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Prism Coupler Surface Sensing PRISM COUPLER SURFACE SENSING by William David Mason A thesis submitted for the degree of Doctor of Philosophy in the University of London. Department of Electrical Engineering, Imperial College of Science and Technology, Exhibition Road, London SW7 2BT. August 1988 1 To my parents and their family. 2 ABSTRACT The mechanism of resonant optical coupling to a surface wave on a thin silver film, with consequent absorption of the optical power, forms the basis of a device for sensing changes in surface characteristics. This "Surface Plasmon Resonance" (SPR) can be detected as a sharp reduction in the reflected intensity over a narrow range of incident angles in an observation of Total Internal Reflection. The advantages of this technique for sensing include : a large fraction of the propagating field resides in the dielectric cladding beyond the silver surface; the con­ finement of the field to a region close to the surface; the coupling resonance occurs over a narrow angular range, typically 0.2 degree with an air cladding and 2.0 degrees with an aqueous cladding. This thesis considers the design and theoretical per­ formance of alternative structures, not involving SPR, that migh,t offer improvements in sensitivity or in material characteristics. The excitation process of SPR relies essentially on a "Prism Coupler" phenomenon, similar to that used routinely for excitation of dielectric waveguides, where a planar waveguide is separated from a high index coupling prism by a low index buffer layer. For such a waveguide excitation there is no inherent loss mechanism, a corollary of which is that the coupling resonance can be made arbitrarily sharp to give improved resolution. Although practical considerations limit the ultimate resolution attainable, 3 angular widths of order 0.1 degree have been obtained with both air and agueous clad polymer guides. In addition to the angular resolution, the fractional power in the sensing cladding medium is a crucial factor in determining the overall device sensitivity. A dielectric waveguiding structure fabricated from Magnesium Fluoride and Zinc Sulphide can have a large fractional power in the sensing cladding (19% with air cladding and 37% with an aqueous cladding). For the polymer structures the power in the sensing cladding is much lower in both cases because the guiding layer is much wider resulting in stronger confinement. The values for the polymer struc­ ture used are 2% with an air cladding and 4% with an aqueous cladding. Thus the ZnS/MgF2 configuration is sub­ stantially more attractive for sensing applications. The overall sensitivity to surface layer changes for ZnS/MgFz dielectric structure with an air cladding is com­ parable to that for SPR, but with an aqueous cladding, it is shown that this dielectric system would be substan­ tially more sensitive. 4 ACKNOWLEDGEMENTS I wish to express sincere thanks to my supervisor Dr John Cozens for his innovation, encouragement and patience. I would also wish to thank Professor Mino Green and Dr Keith Leaver for their helpful suggestions and discussions and to thank my colleagues in the Optics group for their willingness to advise and explain. I am also indebted to the S.E.R.C for financial support throughout my course of study. 5 CONTENTS Page Abstract 3 Acknowledgements 5 Table Of Contents 6 List Of figures 11 Chapter 1 Optical Surface Measurement Techniques 16 1.1 Total Internal Reflection Spectroscopy 16 1.2 Ellipsometry 19 1.3 Thin Film Waveguide Measurement 20 1.4 Surface Plasmon Excitation 23 1.4.1 Applications Of Surface Plasmon Excitation 26 1.5 The Prism Coupler Dielectric Sensor 28 Chapter 2 Optical Excitation Of Thin Film Waveguides 32 2.1 Coupled Mode Theory Of Prism Coupler Operation 32 2.2 Solutions Of Maxwells Equations For The Prism Coupler Geometry 35 2.3 The Theory Of Prism Coupler Operation Using Modified Fresnel Coefficients 40 2.4 Representation Of The Amplitude Field Solutions In The Waveguiding Layer As A Resonance Phenomena 42 2.5 Excitation By A Beam Of Finite Width 44 2.6 Physical Description Of Power Transfer 47 2.7 "Leaky" Mode Field Distribution Matching 50 6 2.8 Prism Coupler Experimental Observations 52 2.9 Device Operation 53 Chapter 3 Silicon Oxide Waveguide Structures 55 3.1 Initial Choice Of Thin Film Structure 55 3.2 Substrate Preparation 56 3.3 Magnesium Fluoride Deposition 59 3.4 Reviewing The Design Of The Structure 62 3.5 Deposition Of Silicon Oxide 64 3.5.1 Summary Of The Reviewed Literature On Silicon Oxide Deposition 65 3.5.2 Experimental Deposition Results 67 3.6 Excitation Of The Guiding Structure 71 3.6.1 Experimental Arrangement 71 3.6.2 Experimental Observation 7 3 3.7 Expected Observations For Ideally Flat Films 79 3.7.1 Focusing The Beam 80 3.7.2 Expanding The Beam 82 3.8 Quantitative Measurement Of Mode Indices 83 3.9 Suitability Of Silicon Oxide Films 83 Chapter 4 Fabrication Of "Lossless" Polymer Structures 85 4.1 Choice Of Deposition Technique 85 4.2 Fabrication Details - The Spin Coating Technique 87 4.2.1 The Proposed Polymer Structure 91 4.2.2 Determination Of Film Refractive Indices 91 7 4.3 Locating The Resonance Experimentally 93 4.3.1 Detection Of Excitation By Phase Shifts 94 4.4 An Alternative Babinet Compensator 95 4.5 Transmission Through The Output Plane Polariser 95 4.6 Experimental Demonstration 96 4.6.1 Experimental Agreement With Theoretical Curves 100 4.7 Predicted Appearance Of The Output Spot 104 4.8 Experimental Observations Of The Output Spot 107 4.9 Explanation Of The Effect Of Non-Uniformity 108 Chapter 5 Optimisation Of The Sensing Structure 112 5.1 Sensitivity Of The Polymer Structure 112 5.2 Choice Of Layer Thickness 112 5.3 Sensitivity To Guide Width Variation 118 5.4 Comparison With A Critical Angle Sensor 120 5.5 Choice Of Spacing Layer Thickness And Index 121 5.6 Proposed Fabricatable Structure 124 5.7 Possible Use Of The TM Fundamental Mode 126 5.8 Theoretical Performance Compared With The Surface Plasmon Resonance 127 5.8.1 Resonance Width 127 5.8.2 Sensing Field Decay lengths 131 5.8.3 Theoretical Mode Index Shifts 132 5.9 Predicted Resonance Shift Comparison 134 5.10 Use Of Bimodal Structures 138 5.11 Use Of Higher Order Modes 143 8 5.12 Practical Fabrication Of Two Mode Structure 143 Chapter 6 Experimental Fabrication Of The Nominally Optimised Structure 145 6.1 Deposition Of ZnS Films By Evaporation 145 6.2 Properties Of ZnS Films 148 6.2.1 Microstructure 148 6.2.2 Stress Stability 149 6.2.3 Stability Of Zinc Sulphide On Magnesium Fluoride 151 6.2.4 Refractive Index Inhomogeneity 152 6.3 Estimation Of The Error Due To Refractive Index Inhomogeneity 153 6.4 Film Evenness 154 6.5 Buffer Thickness Variation 155 6.6 Measured Thickness Variation 155 6.7 The Source Of Scattering 160 6.8 Deposition Of Zinc Sulphide Onto PMMA 160 6.9 The Fabricated Structure 161 Chapter 7 "Prism Coupler Surface Sensing" Applications 167 7.1 Qualitative Illustration Of Surface Sensing 167 7.2 Quantitative Demonstration Of Thin Film Surface Sensing 172 7.3 Demonstration Of The Resolution Of The Device 174 7.4 Comparison Of Dielectric Structure And SPR Sensitivities 179 9 7.5 Suggestion For Future Work 1 8 0 7.6 Conclusions 183 References 186 10 LIST OF FIGURES Page Fig 1.1(a) Resonant field enhancement in TIR spectroscopy using an optical cavity layer N3 and spacing layer N2 ........... 18 Fig 1.1(b) Typical mode of operation of ellipsometric thin film measurement ................... 18 Fig 1.1(c) Thin film measurement by coupling into an optical waveguide layer ......... 22 Fig 1.1(d) Surface plasmon excitation on a silver film with a coating adlayer ............. 22 Fig 1.2 Comparison of field amplitudes for surface plasmon excitation and dielectric waveguide excitation ................................. 29 Fig 1.3 "Prism coupler surface sensor" experimental arrangement ................................ 31 Fig 2.1 Coupled mode explanation of prism coupler excitation .................... 33 Fig 2.2 Four media planar structure representing a prism coupler configuration ................ 36 Fig 2.3(a) Convention used for modified Fresnel coefficients ............................ 36 Fig 2.3(b) Representation of prism coupler excitation using modified Fresnel coefficients ............................ 36 Fig 2.4 Excitation of waveguide mode resonances for increasing gap thickness ................... 43 Fig 2.5 Spatial power redistribution of the reflected spot on waveguide structure excitation ................................. 46 11 F i g 3.1(a) Movement of low intensity band across the output spot .................. 74 Fig 3.1(b) Scattered and reflected output intensity from a structure using evaporated inorganic layers ..................... 74 Fig 3.2 Reflected output intensity for an evaporated structure with theoretical mode index width 10~4 ..... 76 Fig 3.3 Reflected spot profile on excitation of a scattering waveguide structure ............ 78 Fig 3.4 Appearance of the reflected laser spot at resonance for focused and unfocused beams at various azimuths ................. 81 Fig 4.1(a) TE and TM polarisation resonances for an air clad polymer structure ............. 98 Fig 4.1(b) Experimental reflectance for the structure of Fig 4.1(a) ................ 99 Fig 4.2(a) TE and TM polarisation resonances for an aqueous clad polymer structure .......... 101 Fig 4.2(b) Experimental reflectance for the structure of Fig 4.2(a) ................. 102 Fig 4.3 Appearance of the laser spot on reflection from a polymer structure for different polariser arrangements ..................... 106 Fig 4.4 Invariance of the output spot for azimuthal rotation ................................... 106 Fig 4.5 Surface contour of a polystyrene guide ..... 109 Fig 5.1 Schematical power distributions for surface plasmon and dielectric waveguiding structures ....................
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