Topology-optimized terahertz broadband polarization-independent reconfigurable multifunctional Dirac-semimetal-coding metasurface.

In this paper, a broadband reflective terahertz polarization-independent reconfigurable multifunctional coding metasurface based on topological optimization using Dirac semimetals as controllable materials, which can realize flexible control of beam steering and vortex beam generation in the frequency range of 1.54-1.58 THz, is presented. First, the metasurface unit is topologically optimized using the NSGA-II multi-objective optimization algorithm. By applying the bias voltage to dynamically adjust the dielectric constant of the Dirac semimetals, the metasurface unit is capable of polarization-independent 2-bit coding in the frequency range of 1.54-1.58 THz. Then, the array arrangements of the coding metasurface are reverse-designed to achieve beam steering and vortex beam generation. The results show that for beam steering, not only can polarization-independent steering of both single- and multi-beams be realized at continuous arbitrary angles in the range of an elevation angle of 40° and an azimuth angle of 360° but the elevation angle and azimuth angle of each beam in the multi-beam can be controlled independently, which improves the flexibility of terahertz beam steering. For the vortex beam, the single- and multi-vortex beams can be generated in the range of an elevation angle of 40° and an azimuth angle of 360°, with topological charges =±1 and ±2, and the generation angle of each vortex beam in the multi-vortex beam can be controlled independently. Therefore, the proposed terahertz Dirac-semimetal-coding metasurface can realize flexible reconfigurable functions in a certain frequency range and has certain application prospects in the fields of terahertz broadband communication, vortex radar, and phased array radar.