Geometric metasurface for multiplexing terahertz plasmonic vortices

Surface plasmon polaritons carrying orbital angular momentum (OAM), namely, as plasmonic vortices, have attracted considerable attention in optical trapping, quantum information processing, and communications. The previous studies of near-field OAM are limited to generate only one single plasmonic vortex, which inevitably degrades further on-chip applications. Geometric metasurfaces, two-dimensional counterpart of metamaterials, enable the unprecedented capability in manipulating the phase, polarization, and amplitude of electromagnetic waves, providing a flexible platform in controlling plasmonic vortices. Here, we propose and experimentally demonstrate an approach to realize the multiplexing of terahertz (THz) plasmonic vortices based on geometric metasurfaces. Under the illumination of circularly polarized THz waves, multiple plasmonic vortices with identical topological charges are generated at the metal/air interface. Furthermore, the conversion from spin angular momentum to multiple plasmonic OAM, i.e., multiple plasmonic vortices with different topological charges, is also demonstrated. Geometric metasurfaces consisting of paired air-slits with different in-plane orientations are designed to demonstrate these characteristics. Our proposed approach may open an avenue for on-chip applications with increasing information capacity.