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.

Metasurfaces have been attracted extensive attention due to their powerful function, especially the manipulation of the polarization state of electromagnetic wave in many different areas has caused a lot of research interest. Broadband transmission polarization converter based on V-shaped element array in terahertz band was designed and analyzed, which consists of grating-V-shaped metasurface-grating. The top and bottom layers are a pair of crossed grating, and the middle layer is V-shaped metasurface, and the layers are separated by polyimide. The structure parameters of the polarization converter were optimized by CST microwave studio, changes of which can result in narrow band or low transmission. Cross-polarization transmission and polarization conversion rate can reach more than 80% and 99% in the frequency range from 0.35 THz to 1.11 THz. By studying the electric field distribution of the substrate under the metasurface of the V-shaped, found the real part of the cross-polarization electric field between adjacent V-shaped presents similar values in the frequency range from 0.35 THz to 1.11 THz, resulting in high cross-polarization transmission. However, the real part of the cross-polarization electric field between adjacent V-shaped presents opposite values, resulting in low cross-polarization transmission at 1.40 THz. At the same time, the responses of the single layer structure of the V-shaped array and the bi-layer structure of the grating placed behind the V-shaped array to vertically incident x-polarized terahertz waves are investigated respectively, the results show that the single-layer V-shaped array can convert part of linearly polarized incident light into cross-polarization light, however, in the bi-layer structure, Fabry-Perot cavity is formed between the V-shaped array and the grating, and the cross polarization transmission increases. This indicates that the V-shaped array provides the ability of polarization conversion, and the existence of the grating makes the F-P cavity inside the structure create conditions for the back and forth reflection of terahertz waves. The combined action of the V-shaped metasurface and orthogonal grating, resulting in a high polarization conversion rate.