基于几何相位超表面的高效独立双频点圆偏振太赫兹波束调控

为了克服超表面普遍具有的波长依赖性，提出了一种基于几何相位的多功能超薄超表面，在双频点处对透射圆偏振太赫兹波实现独立波前调控。该超表面单元由表层金属层和中间介质层组成，其中表层金属图案相同，均是由双C型开口环谐振器、中间金属圆环和长方形金属片谐振器构成。通过分别旋转表层金属谐振器，可以控制交叉偏振透射光具有相同的振幅和不同的相位。将单元结构按照特定的规律排列，可对入射波的波前实现任意调控，例如，在低频f₁=0.701 THz，分别实现了携带拓扑电荷数+1、+2、+3、+4的涡旋波束，其纯度均在60%以上；在高频f₂=1.663 THz，实现了对入射圆偏振波的汇聚，且焦距误差仅为0.04。仿真结果表明，设计的超表面在双频点处对电磁波具有良好的调控。

High-efficiency wavefront manipulation based on geometric phase metasurface for circularly polarized terahertz wave at two frequencies independently

In order to overcome the common frequency-dependent behavior of metasurface (MS), an ultrathin and multifunctional MS that based on geometric phase is proposed to control wavefront independently of transmitted circular polarized (CP) terahertz (THz) waves at two different THz frequencies. The unit-cell of MS consists of two layers metal structure separated with dielectric substrate layer. The metal layer pattern is the same, which is made of double-C-shaped-ring (DCSR) resonator structures and the middle metal ring and rectangular metal patch (RMP) resonator structure. By rotating the metal resonators ring on the top layer separately, the cross-polarized transmitted wave can be controlled to have the same amplitude and correspondingly different phases. By appropriately arranging the array of unit-cell, the designed MS has ability to arbitrarily manipulate the wavefront. For example, the vortex beam, which carry orbital angular momentum (OAM) with topological charge of l = +1, +2, +3 and +4, can be generated and the corresponding all mode purities are greater than 60% at the lower frequency of f₁ = 0.701 THz; the CP wave focusing effect can be achieved and its error is only 0.04 at the higher frequency of f₂ = 1.663 THz. The simulation results show that the designed MS is good at manipulating electromagnetic waves at two different THz frequencies.