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Hollow Core Bragg Waveguide Design and Fabrication for Enhanced Raman Spectroscopy

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Hollow Core Bragg Waveguide Design and Fabrication for Enhanced Raman Spectroscopy Hollow Core Bragg Waveguide Design and Fabrication for Enhanced Raman Spectroscopy by Janahan Ramanan A thesis submitted in conformity with the requirements for the degree of Master of Applied Science Graduate Department of Electrical and Computer Engineering University of Toronto © Copyright by Janahan Ramanan 2011 i Hollow Core Bragg Waveguide Design and Fabrication for Enhanced Raman Spectroscopy Janahan Ramanan Master of Applied Science Graduate Department of Electrical and Computer Engineering University of Toronto 2014 Abstract Raman spectroscopy is a widely used technique to unambiguously ascertain the chemical composition of a sample. The caveat with this technique is its extremely weak optical cross- section, making it difficult to measure Raman signal with standard optical setups. In this thesis, a novel hollow core Bragg Reflection Waveguide was designed to simultaneously increase the generation and collection of Raman scattered photons. A robust fabrication process of this waveguide was developed employing flip-chip bonding methods to securely seal the hollow core channel. The waveguide air-core propagation loss was experimentally measured to be 0.17 dB/cm, and the Raman sensitivity limit was measured to be 3 mmol/L for glycerol solution. The waveguide was also shown to enhance Raman modes of standard household aerosols that could not be seen with other devices. ii Acknowledgements I would first like to acknowledge my supervisor, Professor Amr S. Helmy for giving me the opportunity to take up a project focused on Raman detection and sensing. Throughout the two years he has been a constant source of motivation, and for those days where nothing seemed to be working in the clean room, it was exactly what I needed. I would like to thank Rajiv, Steve, Kaustav, and Nima for providing their expertise along the way. It starts off with a goal, and a bunch of unknowns that need to be overcome. Knowing that there are people, with which you can share and bounce ideas off, is an invaluable asset, and one I cherish most in the Helmy group. Some things go without saying, but I feel the need to mention it here: I am truly grateful for the support I have received from my parents, Shan and Malar, who have always put my needs over their own. My sisters, Thammi and Bhamini, for their unwavering encouragement and comic relief. My brother-in-law, Edmund who continuously demands, and inspires me to question what the world should be like. Finally, my darling nephew and niece, Arvind and Divya, who are already beginning to develop an insatiable curiosity for the world. To my friends-in-arms, Anith, Rosanah, Kaustav, and Rajiv, who survived the two years with me, the experiences we have shared, will be those I remember fondly. iii Table of Contents Table of Contents iv List of Tables ix List of Figures xi List of Appendices xvii List of Acronyms xviii 1 Introduction 1 1.1 Motivation ............................................................................................................................ 1 1.2 Sensing Applications ............................................................................................................ 2 1.3 Current State-of-the-Art Optical Techniques ....................................................................... 2 1.3.1 Refractive Index Sensing ......................................................................................... 3 1.3.2 Fluorescence Detection ............................................................................................ 3 iv 1.3.3 Vibrational Spectroscopy ........................................................................................ 4 1.4 Challenges with Raman Sensing .......................................................................................... 5 1.5 Surface Enhanced Raman Spectroscopy .............................................................................. 5 1.6 Total Internal Reflection Based Raman Enhancement ........................................................ 6 1.7 Thesis Objective ................................................................................................................... 7 1.8 Thesis Organization .............................................................................................................. 9 2 Raman Spectroscopy 11 2.1 Fundamentals of Raman Spectroscopy .............................................................................. 11 2.2 Directionality of Scattered Photons .................................................................................... 13 2.3 Raman Cross Section ......................................................................................................... 15 2.4 Measured Raman Signal ..................................................................................................... 16 2.5 Raman Collection in a Waveguide ..................................................................................... 17 2.6 Raman System Overview ................................................................................................... 20 3 Optical Waveguiding in Low Index Materials 23 3.1 Introduction ........................................................................................................................ 23 3.2 Index Guiding ..................................................................................................................... 23 3.3 Photonic crystals ................................................................................................................. 26 3.3.1 1-D Photonic crystals........................................................................................... 26 3.4 Bragg Reflection Waveguides ............................................................................................. 31 3.4.1 TE Mode Analysis .................................................................................................. 32 v 3.4.2 Raman Loss Modeling ......................................................................................... 38 3.5 Hollow Core Bragg Groove Reflection Waveguide Structure ............................................ 40 4 Photonic Crystal Fiber Aerosol Sensing 44 4.1 Introduction ........................................................................................................................ 44 4.2 Aerosol coupling into Hollow Core-PCF ........................................................................... 45 4.2.1 Coupling methods .................................................................................................. 45 4.2.2 Aerosol size distribution ........................................................................................ 46 4.3 Aerosol Raman Measurement ............................................................................................ 47 5 Fabrication of Hollow Core Waveguides 55 5.1 Introduction ........................................................................................................................ 55 5.2 Process Overview ............................................................................................................... 57 5.3 KOH etching ....................................................................................................................... 58 5.4 Plasma Enhanced Chemical Vapor Deposition ................................................................... 63 5.4.1 Chamber properties................................................................................................. 63 5.4.2 Film Conformality .................................................................................................. 65 5.4.3 Film Characterization ............................................................................................. 66 5.4.4 Film Uniformity ...................................................................................................... 68 5.5 SU-8 adhesive bonding ..................................................................................................... 71 5.5.1 Overview ................................................................................................................ 71 5.5.2 SU-8 spin deposition .............................................................................................. 72 vi 5.5.3 Bonding .................................................................................................................. 72 5.6 Sample Dicing ..................................................................................................................... 74 6 Bragg Reflection Waveguide Characterization 76 6.1 Overview ............................................................................................................................ 76 6.2 Waveguide Loss Characterization ...................................................................................... 76 6.3 Raman Spectroscopy Characterization ............................................................................... 80 6.3.1 Measuring Raman Enhancement ........................................................................... 80 6.3.2 Silica Groove Waveguide Characterization .......................................................... 81 6.3.3 Bragg Groove Reflection Waveguide Enhancement ............................................. 82 6.3.4 Aerosol enhancement ............................................................................................ 84 6.3.5 Fabrication Variability ..........................................................................................
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