基于反射型超表面的太赫兹偏折涡旋波束生成

太赫兹涡旋波束可以提高雷达通信系统通信容量及成像系统的分辨率，如何有效地产生这种波束成为近期研究热点之一。为了克服传统方式的缺点，该文设计加工了5个工作在太赫兹频段的反射型超表面，它们可以产生±1、±2和3共5个不同模态的涡旋波束。为了避免馈源对涡旋波束的遮挡，通过平面反射阵原理控制了波束的偏转方向。超表面单元为3层结构，其中，上层为金属结构，控制上层结构中8个枝节的长度，可以在基本不改变超表面单元反射系数的情况下，调整它的反射相位。中间层为介质层，为了使超表面单元有较高的反射系数，介质层下方为一金属地。超表面单元仿真显示，其同极化反射率在90%以上，相位分布也满足超表面设计需求。超表面的仿真及测试结果表明，在340 GHz附近，不同超表面在设计的方向上产生了对应模态的涡旋波束，并且涡旋波束中的主模态能量占比最高。 

Deflective Vortex Beam Generation Based on Metasurfaces in the Terahertz Band

Terahertz vortex beams can be used to improve the communication capacity of radar communication systems and the resolution of imaging systems. This paper presents a deflective vortex beam generation method based on a reflective metasurface working in the terahertz band. Without the limitations of traditional methods, metasurfaces are a good candidate to generate beams carrying an orbital angular momentum in the terahertz band. First, we designed and simulated a unit cell of the metasurface. The unit cell of our design consists of two metallic (gold) layers and one dielectric layer. An almost 360° phase shift was acquired by adjusting the length of the eight stubs of the top layer. The unit cell of the metasurface was simulated by CST Microwave Studio, and the simulation results showed that the co-polarization reflection efficiencies of the unit cells were more than 90%. To avoid performance degradation due to blockage of the feed horn, we controlled accurately the directions of vortex beams based on the concept of reflectarray. To verify the performance of our design, we simulated and measured five reflective metasurfaces. The results of simulation and measurement showed that these metasurfaces could generate five deflective vortex beams in the terahertz band. The topological charges of these beams are ±1, ±2, and 3, which account for the highest energy proportion in different vortex beams.