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Title:Creation of 3-dimensional micro-optical materials
Author(s):Bacon-Brown, Daniel
Director of Research:Braun, Paul V
Doctoral Committee Chair(s):Braun, Paul V
Doctoral Committee Member(s):Eden, Gary; Schleife, Andre; Shim, Moonsub
Department / Program:Materials Science & Engineerng
Discipline:Materials Science & Engr
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):gradient refractive index
tunable refractive index
interference lithography
porous silicon
Abstract:Structure at or below the wavelength of light can greatly modify the optical properties of materials. Control over these aspects can introduce a variety of optical phenomena, such as photonic band gaps, negative refractive index, transformation optics, and sub-diffraction limit focusing, to name a few. These phenomena have great implications for our ability to control light, whether it is in its spectral emission and absorption, in directional control through waveguides, or in its ability to propagate through a material at all. This dissertation will cover a variety of topics in this domain. Chapter 1 will be a brief overview of how light behaves in response to structures at different scales: features much larger than, comparable to, or smaller than the wavelength of the light. This will lay groundwork for more advanced concepts to be introduced later on, but if you are already familiar with the field of modern optics, you may wish to skip the basics introduced here. Chapter 2 will be focused on multi-beam interference lithography, a technique whereby the interference pattern of beams of light is converted into a pattern of solid photoresist, often used to create photonic crystals. The mathematical relations between the interfering beams and the fabricated pattern are discussed in detail. The problem of reflectance at the interface between photoresist and substrate is shown to cause problems for interference lithography glancing angles of incidence, and a tunable polymer blend antireflection coating is proposed and demonstrated to eliminate this issue. Chapter 3 discusses proximity-field nanopatterning, a variation of interference lithography where a diffractive mask generates the beams of light that create the interference pattern. The design space of this technique is examined for a cubic lattice and multilevel phase masks are shown to be greatly expand the possible design space compared to the more conventional binary phase mask. Chapter 4 presents work on using proximity-field nanopatterning to create arrays of metallic helices to function as broadband circular polarizers. While this project was ultimately unsuccessful, a better understanding of the fabricability limits was developed. Concepts of concentration fluctuations and shot noise were borrowed from EUV lithography and used to create more rigorous models of the interference lithography process. Chapter 5 discusses our recent discovery that direct laser writing can be used inside the pores of porous silicon and porous silicon oxide, and that the local fill fraction of resist can be controlled. This method opens up great possibilities for a wide variety of micro-optics. Nearly arbitrary 3D structures can be defined with a refractive index over a wide range and with no direct connection to the substrate. A variety of focusing optics are created, including with a gradient refractive index. This method is used to shift the bandgap position of PSi DBR’s across the visible spectrum, and is used to embed true-color images into PSi with pixel sizes on approximately a micron in width. The ability to combine 3D spatial control with gradient refractive index control on such fine scales shows great potential for new micro-optics.
Issue Date:2019-05-21
Rights Information:Copyright 2019 Daniel Bacon-Brown
Date Available in IDEALS:2019-11-26
Date Deposited:2019-08

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