Dual-Band High Efficiency Terahertz Meta-Devices Based on Reflective Geometric Metasurfaces

The wavelength-dependent behavior of the metasurface is one of the severe disadvantages in the metasurface-based applications, which greatly limits its applications. In this work, a novel reflective building block (unit cell) has been proposed to work at two arbitrary terahertz frequency bands with independent phase control at each band. The geometric phase building block can reflect the incident circularly polarized wave and convert it to the wave with opposite helicity and high conversion efficiencies at both bands. As proof of concept demonstrations, a dual-band cylindrical meta-lens and two vortex beam generators operating at 0.45 and 0.7 THz are investigated to verify the performance of the proposed building block. The simulated results of the metalens agree very well with the theoretical calculations, which are consistent with the design goals. Moreover, the designed vortex metasurfaces reveal a convenient and low-cost way to realize the vortex beam carrying different/same orbital angular momentum modes at two bands.