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Broadband and high efficiency terahertz metasurfaces for anomalous refraction and vortex beam generation


 The applications of metasurfaces are currently a highly active field of research due to their extraordinary capacity to manipulate electromagnetic waves. The ultra-thin characteristics of metasurfaces allow the miniaturization and integration of metasurface devices. However, these devices typically suffer from a low efficiency and narrow bandwidth conditions. In this paper, we design eight multilayered unit cells with similar amplitudes and a phase with an interval of π/4, which convert the polarization states of the terahertz (THz) waves between two orthogonal directions. The average cross-polarized transmission amplitude of these cells is around 0.9 within the ultra-broad frequency regions ranging from 0.5 to 1.4 THz. Furthermore, unit cells are used to construct both an ultra-thin anomalous refraction metalens and a vortex phase plate. Our simulation results show that the anomalous refraction for the transmitted linear polarization component is comparable to the theoretical predictions, and the maximum error is determined to be below 4.8%. The vortex phase plate can also generate an ideal terahertz vortex beam with a mode purity of 90% and above. The distributions of longitudinal electric field, intensity and phase illustrate that the generated vortex beam has excellent propagation characteristics and a weak divergence. Simulations of the two types of metasurface devices, based on the eight unit cells, exhibit very high efficiencies in a wide bandwidth. Our research will assist in the improvement for the practical applications of metasurfaces. It also provides a reference for the design of high efficiency and broadband devices that apply to other frequency ranges.