The physical origin of the reflectance features of structurally-colored colloidal glasses

We use colloidal glasses as a model system to study the physics in structurally-colored systems in nature, such as the blue feather barbs of the plum-throated Cotinga. Both are composed of spheres in an arrangement with short-range order and long-range disorder, and in both cases the color results from interference rather than absorption. Unlike biological systems, however, colloidal glasses can be fabricated in the lab, and we can tune their reflectance spectra. While the location of the primary structural resonance in their spectra can be predicted using a single-scattering theory, other features of the spectra, such as the increase in reflection at short wavelengths, cannot be explained by single scattering alone. To understand the effect of multiple scattering on the color, we do co- and cross-polarized measurements of the reflection spectra, and we interpret these measurements using Monte Carlo simulations of photons scattering inside the disordered structure.