High-efficiency spin-selected multi-foci terahertz metalens

Metasurfaces have demonstrated extraordinary abilities in modulating light waves and, therefore, have been used in many applications. In this paper, we demonstrate multifocal metalens with high focusing efficiency controlled by circular polarization multiplexing. Under normal incidence of linearly polarized light, the metalens can generate transversely distributed four focal points, two of them support left-circularly polarized (LCP) conversion and the rest correspond to right-circularly polarized (RCP) conversion. The total focusing efficiency of the metalens as high as 60.38% is obtained. Excellent extinction is obtained for the focused beam, which achieves a focused polarization component energy of more than 96% in the focused region. In addition, we designed an oblique incidence metalens using the same structure and method. For 30 degrees obliquely incident terahertz waves, high total focusing efficiency of 51.12% in the designed metalens is achieved. Focused polarization component energy exceeds 92% in the focused region. This provides potential new applications for polarization imaging and polarization detection.