基于双开口谐振环超表面的宽带太赫兹涡旋光束产生

提出了一种基于双开口谐振环单元结构超表面的太赫兹宽带涡旋光束产生器.该结构由金属-电介质两层构成,位于顶层的是基于双开口谐振环单元结构的超表面,底层为介质层.对单元结构阵列进行数值仿真,圆偏振的入射光可以被转换成相应的交叉偏振透射光,通过旋转表层金属谐振环,可以控制交叉偏振透射光具有相同的振幅和不同的相位.这些单元结构按照特定的规律排列,可以形成用以产生不同拓扑荷数的涡旋光束的涡旋相位板.以拓扑荷数1和2为例,设计了两种涡旋相位板,数值分析了圆偏振波垂直入射到该涡旋相位板生成交叉圆偏振涡旋光束的特性.结果表明,在1.39—1.91 THz的频率范围内产生了比较理想的不同拓扑荷数的涡旋光束,且透过率高于20%,最高可达到24%,接近单层透射式超表面的理论极限值.

Broadband terahertz vortex beam generation based on metasurface of double-split resonant rings

Terahertz vortex beam generators have potential applications in optical micro-manipulation, terahertz communications and many other fields. A broadband vortex beam generator in a terahertz frequency range is proposed based on the metasurface of double-split resonant rings' array. The designed structure consists of two layers, i.e., the top layer, which is a metasurface of double-split resonant rings, and the bottom layer, which is the dielectric layer of polymide. The numerical simulation of the cell structure array is performed by using the CST microwave studio. In order to obtain the best performance, the structure parameters of metasurface are continuously optimized and a set of optimal geometric parameters is finally determined. The simulation results show that the circularly polarized incident light can be converted into corresponding cross-polarized transmitted light. By rotating the metal resonant ring on the top layer, the cross-polarized transmitted light can be controlled to have the same amplitude and correspondingly different phases. The relationship between the phase change and the angle of rotation conforms to the P-B phase principle. These cell structures are arranged according to a specific order and can form the vortex phase plates for generating the vortex beams with different topological charges. Taking the topological charge numbers 1 and 2 for example, two kinds of vortex phase plates are designed. The characteristics of the circularly cross-polarized vortex beams generated by a circularly polarized wave perpendicularly incident on the vortex phase plates are numerically analyzed. The results show that the ideal vortex beams with different topological charge numbers are generated. The characteristics of vortex beams appear to be consistent with those theoretical results. Moreover, the vortex beams can be generated in a frequency range from 1.39 THz to 1.91 THz. The operating bandwidth is much wider than the previously obtained result of the transmission terahertz vortex phase plates. The transmission is higher than 20%, and the maximum value of transmission can reach 24%, which is close to the theoretical limit value of the single-layered transmission-type metasurface. This work provides a reference for generating the terahertz vortex beams based on metasurface. It is expected to possess a practical application in generating the device of terahertz vortex beam.