Terahertz vortex beam generation based on reflective metasurface

Abstract

The electromagnetic wave with spiral wavefront is a vortex beam carrying orbital angular momentum. The phase singularity of the vortex beam has special applications in the fields of particle manipulation and communication. In this paper, a terahertz (THz) wide-band vortex beam generator based on reflective metasurface is proposed and simulated. The device consists of a metasurface-dielectric-metal three-layer structure, and the top layer is a metasurface composed of two orthogonal I-shaped metal structural units. The intermediate layer of polyimide medium, and the bottom layer is of metal as a reflecting plate. The CST microwave studio is used to simulate the reflection performance of unit cell. The structure parameters are optimized to obtain the better performance. A set of optimed structure parameters is determined. According to the phase principle of Pancharatnam-Berry (P-B), by rotating the angle of the top-layer I-type metal structure, the reflection amplitudes of the unit cell structure at different rotation angles are required to approximately equal while the phase changes linearly with rotation angle and reaches a range of 2lπ for the topological charge number l. These cell structures are arranged according to the phase principle mentioned above. The metasurfaces of different topological charge numbers are designed to generate the corresponding vortex beams. In this paper, the metasurfaces with topological charge numbers 1 and 2 are designed. The reflection amplitude and phase of the circularly polarized THz beam incident vertically on the metasurface are simulated by using CST microwave studio. The simulation results show that in a frequency range of 0.8−1.4 THz, the metasurface can convert the circularly polarized terahertz beam into a vortex beam with a different topological charge number. In addition, in order to illustrate that the metasurface designed can produce a higher topological charge number of vortex beam, a metasurface with a topological charge number of 3 is designed as an example. The reflection amplitude and phase of the circularly polarized THz beam at a frequency of 1.1 THz is simulated. The results show that the designed metasurface can produce a vortex beam with a topological charge number of 3. The higher topological charges of vortex beam can also be generated according to the corresponding phase arrangement. The device has a relatively wide operating bandwidth, simple structure, high conversion efficiency, and has the potential application in terahertz vortex beam generation.