Light manipulation with photonic fibers and optical light guides : dynamic structural color and light distribution in microalgae cultures

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.