Light Transport through Disordered Materials: Experimental and Computational Approaches

Abstract

Disordered materials are abundant in our world, and they can have complex interactions with light. Understanding these interactions is important for both engineering new optical materials and for developing new methods of imaging materials. To engineer new optical materials, I use a combination of experiments and computational modeling to investigate how light moves through disordered materials that both scatter and absorb. For imaging disordered materials, I demonstrate that a fundamental understanding of how light scatters from the interface allows us to extract information from interferometric scattering imaging experiments and use that information to understand the dynamics of the system. In the first half of this thesis, I examine the interaction between absorption and scattering in disordered systems and explore how understanding this interaction can be used to tailor their optical properties. I first examine inverse photonic glasses composed of a porous polymer film with varying amounts of embedded absorber. In these systems, scattering can increase the optical path length and potentially increase absorption. I quantify absorption enhancement by comparing simulated optical properties of this system to those of a non-scattering absorbing polymer film and a layered system with spatially separated absorption and scattering. I compare the calculated absorption enhancement to the theoretical limits and to measurements of experimental systems. These comparisons allow us to propose design rules for optimizing absorption enhancement within photonic glasses. Next, I explore the optical properties of microgeode systems. Microgeodes are hollow silica shells encapsulating silicon nanowires. The multiple refractive indices and length scales within these systems in principle allow for the tailoring of optical properties across a wide spectral range. To explore how the response can be tailored, I characterize the optical properties of individual microgeodes and bulk collections of microgeodes, and I describe how the interaction of absorption and scattering of the different components affects the optical properties. In the second half of the thesis, I use interferometric scattering microscopy (iSCAT) to measure the structure and dynamics of disordered films. iSCAT is a noninvasive, label-free imaging technique that records the interference of scattered light with a reference plane wave, similar to interferometry but without the need for extensive alignment of the instrument. I show how iSCAT reveals heterogeneous dynamics of surfactants breaking down phase-separated grease films and how it can provide insights into these dynamics. Lastly, I develop an analysis technique to extract quantitative film topography from recorded iSCAT interference data, adapting both holographic reconstruction and interferometry techniques to do so.