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Title:Realization of a 3D gradient index medium in porous silicon
Author(s):Ocier, Christian H
Director of Research:Braun, Paul V
Doctoral Committee Chair(s):Braun, Paul V
Doctoral Committee Member(s):Huang, Pinshane Y; Shoemaker, Daniel; Li, Xiuling
Department / Program:Materials Science & Engineerng
Discipline:Materials Science & Engr
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):Optical Materials
Optical Components
Materials Transformation
Diffractive Optical Elements
Abstract:Porous silicon (PSi) and derivative materials like thermally oxidized porous silicon dioxide (PSiO2) are effective medium composites composed of anisotropic columns of solid material (Si or SiO2) and void. During its electrochemical porosification, the material’s refractive index is defined by modulating the current density along the etch direction to change the porosity in either periodic or continuous fashion, making PSi an excellent materials platform for a variety of photonic and gradient refractive index (GRIN) optical elements. After a porosity-dependent index profile is etched into the PSi element, the optical response can be further modified by introducing secondary materials into the void volume of the mesoporous structure, thereby changing the filled region’s effective refractive index. This dissertation presents strategies to fabricate demonstrate novel optical elements by modifying the effective medium properties of PSi-based GRIN composites. PSi has been extensively used in the literature for fabricating optical superlattices and elements like microcavities, optical sensors and Bragg reflectors. The absorptive properties of the silicon host, however, reduce the transmission efficiencies of such optical elements in the visible spectrum. Reducing absorption is accomplished by transforming PSi into PSiO2 via thermal oxidation, which extinguishes the extinction coefficient in the visible spectrum while maintaining the porosity gradient that imparts an index profile onto these elements. The lower index contrast of PSiO2 and air is addressed by introducing TiO2 into the void volume, generating a hybrid, transparent PSiO2/TiO2 composite that is used to form highly transmissive superlattice elements and phase shifting lenses. This study demonstrates the potential of applying such composites for transparent GRIN devices. PSi and PSiO2 are structurally anisotropic materials, and by that extension also manifest birefringence based on how light is polarized relative to the microstructure’s elongated axis. When polarized light interacts with a PSi optical element, the response is affected by both the spatially changing refractive index and birefringence profiles across the element volume. Spectroscopic ellipsometry is used to quantify the birefringent refractive indices of PSi, PSiO2, and PSiO2/TiO2 composite films of varying porosity. These optical constants guided the design of birefringent lenses with unusual beam diverging and bifocusing properties not found in elements made from conventional birefringent materials. The most important work of this dissertation and the capstone of my doctoral work introduces a novel lithographic approach known as SCRIBE (Subsurface Controllable Refractive Index via Beam Exposure), a technique that generates microscale subsurface elements inside PSi and PSiO2. Unlike previous processing techniques, SCRIBE can define microscale 3D geometries and refractive index profiles simultaneously by focusing a femtosecond pulsed laser into PSi filled with photoresist. Voxel-based 3D objects of arbitrary shape form the basis of subsurface optical elements, and the use of laser power to modulate the effective refractive index arbitrarily within PSi enables unprecedented control over the 3D index profile. This enables the fabrication of multicomponent lenses with elements of differing optical dispersions, 3D GRIN lenses including the world’s smallest known Luneburg lens, 3D subsurface optical waveguides, and even 3D, multiplanar nanophotonic elements, and complex diffractive optics. This work paves a new path for designing novel subsurface optical elements with future applications in imaging and point spread function modification.
Issue Date:2021-02-22
Rights Information:Copyright 2021 Christian Ocier
Date Available in IDEALS:2021-09-17
Date Deposited:2021-05

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