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.

Multiple-pathways light modulation in Pleurosigma strigosum bi-raphid diatom

“…radially symmetric) diatom frustules include photonic crystal-like behavior of Coscinodiscus granii girdles 23 , 24 and Melosira variance girdles and valves 25 ; diffraction-induced optical lensing in Coscinodiscus wailesii 26 , Arachnoidiscus sp. 27 , Coscinodiscus centralis 28 , 29 , and C. granii 30 valves; manipulation of the polarization state of the incoming radiation (optical activity) in Aulacodiscus oregonus 31 and Arachnoidiscus ehrenbergii 32 valves; frustule photoluminescence in Thalassiosira rotula 33 , Coscinodiscus concinnus 34 , C. wailesii 35 and several other species 36 , 37 ; ultraviolet radiation (UVR) selective screening by C. wailesii 35 and Coscinodiscus radiatus 38 valves.…”