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.