Light Manipulation with Photonic Fibers and Optical Light Guides: Dynamic Structural Color and Light Distribution in Microalgae Cultures
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Light Manipulation with Photonic Fibers and Optical Light Guides: Dynamic Structural Color and Light Distribution in Microalgae Cultures by Joseph D. Sandt B.S. Mechanical Engineering University of Kansas, 2013 S.M. Mechanical Engineering Massachusetts Institute of Technology, 2015 Submitted to the Department of Mechanical Engineering in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy at the Massachusetts Institute of Technology February 2020 ©2020 Massachusetts Institute of Technology. All rights reserved. Signature of Author Department of Mechanical Engineering January 15th, 2020 Certified by Mathias Kolle Professor of Mechanical Engineering Thesis Supervisor Accepted by Nicolas Hadjiconstantinou Professor of Mechanical Engineering Chairman, Committee on Graduate Students 2 Light Manipulation with Photonic Fibers and Optical Light Guides: Dynamic Structural Color and Light Distribution in Microalgae Cultures by Joseph D. Sandt Submitted to the Department of Mechanical Engineering on January 15th, 2020 in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy in Mechanical Engineering ABSTRACT Optical and photonic fibers represent versatile systems for light manipulation. They are used to guide, reflect, emit, and absorb light, and can be designed to alter the light’s spectral composition in any of these light-matter interactions. Additional functionality arises from the combination of these effects in single fibers, and the ability to employ fibers as individual strands, or as woven networks. Two distinct light-manipulating-fiber systems are the focus of this thesis: (1) photonic fibers, which have vivid structural color that changes reversibly in response to mechanical or electrical stimuli, and (2) leaky light guides, which emit light along their length when illuminated from one end. Mechanochromic fibers that convert a mechanical perturbation into an optical response can be used, standalone or integrated into textiles, as easy-to-read strain sensors. Such fibers respond to elongation with a gradual shift in their reflected color through the visible range of light. In particular, their use in compressive bandages – discussed in detail in this thesis – could greatly improve the efficiency of compression therapy for chronic venous ulcers and other vascular maladies. Electrochromic fibers exploit the electrochemically-tunable absorption of poly(3,4-ethylenedioxythiphene) polystyrene sulfonate, a common conducting polymer, to design devices that can be flipped between a vivid, structurally colored state, and a dull, absorption-colored state. Custom optical multilayer and lumped parameter models are used to analyze the behavior of these fibers. Leaky light guides, by distributing light throughout volumes of algae culture, could yield greater productivity in microalgae cultivation, while lowering energy requirements. The combination of these factors could enable the economically favorable generation of algal biomass for fuels, feedstock, pharmaceuticals, and many other uses. A passive system for distributing light throughout culture volumes, by selectively scattering light out of light-guiding fibers, is developed and implemented. The process of designing and manufacturing these leaky light guides, and their use in a variety of laboratory- scale bioreactors with live microalgae cultures, are described. Thesis Supervisor: Mathias Kolle Title: Associate Professor of Mechanical Engineering 3 Table of Contents Introduction: Stimuli-Responsive Photonic Fibers and Leaky Light Guides ................................................ 6 Chapter 1: Dynamic Structural Color in Stimuli-Responsive Photonic Fibers ............................................. 9 1.1 Sensing with Structural Color .................................................................................................. 9 1.2 Mechanochromic Photonic Fibers ......................................................................................... 11 1.2.1 Sensor Concept .............................................................................................................. 11 1.2.2 Manufacture and Characterization of Mechanochromic Fibers ....................................... 12 1.2.3 Mechanochromic Fibers as Sub-Bandage Pressure Sensors ............................................. 18 1.2.4 Other Applications of Mechanochromic Fibers ............................................................... 23 1.2.5 Summary and Perspective for Project Continuation ........................................................ 24 1.3 Electrochromic Photonic Fibers ............................................................................................. 25 1.3.1 Sensor Concept .............................................................................................................. 26 1.3.2 Manufacture and Characterization of Electrochromic Fibers ........................................... 27 1.3.3 Circuit Modeling of Electrochromic Fibers ...................................................................... 32 1.3.4 Optical Modeling of Electrochromic Fibers ..................................................................... 37 1.3.5 Continuation .................................................................................................................. 43 1.4 Outlook ................................................................................................................................. 44 Chapter 2: Leaky Light Guides for Microalgae Cultivation ...................................................................... 45 2.1 The Promise of Microalgae .................................................................................................... 45 2.2 Optical Concepts.................................................................................................................... 47 2.2.1 Total Internal Reflection and Light Guiding ..................................................................... 47 2.3 Modeling and Realization of Uniform Light Emission ............................................................ 48 2.3.1 Modeling of Light Scattering from Leaky Light Guides ..................................................... 48 2.3.2 Laser Ablation of Transparent, Cylindrical Light Guides ................................................... 50 2.3.3 Manufacture of Uniformly-Emitting Leaky Light Guides .................................................. 56 2.4 Implementation of Leaky Light Guides in Algae Cultures....................................................... 57 4 2.5 Characterization of the Influence of Leaky Light Guides on Microalgae Growth ................... 58 2.5.1 Bubble Column Bioreactors at Arizona State University .................................................. 58 2.5.2 Photoelectrochemical Cells at Arizona State University................................................... 60 2.5.3 In-lab Bioreactors with Chlamydomonas reinhardtii ....................................................... 61 2.6 Discussion .............................................................................................................................. 62 2.6.1 Manufacturability of Leaky Light Guides ......................................................................... 62 2.6.2 Outlook and Next Steps .................................................................................................. 63 Conclusion: Structural Manipulation of Light in Fiber-Based Technologies ............................................. 64 References ............................................................................................................................................ 66 5 Introduction: Stimuli-Responsive Photonic Fibers and Leaky Light Guides Consider light traveling through some uniform, isotropic optical medium (Figure 1a). This medium has some refractive index n, defined as the ratio of the speed of light in a vacuum to the speed of light in the medium, and it might have some extinction coefficient k, associated with attenuation of electromagnetic radiation. Both of these quantities may vary with wavelength, e.g. red light may travel faster than blue light through one medium, or all visible wavelengths besides green light might be absorbed in another. In this uniform, isotropic medium, light will travel in a straight line. For spectral ranges for which the medium is non-absorbing, light will do so indefinitely. This is immensely important to life on Earth (it's how light and heat from the Sun can travel nearly 100 million miles to Earth), but not particularly interesting for many engineering applications. Figure 1: Light and Interfaces. a) Light traveling, in a straight line forever, through some medium with refractive index n1. b) A single, flat interface between regions of distinct refractive index results in the reflection and refraction of an incident ray of light. c) Multiple interfaces between regions of distinct refractive index result in multiple reflections and refracted rays. The separation between the two interfaces, and the refractive indices, can be tuned to strongly reflect particular ranges of visible light. By introducing structure (i.e. inhomogeneity or anisotropy of optical properties) into the medium light is traveling through, it is possible to manipulate and tune visible light in myriad ways by its interaction at interfaces. In one of the simplest cases, light traveling through a volume with some refractive index could encounter a flat interface with a volume