(458f) Extending the Diatom's Color Palette: Bio-Inspired, Colloid-Templated Structures for Vivid Coloration and Optical Sensing

Dielectric micro- and nanostructures have shown promise as exquisitely tunable and nearly lossless media for optical functions in both nature and technology. Herein, the biosilica valve structures of the centric marine diatom family, naturally used for cellular transport and UV-shielding, were computationally investigated with finite-difference time-domain (FDTD) methods as viable photonic elements and their optical behavior explored when comprised of a higher refractive index material (TiO₂). While sharp optical resonances exist within the native silica structure, they broaden and intensify within analogous TiO₂ nanostructures, leading to the possibility of filtering, high reflection, and wavelength-tunable design through proper selection of geometry. Non-dimensionalization of key structural parameters such as pore spacing and frustule thickness yielded a wavelength-invariant design space for intense (>95%) normal-incident reflection and transmission owing to the Fano resonances supported by the perforated, high-index thin film geometry. Furthermore, introducing disorder such as pore size polydispersity yielded varying degrees of modally specific, angle-independent scattering. To verify the computed results, a fabrication technique was developed involving area-scalable application of a colloidal patterning mask and standard nanofabrication. This process yielded non-iridescently colored surfaces across the entire visible wavelength range. This resonant filtering property was further implemented in a thin-film, porous Fabry-Perot refractive index sensor for liquids, which exhibited competitive sensitivity (~650 nm/RIU) and a striking color transition upon analyte infiltration. Such optical surfaces are area-scalable, substrate-agnostic, and may find further use as filters and waveguides in photovoltaics, passive displays, and optoelectronic devices.