High focusing efficiency metalens with large numerical aperture at terahertz frequency

Metalenses have been widely investigated for their features of high design freedom. For practical applications, it is necessary to maximize the efficiency of the metalens. However, it is a great challenge to realize both a high numerical aperture (NA) and high-efficiency metalens in the community. Here, we introduce a method to design a hybrid metalens with a large numerical aperture and high focusing efficiency at terahertz frequency. The hybrid metalens consists of gradient metasurfaces in the central area and metagrating in the peripheral area to achieve high-efficiency beam focusing. To verify this concept, a hybrid metalens with a numerical aperture of 0.95 was designed at λ = 118.8 μm. The simulation results demonstrate that the focusing efficiency of the hybrid metalens is 65.8%. The experimental results show that the designed metalens is able to increase the focusing efficiency from 22.8% to 41.7%. The full widths at half maxima (FWHMs) of the focused spots of the hybrid metalens in the x direction and y direction are 0.72λ and 0.45λ, respectively. The proposed high-efficiency hybrid metalens has promising application prospects in various applications of a complex optical system.