Functional and responsive photonic crystals

Organisms can display color either through pigments, periodic nanostructures or a combination of both. The colors of pigments can be attributed to the selective absorption of portions of the visible light spectrum, while structural color originates from the interaction of periodical nanostructures with visible light. Photonic crystals, which are a class of structures consisting of periodically ordered materials with different refractive indices, possess structural color due to their photonic band gap, a region in which certain optical frequencies are not allowed to propagate inside the crystal due to destructive interference. Their structural color will thus be determined by the wavelengths for which constructive interference occurs upon reflection of the light from the crystal.

Throughout this thesis, both biological (periodic nanostructure of the red Torynorrhina flammea beetle) and synthetic (colloidal photonic crystals) origins of structural color were examined.

The elytra of the red Torynorrhina flammea beetle were examined before and after chemical treatment through linear and nonlinear optical techniques. Previous entomology reports almost exclusively studied insects with linear optical techniques but the additional examination through nonlinear optical techniques has several advantages. These include label-free observation (direct sample information), a large reduction in photodamage and photobleaching outside the focal volume (non-destructive analysis), formation of 3D images without background noise (small focal volume) and a greater penetration depth of the light into the sample due to the use of near-infrared excitation wavelengths. Through these measurements, it became clear that the elytra of both the treated and untreated samples do not display one-photon fluorescence or second harmonic generation but do display multiphoton fluorescence. It was also shown that the chemical treatment affects the color of the elytra, their reflectance spectra, and the multiphoton fluorescence decay times.

Colloidal photonic crystals were explored as a platform for two applications, depending on the material infiltrated inside their interstitial voids. These two applications were a rewritable colloidal photonic crystal paper, created through the infiltration of a PEGDA hydrogel, and the magnification of the Faraday rotation effect depending on the PBG wavelength position through the infiltration of a Faraday active discotic liquid crystal.