Near-Perfect Linear Terahertz Polarization Converter Design Based on Multi-Objective Optimization Algorithm

Polarization conversion is an important technique for converting the polarization state of electromagnetic waves. Based on the Fabry-Pérot (F-P) resonance enhancement in a multilayer metasurface structure, the reverse design of a broadband, highly-efficient and adjustable terahertz polarization converter using a multi-objective particle swarm optimization algorithm is demonstrated. The device is composed of three layers of metasurfaces cascaded together. The outer two layers are a pair of orthogonal grating structures, the middle layer is a sawtooth grating structure with a 45° angle to both sides of the grating, and the medium between the layers is air. Single-layer metasurfaces were prepared using femtosecond laser micro-machining, assembled through a cage structure, and the interlayer spacing was adjusted using two precision translation stages. The device was characterized using time-domain terahertz spectroscopy, and the experimental results were highly consistent with simulations, with a conversion efficiency of over 90%. In addition, the operating frequency of the device can be changed by adjusting the interlayer spacing. The F-P interference effect in the triple-layer structure plays a major role in the excellent characteristics of this device. The device structure can be used in the design of other adjustable/reconfigurable optical devices.