Design, Fabrication and Analysis of Photonic Device Nanostructures

Abstract

This thesis provides an insight to resonant waveguide gratings (RWGs) and thermo-optic coefficients (TOCs) accompanying various organic and inorganic materials. The RWGs structures were designed by Fourier Modal Method (FMM) based on the rigorous diffraction theory and fabricated by Atomic Layer Deposition (ALD), ElectronBeam Lithography (EBL), Nanoimprinting (NIL), Reactive Ion Etching (RIE), and characterized by Scanning Electron Microscope (SEM), X-Ray Diffraction (XRD), and Ellipsometer. Categorizing the materials used for RWGs by employing different fabrication methods in order to facilitate the simplest, cost-effective approach for largescale production of aforementioned devices is accomplished. The first type of gratings was prepared by a simple replication method i.e., Nanoimprinting, where the master stamp was manufactured by EBL and its subsequent development processes on a silicon wafer without contribution of an etching process. The subwavelength grating structures are directly replicated in polymer materials such as Polycarbonate, Cyclic-olefin-copolymer and UV-curable material Ormocomp® followed by a smooth and conformal cover layer of high refractive index and amorphous optical material TiO2 by ALD method. This type of gratings in polymer materials have been demonstrated to exhibit athermal waveguide operation first time, yielding a net spectral shift of a fraction of a nm over a wide range of temperatures (25–85 °C). In addition, such gratings depict excellent polarizationinsensitive properties that can be used in optical communications, bio-sensors and fluorescence-enhancement applications. These demonstrations showed an excellent agreement between the theoretical and experimental results. The measurement results are the first experimental demonstration so far on the realization of polarization-insensitive 1D GMR gratings under normal incidence. The other type of subwavelength grating structures are explicitly fabricated and demonstrated for polarization-independent properties containing TiO2 pillars as a waveguide layer on the fused silica substrate. The degeneracy of both the TE- and TM-modes for second type of non-polarizing grating was further investigated by studying an over-etch effect into the fused silica substrate. A relatively good agreement between the theoretical and experimental results was found after fabrication through a number of processes. The second research activity contributing this study was to compute TOC of organic and inorganic materials experimentally. The TOC was obtained from the experimental data as an ellipsometric measurement followed by using Lorentz-Lorenz relation and optical Models (Cauchy Model) under a least-square approach. The experimentally calculated TOC was directly applied extensively for the design/simulation of athermal waveguide in various photonic applications. This research work includes experimental investigation of TOCs of Ormocomp®, TiO2 thin films of various thicknesses which are explained on the basis of a porosity model to the near-surface-region. It was investigated that thin TiO2 films possess a larger negative index-gradient while thicker ones showed positive indexgradient. The change of signs of TOCs was described on the basis of a surface porosity model. Furthermore, the proposed porosity model was investigated indirectly by deposition of diffusion barrier layers of ALD-Al2O3 with different thicknesses on the surface of ALDTiO2 films. Interesting results were demonstrated to show a decrease in negative TOCs with the increase in ALD-Al2O3 film thicknesses which are explained based on the impermeable properties of ALD-Al2O3 for water molecules. This thesis also reports on TOCs of ALD-Al2O3 films of different thicknesses for the first time.