Nature engineered metasurfaces: spin-to-orbital angular momentum conversion in diatom frustules

Abstract

Evolution provided, through eras, several animal, vegetal, and protist species with sub-micrometric constituent structures able to manipulate light at the nanoscale in non trivial ways. In particular, diatoms are single-celled microalgae enclosed in a porous silica shell, the frustule, perforated by regular patterns of microand nano-pores and whose functionalities comprise mechanical stability, sorting of nutrients from harmful agents and optimization of sunlight harvesting. Photonic properties of frustules include focusing, photoluminescence, and optical activity, among others. In the present work we show preliminary results concerning the ability of single valves of Arachnoidiscus ehrenbergii diatom frustules to manipulate incoming, circularly polarized radiation in such a way to generate light beams provided with orbital angular momentum (optical vortices). The combination of cross-polarization imaging in different spectral ranges, polarization sensitive digital holographic imaging (PSDHI), and interferometry allowed characterizing the valves and detecting the presence of spin-orbit coupling induced by their ultrastructure.