-295 352-357 29. Malitson 1965 in Handbook of Optics Sponsored by the Optical Society of America edited by W Driscoll (McGraw-Hill : New York 1970) chap 7 p 100 30. S Chiao B. G Bovard and H. A. Macleod Optical-constant calculation over an e x tended spectral region : application to titanium dioxide film," Applied Optics vol. 34, 1995 pp 7355-7360 31 J. R Sambles G W Bradbery, and F. Yang Optical excitation of surface plasmons : an introduction," Contemporary Physics vol. 32, 1991 pp 173-183 32 H E. de Bruijn R. P H Kooy man and J. Greve "Choice of metal and wavelength for surface-plasmon resonance sensors : some considerations ," Applied Optics vol. 31 1992 pp. 440-442 33 Y. Wang Voltage-induced color-selective absorption with surface plasmons," Applied Physics Letters vol. 67 1995 pp. 2759-2761. 34. Cargille Certified Refractive Index Liquids (R. P. Cargille Laboratories Inc., Cedar Grove New Jersey).

PAGE 162

153 35. M N Weiss R. Srivastava, and H Groger, "Experimental investigation of a surface plasmon-based integrated-optic humidity sensor Electronics Letters, vol. 32, 1996 pp 842-843. 36. Z Fan and D J. Harrison, "Permeability of glucose and other neutral species through recast perfluorosulfonated ionomer films," Analytical Chemistry vol. 64 1992 pp. 1304-1311. 37. J. D. Jackson Classical Electrodynamics (Wiley : New York, 1962). 38 Y. Sadaoka M Matsuguchi, Y. Sakai Y. Murata "Optical humidity sensing characteristics of composite thin films of hydrolysed nation-dye with a terminal Nphenyl group," Journal ofMaterials Science, vol. 27 1992, pp 5095-5100. 39. H Franke, D Wagner, T. Kleckers, R. Reuter H. V. Rohitkumar and B. A. Blech, Measuring humidity with planar polyimide light guides," Applied Optics, vol. 32 1993, pp 2927-2935 40. K E Petersen Silicon as a mechanical material Proceedings of the IEEE, vol. 70 1982 pp 420-457 41 U Schnakenberg W Benecke, and P. Lange, "TMAHW etchants for silicon micromachining Transducers '91, International Conference on Solid State Sensors and Actuators San Francisco CA, 1991 pp 815-818. 42. K. Imoto H Sano, and M Miyazaki Guided-wave multi/demultiplexers with high stopband rejection," Applied Optics, vol. 26 1987, pp 4214-4219 43 M. J. Rand and R. D Standley "Silicon oxynitride films on fused silica for optical waveguides ," Applied Optics, vol. 11, 1972 pp 2482-2488 44. V. Ramaswamy and H. P Weber, Low-Loss Polymer Films with Adjustable Refractive Index, Applied Optics vol. 12, 1973 pp 1581-1583. 45 T. Kurokawa N Takato and Y. Katayama, "Polymer optical circuits for multimode optical fiber systems ," Applied Optics vol. 19 1980 pp 3124-3129 46. R. Ulrich and H. P. Weber Solution-Deposited Thin Films as Passive and Active Light-Guides," Applied Optics vol. 11, 1972 pp 428-434 47. S Imamura R. Yoshimura T. Izawa Polymer channel waveguides with low loss at 1.3 m," Electronics Letters vol. 27, 1991 pp 1342-1343

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154 48. K. K Chakravorty, "Ultraviolet defined selective in-diffusion of organic dyes in polyimide for applications in optical interconnection technology," Applied Physics Letters, vol. 61, 1992, pp 1163-1165 49 M J. Goodwin, R. Glenn, and I. Bennion, "Organic nonlinear optical waveguides formed by solvent-assisted indiffusion," Electronics Letters, vol. 22, 1986, pp 789-790 50 J.M. Cech, A. F. Burnett, and L. Knapp, "Pre-imidized photoimageable polyimide as a dielectric for high density multichip modules," Polymer Engineering and Science, vol. 32, 1992 pp. 1646-1652. 51. K. K Chakravorty, C. P. Chien, J.M. Cech, M. H. Tanielian, and P L. Young "High-density interconnection using photosensitive polyimide and electroplated copper conductor lines," IEEE Transactions on Components Hybrids and Manufacturing Technology, vol. 13, pp. 200-206. 52 V White, R. Ghodssi, C Herdey D D. Denton, and L. McCaughan, "Use of photosensitive polyimide for deep x-ray lithography," Applied Physics Letters vol. 66, 1995, pp 2072-2073. 53. R. Selvaraj, H. T. Lin J. F. McDonald, Integrated optical waveguides in polyimide for wafer scale integration," Journal ofLightwave Technology vol. 6 1988,pp. 1034-1043 54 T. C Kowalczyk, T. Kosc K. D. Singer, P.A. Cahill, C H Seager, M B. Meinhardt, A. J. Beuhler and D. A. Wargowski, "Loss mechanisms in polyimide waveguides ," Journal of Applied Physics, vol. 76, 1994 pp 2505-2508. 55. T. Matsuura S Ando, S. Matsui S Sasaki, and F Yamamoto "Heat-resistant singlemode optical waveguides using fluorinated polyimides," Electronics Letters vol. 29 1993, pp 2107-2109 56 R. Reuter, H Franke, and C Feger, "Evaluating polyimides as lightguide materials, Applied Optics, vol. 27, 1988, pp. 4565-4571. 57 J. W. Wu J F. Valley, S Ermer E S Binkley, J. T. Kenney G F. Lipscomb, and R. Lytel "Thermal stability of electro-optic reponse in poled polyimide-systems, Applied Physics Letters, vol. 58 1991 pp 225-227 58 Probimide 400 series product brochure (OCG Microelectronics : East Providence Rhode Island 1994).

PAGE 164

155 59. S Wolf and R. N Tauber Silicon Processing for the VLSI Era, volume J -Process Tech no) ogy (Lattice Press: Sunset Beach, California 1986). 60 J. C Coburn M T. Pottiger and C A. Pryde, Structure development i n polyimide films Material Research Society Symposia Proceedings vol. 308, 1993 pp. 475-487 61. A. J. Beuhler, D A. Wargowski K. D Singer and T. Kowalczyk, Fabrication of low loss polyimide optical waveguides using thin-film multichip module process technology ," IEEE Transactions on Components Hybrids, and Manufacturing Technology-part B vol. 18 1995 pp 232-234 62. M J. Rooks, H. V Roussell and L. M Johnson Polyirnide optical waveguides fabricated with electron beam lithography ," Appl i ed Optics vol. 29, 1990 pp. 3880-3881. 63 G Stewart and B Cutshaw, Optical waveguide modelling and design for evanescent field chemical sensors ," Optical and Quantum Electronics vol. 26, 1994 pp. S249-S259 64. R. Srivastava C Bao, and C Gomez-Reino, "Planar surface waveguide evanescent wave chemical sensors," Sensors and Actuators A, v ol. SNA 05 1 / 2-3 1996 pp. 165-171. 65 C. Piraud, E Mwarania, G Wylangowski J. Wilkinson K O 'Dwyer, and D J. Schiffrin Optoelectrochemical thin-film chlorine sensor employing evanescent fields on planar optical waveguides ," Analytical Chemistry vol. 64, 1992 pp. 651-655. 66 E Sensfelder J. Burck, and H. J. Ache Determination of hydrocarbons in water by evanescent wave absorption spectroscopy in the neari nfrared region ," Fresenius Journal of Analyt i cal Chemistry vol. 354, 1996 pp. 848-851 67. J. Mayer, J. Burck, and H.J. Ache, Optimisation of an i ntegrated optical evanescent wave absorbance sensor for the determination o f chlorinated hydrocarbons in water," Fresenius Journal of Analytical Chemistry vol. 354, 1996 pp 841-847 68 R. Klein and E Voges Integrated-optic ammonia sensor," Fresenius Journal o f Analytical Chemistry vol. 349, 1994 pp. 394-398 69. E E Hardy, D J. David, N S Kapany F. C Unterleitner "Coated optical guides for spectrophotometry of chemical reactions," Nature, vol. 257 1975 pp. 666-667

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Do 70 A. Messica, A. Greenstein A. Katzir U. Schiessl and M. Tacke "Fiber-optic evanescent wave sensor for gas detection Optics Letters vol. 19 1994 pp. 11671169 71. J E Lee and S S. Saavedra "Evanescent sensing in doped sol-gel glass films, Analytica Chimica Acta vol. 285, 1994 pp 265-269 72 M N. Weiss, R. Srivastava P Lo, and H Groger Rapid detection of aqueous ammonia at sub-part-per-million levels using Nafion-clad polyimide waveguides ," submitted to Sensors and Actuators B 73 W D Sample "Water quality is essential in holding fish in vats and hauling tanks," Aquaculture Magazine, 1994 pp 6872 74 N R. Bromage and C. J. Shepherd in Intensive Fjsh Fanning C. J. Shepherd and N R. Bromage Eds. (BSP Professional Books Oxford 1988) 75 Erickson, R. J., An evaluation of mathematical models for the effects of pH and temperature on ammonia toxicity to aquatic organisms," Water Research vol. 19 1985 pp 1047-1058 76 Meade, J. W Allowable ammonia for fish culture, The Progressive Fish Culturist vol. 47 1985 pp 135-145 77. J. Colt, Aquacultural production systems," Journal of Animal Science vol. 69 1991 pp. 4183-4192 78 J. F. Guiliani H. Wohltjen and N L. Jarvis, Reversible optical waveguide sensor for ammonia vapors Optics Letters vol. 8 1983 pp 54-56 79 D J. David, M. C Wilson and D S Ruffin Direct measurement of ammonia in ambient air, Analytical Letters vol. 9 1976 pp. 388-404 80 L. L. Blyler, JR. R. A. Lieberman L G Cohen J A. Ferrara and J B. Macchesney Optical fiber chemical sensors utilizing dye-doped silicone polymer claddings," Polymer Engineering and Science vol. 29 1989 pp. 1215-1218. 81. V Chemyak, R. Reisfeld R. Gvishi and D Venezky Oxazine 170 in sol-gel glass and PMMA films as a reversible optical waveguide sensor for ammonia and acids ," Sensors and Materials vol. 2 1990 pp 117-126

PAGE 166

157 82. I. P. Kaminow, H.P. Weber, and E A. Chandross, "Poly (methy methacrylate) dye laser with internal diffraction grating resonantor," Applied Physics Letters vol 18, 1971, pp 497-499. 83. A. Tagaya, Y. Koike, E. Nihei, S Teramoto K Fujii, T. Yamamoto and K. Sasaki "Basic performance of an organic dye-doped polymer optical fiber amplifier Applied Optics vol. 34 1995, pp 988-992 84 A. Mukherjee, "Two-photon pumped upconverted lasing in dye doped polymer waveguides ," Applied Physics Letters vol. 62 1993, pp. 3423-3425 85. W G. Grot, Perfluorinated ion-exchange polymers and their use in research and industry," Macromolecule Symposia, vol. 82, 1994 pp 175-184 86. R. J. Churchill K. P. Lo H.P. Groger S Luo Self-assembled thin film sensors for aqueous process control, SBIR phase II final report to United States Department of Agriculture, grant #93-33610-9096, 1995 87 J. Zen and G Patonay "Near-infrared fluorescence probe for pH determination ," Analytical Chemistry vol. 63, 1991 pp 2934-2938 88. D S Ballantine Jr., D Callahan G J. MaClay and J. R. Stetter An optical waveguide acid vapor sensor ," Talanta vol. 39 1992 pp 1657-1667 89 P J Kinlen and D C. Silverman "Annealed perfluorinated cation exchange polymers for corrosion resistant coatings Journal of the Electrochemical Society vol. 140 1993, pp. 3140-3145 90 F. Moussy and D J. Harrison, Prevention of the rapid degradation of subcutaneously implanted Ag I AgCl reference electrodes using polymer coatings ," Analytical Chemistry vol. 66, 1994, pp 674-679 91. K. H. Drexage Fluorescence efficiency oflaser dyes, National Bureau of Standards Special Publication 466 : Standardization in Spectrophotometry and Luminescence Measurements 1977 pp. 33-40 92 G Gebel P Aldebert, and M Pineri Structure and related properties of solution cast perfluorosulfonated ionomer films ," Macromolecules vol. 20 1987 pp. 1425-1428 93. R. B Moore ill and C R. Martin "Chemical and morphological properties of solution-cast perfluorosulfonate ionomers Macromolecules vol. 21, 1988 pp 1334-1339

PAGE 167

158 94. J. F Giuliani and T. W. Barrett, "The effect of ammonia ions on the absorption and fluorescence of an oxazine dye," Spectroscopy Letters, vol. 16, 1983 pp 555-563 95 Y Sadaoka, Y Sakai, X. Wang, "Optical properties of fluorescent dye-doped polymer thin film and its application to an optochemical sensor for quantification of atmospheric humidity," Journal of Material Science, vol. 29, 1994, pp 883-886. 96. K. Emerson, R.R. Russo, R E. Lund, and R. V Thurston, "Aqueous ammonia equilibrium calculations : effect of pH and temperature," Journal of Fisheries Research Board of Canada vol. 32, 1975, pp. 2379-2383 97 C Easter, "Water chemistry characterization and component performance of a recirculating aquaculture system producing hybrid stripped bass," Master's Thesis, Virginia Polytechnic Institute and State University, 1992, Blacksburg, Virginia. 98 M D DeGrandpre, L. W Burgess P L. White, and D.S. Goldman, "Thin film planar waveguide sensor for liquid phase absorbance measurements," Analytical Chemistry, vol. 62, 1992, pp 2012-2017. 99 M. N. Weiss, R. Srivastava and H. Groger, Fluorescence-excited evanescent wave absorption sensors," submitted to Applied Optics 100 S. Albin A. L. Briant C 0 Egalon R. Rogowski and J. S. Nankung, in Chemical, Biochemical, and Environmental Fiber Sensors m R. A. Lieberman ed. (SPIE : Bellingham, Washington, 1992) vol. 1796, pp. 393-401. 101. D Marcuse Launching light into fiber cores from sources located in the cladding ," IEEE Journal ofLightwave Technology vol. 6, 1988, pp. 1273-1279 102. C 0. Egalon and R. Rogowski "Theoretical model for a thin cylindrical film optical fiber fluorosensor," Optical Engineering, vol. 31, 1992 pp. 237-244 103 C 0 Egalon R. Rogowski, and A. C Tai, "Excitation efficiency of an optical fiber core source," Optical Engineering vol. 31, 1992 pp. 237-244 104. R. A. Lieberman K E. Brown in Chemical, Biochemical, and Environmental Applications of Fibers, (SPIE : Bellingham, Washington, 1988) vol. 990 pp. 104110 105. Exciton laser dye catalog (Exciton Inc.: Dayton, Ohio, 1992) 106. R. Scheps "Near-IR dye laser for diode-pumped operation, IEEE Journal of Quantum Electronics vol. 31, 1995 pp 126-134

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107. J. Hoffnagle, L Ph Roesch N Schlumpf, and A. Weis, "CW operation oflaser dyes styryl-9 and styryl-11 Optics Communications vol. 42, 1982, pp. 267-268 108. P Bado, C Dupuy, K. R. Wilson, R. Boggy, J. Bowen, and S Westra, High efficiency picosecond pulse generation in the 675-930 nm region from a dye laser synchronously pumped by an argon-ion laser," Optics Communications, vol. 43, 1983 pp. 241-243. 109 B. E A. Saleh and M C. Teich, Fundamentals of Pbatanics (John Wiley and Sons Inc. : New York, 1991) 110. R. DeMeis, Gallium nitride LEDs shine over visible spectrum ," Laser Focus World vol. 32, 1996, pp 18-22 111. J. J. Pan and Y. Shi, Polarizing beamsplitter trims noise in EDF As, Laser Focus World vol. 32, 1996 pp 93-99. 112. D. M Spirit L. C Blank D L. Williams S. T. Davey B J. Ainslie 5 Gbit/s + 10 dBm lossless transmission in 10 km distributed erbium fibre amplifier Electronics Letters vol. 26 1990 pp 1658-1659 113. A.H. Gnauck and C R. Giles 2 5 and 10 Gb/s transmission experiments using a 137 photon/bit erbium-fiber preamplifier receiver IEEE Photoonics Technology Letters vol. 4 1992 pp 80-82. 114 R. E Hermes T. H. Allik, S Chandra J. A. Hutchinson, "High-efficiency pyrromethene doped solid-state dye lasers Applied Physics Letters vol. 63, 1993 pp 877-879. 115 D A. Gromov K M Dyumaev A. A. Manenkov A. P. Maslyukov G A. Matushin V S. Nechitailo and A. M Prokhorov Efficient plastic-host dye lasers ," Journal of the Optical Society of America B vol. 2 1995 pp 1028-1031. 116 M S Chang C Hu, and J. R. Whinnery Light amplification in a thin film ," Applied Physics Letters vol. 20 1972 pp. 313-314 117 K Saski T. Fukao T. Saito and 0 Hamano Thin-film waveguide evanescent dye laser and its gain measurement ," Journal of Applied Physics vol. 51, 1980 pp 3 090-309 2 118 Y. Sorek R Reisfeld I. Finkelstein and S. Ruschin Light amplification in a dye doped glass planar waveguide ," Applied Physics Letters vol. 66 1995 pp 11691171.

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160 119. E .T. Knobbe, B. Dunn, P. D Fuqua, and F Nishida, "Laser behavior and photostability characteristics of organic dye doped silicate gel materials," Applied Optics, vol. 29, 1990 pp 2729-2733 120 F. Salin, G. Le Saux P. Georges, A. Brun C Bagnall, and J. Zarzycki, "Efficient tunable solid-state laser near 630 nm using sulforhodamine 640-doped silica gel, Optics Letters, vol. 14 pp 785-787 121. M Canva, P. Georges, J. F. Perelgritz, A. Brum, F. Chaput, and J.P. Boilot, "Peryleneand pyrromethene-doped xerogel for a pulsed laser, Applied Optics, vol. 34, 1995, pp 428-431. 122 M D. Rahn and T. A. King "Comparison oflaser performance of dye molecules in sol-gel, polycom ormosil, and poly (methyl methacrylate) host media," Applied Optics vol. 34, 1995 pp. 8260-8271. 123. M. N. Weiss R. Srivastava, R.R. B Correia, J. F. Martins-Filho, and C de Araujo "Measurement of optical gain at 670 nm in an oxazine-doped polyimide planar waveguide," to be published in Applied Physics Letters 124 J. T. Verdeyen, Laser Electronics, 2nd edition (Prentice Hall : Englewood Cliffs, New Jersey 1989). 125. C. Wu "Thin p-clad InGaAs single quantum well lasers," Ph. D Dissertation University of Florida 1996

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BIOGRAPIIlCAL SKETCH Martin Weiss was born in Staten Island, New York, on September 26, 1969 In 1991, he received the Bachelor of Science degree in electrical engineering from the Cooper Union for the Advancement of Science and Art, New York. He went on to attend the University of Florida, Gainesville, where he received the Master of Science degree in 1993. He is presently working on the development of novel optical waveguide devices for chemical sensing applications. His current research interests include polymer waveguides, surface plasmon resonance, waveguide amplifiers and lasers, and optical sensors 161

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I certify that I have read this study and that in my opinion it conforms to acceptable standards of scholarly presentation and is fully adequate, in scope and quality, as a dissertation for the degree of Doctor of Philosophy J?s25:,u' Ramakant Srivastava, Chairman Professor of Electrical and Computer Engineering I certify that I have read this study and that in my opinion it conforms to acceptable standards of scholarly presentation and is fully adequate, in scope and quality, as a dissertation for the degree of Doctor of Philosophy Peter S Zory Professor of Electrical and Computer Engineering I certify that I have read this study and that in my opinion it conforms to acceptable standards of scholarly presentation and is fully adequate in scope and quality as a dissertation for the degree of Doctor of Philosophy ~ ~/ZM Toshikazu Nishida Professor of Electrical and Computer Engineering I certify that I have read this study and that in my opinion it conforms to acceptable standards of scholarly presentation and is fully adequate in scope and quality, as a dissertation for the degree of Doctor of Philosophy Ewen M Thomson Professor of Electrical and Computer Engineering

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I certify that I have read this study and that in my opinion it conforms to acceptable standards of scholarly presentation and is fully adequate, in scope and quality, as a dissertation for the degree of Doctor of Philosophy. t ames D. Winefordn r Graduate Research Professor of Chemistry I certify that I have read this study and that in my opinion it conforms to acceptable standards of scholarly presentation and is fully qu.a,te, in scope and quality, as a dissertation for the degree of Doctor of PhilosoP, This dissertation was submitted to the Graduate Faculty of the College of Engineering and to the Graduate School and was accepted as partial fulfillment of the requirements for the degree of Doctor of Philosophy. December 1996 l ~--Winfred M Phillips Dean, College of Engineering Karen A. Holbrook Dean, Graduate School

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LO 1780 1996 t w~3 1,11,nrTQN SCIENCE liBilARY, UN IV E RSITY OF FLORIDA 111111111111 111 111111111111111111111111111 1 111111 111111111111111 3 1262 08554 2420


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