Design and analysis of terahertz trilayer achromatic grating metasurface wave plate

Grating metasurface is a structure formed by combining a grating with metasurface, and high efficiency polarization control can be achieved by using the grating as a unit structure with different rotation angles. By stacking single-layer wave plates as single-frequency modulators, a broadband modulator with a multilayer structure can be formed, but the optimization of the structural parameters is more complicated. In this paper, the traditional multilayer wave plate optimization process is improved and an accurate and fast nested optimization process is designed. The process combines theoretical analysis and electromagnetic simulation, and this paper uses the process to optimize a novel trilayer silicon grating metasurface wave plate, which is capable of achieving the achromatic conversion from linear polarized wave to left-handed circularly polarized wave within 0.8-1.8 THz. By analyzing the factors affecting the performance and the electromagnetic field, it can be concluded that the polarization conversion capability of the trilayer grating metasurface wave plate originates from the anisotropy of the grating structures, while the enhancement of the broadband achromatic performance originates from the wavevector coupling between the unit structures and the resonance effect between the grating layers. Finally, we also discuss the fabrication of multilayer structured wave plates. The study in this paper provides an innovative exploration of grating metasurface wave plates, which is important for the future design of multilayer achromatic ultrabroadband wave plates.