Measuring and Modeling Light Scattering in Disordered Systems for Applications in Structural Color

Abstract

Structural color comes from constructive interference between waves scattered from a material with refractive-index variations at the scale of visible light. When the index variation is periodic, as in photonic crystals, the structural color is angle-dependent or iridescent. But when the index variation has only short-range order, the structural color is independent of angle. Angle-independent structural colors appear matte and homogeneous, often indistinguishable from colors that come from absorbing pigments. This type of coloration is found in many species of birds and has been mimicked in disordered assemblies of colloidal particles.

The light scattering in these structures is complex. Light can scatter many times before exiting the material, a phenomenon known as multiple scattering. Analytical calculations for multiple scattering can be time-consuming and computationally intensive, but accounting for multiple scattering is necessary to quantitatively predict a color for a given structure.

In this thesis, I explore the effects of multiple scattering in disordered structurally colored materials. Using a combination of theory and experiment, I explain the physical origin of the spectral features in disordered colloidal assemblies. I then develop a Monte Carlo model that simulates multiple scattering in films of colloidal particles and calculates the reflectance spectra of the films. The simulation is fast, and it gives not only the predicted color of our systems but also information about the trajectories of multiply scattered light within the sample. The reflectance spectra predicted by our model agree well with experiments.

Because many applications require more versatile formulations than films, I also explore the scattering in photonic balls---nanostructured spheres on the order of tens of micrometers that show color. These balls can be packed into films or dispersed in solution. I show that a multiscale model that captures the light transport in packings of photonic balls accurately predicts their reflectance spectra. Then, by tracking individual trajectories as they move through the structures, I investigate the physics of light scattering responsible for different reflectance features. I extend the model to include simulations of individual fields, allowing polarization spectra to be simulated. These spectra can be compared to experiment, yielding insights into multiple scattering in both homogeneous and photonic-ball films.