Transmissive Digital Coding Metasurfaces for Polarization-Dependent Dual-Mode Quad Orbital Angular Momentum Beams.

In wireless communication systems, a multibeam can be used to increase the number of spatial channels by space-division multiplexing. Furthermore, the multimode is used to enhance the channel capacity by mode-division multiplexing. However, few of the previously reported methods cannot achieve independent controls of orbital angular momentum (OAM) states by transmissive metasurfaces in both space-division and mode-division multiplexing simultaneously. To expand the wireless communication channel, a multilayer transmissive digital coding metasurface with a single emitting source is demonstrated for quad-OAM beam generation with a dual mode. By changing the geometry of the cross dipole for a unit cell, the polarization-dependent 3-bit phase responses are obtained to flexibly manipulate the multi-OAM beams with different modes in preset directions simultaneously. Two types of metasurfaces are designed and fabricated to realize four OAM beams with two topological charges in different directions by encoding the phase sequence in - and -directions, which is validated by both theoretical analyses and experimental results. This scheme of transmissive digital coding metasurface provides a simple way to the multiplexing, multichannel, and multiplatform communication and imaging systems.