Two cascaded metasurfaces capable of generating vortex beams with distinct topological charges

In this paper, two simple cascaded metasurfaces capable of generating vortex beams with distinct topological charges (+/- 1, +/- 3) are proposed and designed. The first metasurface is a plasmonic metasurface which is composed of 21x21 elliptic nanoslits arrays perforated in an Au film. These nanoslits act as localized space-variant linear polarizers and introduce Pancharatnam-Berry phase. The second one is an all-dielectric metasurface which consists of 21x21 elliptic silicon nanopillars arrays on SiO2 substrate. These nanopillars work as space-variant half-wave plates and result in a space-variant effective birefringence. The cascaded metasurfaces can operate at 1550 nm and the weight spectra of the desired topological charge can exceed 0.95. The cascaded metasurfaces are designed as transmission-type which makes them applied to optical fiber system. Multiple topological charges correspond to more channel capacity and different propagation directions of vortex beams are more suitable for spatial mode multiplexing. Our scheme may be a potential candidate in integrated optical communication system in future.