Utra-thin single-layered high-efficiency focusing metasurface lens

Abstract

For potential applications of metasurfaces in lens technologies, we propose a cross circularly polarized focusing metasurface which is capable of transforming a circularly polarized wave into cross-polarized wave and simultaneously focusing electromagnetic wave. A helicity-dependent phase change is introduced into the transmission metasurface cell, which is a single layer with a thickness of 1.5 mm and can be engineered by assembling along the spatial orientation of each Pancharatnam-Berry phase element. The phase change of the Pancharatnam-Berry phase element is analyzed theoretically, and the efficiency of the designed element is simulated under the irradiation of differently polarized waves. A phase gradient metasurface with a phase difference of 60 between neighbouring cells is designed. When simulated in CST Microwave Studio, the gradient metasurface is observed to have a ability to refract right-hand circularly polarized waves in +x direction and left-hand circularly polarized waves in -x direction but with an identical refraction angle of 33.8, which is in good accordance with the angle calculated from the general refraction law. Then we design a focusing metasurface with a size of 90 mm90 mm and 1515 cells. When the focusing metasurface lens is irradiated by left-hand circularly polarized wave, the refracted right-hand circularly polarized wave is focused at a point 40 mm away from the lens center. However, when the metasurface lens is impinged by the right-hand circularly polarized wave, the refracted left-hand circularly polarized wave is diffracted. This ultimately accords with different phase responses under different polarized waves when the metasurface cell is rotated. Furthermore, the metasurface lens diffracts the incident wave when impinged by right-hand circularly polarized wave, which validates the design principle. The beam-width at the focal spot and the focal depth are also calculated. The simulation results indicate that the beam-width at the focal spot is approximately equal to three quarters of the operating wavelength. Therefore, the circularly polarized wave refraction focusing metasurface has a good performance for focusing the refracted waves. In addition, the proposed focusing metasurface is simulated separately at f=14 GHz and f=16 GHz, and the results show a good focusing effect, which demonstrates the bandwidth characteristic of the focusing metasurface lens. This designed metasurface lens is thin, single-layered, and highly effective, and it is also convenient to fabricate. Moreover, the metasurface lens has an advantage over the conventional lens, which has potential applications in manipulating electromagnetic waves and improves the performance of lens.