单层超薄高效圆极化超表面透镜

针对超表面在透镜方面的应用, 本文设计了一种交叉极化透射聚焦超表面, 实现了将圆极化波转化为交叉极化波的同时聚焦电磁波的功能. 设计了一款旋转型单元, 单元为一层且厚度仅为1.5 mm, 分析了旋转型单元提供不同相移的原理并设计了相邻单元相移差为60°的相位梯度超表面, 在中心频率f=15 GHz附近发生奇异折射, 折射角与理论计算结果一致, 验证了设计单元的有效性, 基于该单元设计了尺寸为90 m mm×90 mm、单元数为15×15 的透射型聚焦超表面, 在中心频率f=15 GHz附近, 左旋圆极化平面波照射时, 透射波聚焦于L=40 mm 的实焦点且透射波为照射波的交叉极化波. 该超表面透镜效率高、厚度薄且为单层, 易于加工, 相对于传统透镜, 优势明显, 在操控电磁波、改善透镜性能方面有潜在应用价值.

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

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 mm×90 mm and 15×15 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.