Optically reconfigurable multi-channel orbital angular momentum generator based on 3-bit coding metasurface

In this paper, an optically reconfigurable coded metasurface based on photosensitive silicon material for generating multi-channel, multi-mode vortex beams in the terahertz band is proposed. The top layer of the proposed metasurface is a skeletonized metal ring with eight photosensitive silicon patches equidistantly filled in the middle; the bottom structure is a metal plate for reflecting electromagnetic waves. The coding process of the eight photosensitive silicon patches and the use of a spatially structured light source for aligned irradiation can realize the reflection phase reconstruction and form a 3-bit reconfigurable metasurface. Based on the principle of coded metasurface convolution and the principle of superposition, the phase distributions of 3-bit metasurface arrays for single-channel and multi-channel vortex beams are calculated. Under the excitation of a right circularly polarized plane wave, a vertically reflected +1 order vortex wave, a +2 order vortex wave reflected in the tilted direction, +2 and +3 order vortex waves in two directions, and vortex beams carrying orbital angular momentum of orders −2, +1, +2, and +3 in four different directions, respectively, are generated. Our work provides a new way to design a programmable and reconfigurable metasurface, and the designed novel optically controlled reconfigurable metasurface is potentially valuable in the fields of dynamic imaging, terahertz communication, and so on.