基于可调石墨烯超表面的宽角度动态波束控制

Dynamic beam-steering in wide angle range based on tunable graphene metasurface

可调控超表面可用于动态控制空间波束的方向,具有很高的应用价值.石墨烯是一种可调的二维材料,它的电导率可以通过外加电压控制,利用这一特性可设计基于石墨烯的可调控超表面.超表面控制反射波束时的理论依据是广义的斯涅耳反射定律.反射角度可通过沿超表面的相位梯度进行控制.但是这种方法有局限性,当超表面单元固定时,反射角度只能取有限个离散的值.本文设计了基于石墨烯的可调超表面,并采用一种基于卷积运算定理的波束控制方法,实现了反射波角度的大范围动态控制.在1.75 THz垂直入射平面波激励下,反射角度可以从5?变化到70?,间隔小于10?.数值模拟结果与理论计算结果一致.

Metasurfaces, the two-dimensional counterparts of metamaterials composed of subwavelength building blocks, can be used to control the amplitude, phase, and polarization of the scattered wave in a simple but effective way and thus have a wide range of applications such as lenses, holograms, and beam steering. Among these applications, metasurfacebased beam steering is of great importance for antenna engineering in communication systems, because of its low loss and easy manufacture.The capability of beam steering is mainly controlled by the phase profile which is determined by the phase shift applied to the wave scattered by each of unit cells that constitute the metasurface. It should be noted that the required phase profile achieved by distributing the unit cells with different phase responses can operate well only at a certain frequency. The guidance in determining the required phase profile to steer the beam into a certain direction is the generalized Snell＇s law. According to this law, the reflection angle of the wave reflected by the metasurface interface depends on the linear phase gradient along the metasurface. Therefore, by forming different linear phase gradients covering the whole phase shift 2π periodically, one can steer the reflected waves to different angles. However, the obtained reflection angles are limited because the phase gradient of a metasurface is limited by the unit cell size, which cannot be infinitely small. Recently, a new pattern shift theory based on the convolution theorem has been proposed to realize wide angle range steering, enabling flexible and continuous manipulation of reflection angle. Because the electric field distribution and the scattering pattern in the far-field region are a Fourier transform pair, we can pattern the electric field of the metasurface to control the scattered waves of far field. Specifically, the multiplication of an electric distribution by a gradient phase sequence leads to a deviation of the scattering pattern from its original direction to a certain extent in the angular coordinate. However, we have not considered the tunability of metasurfaces so far, which is required in applications. The ways to reach tunability in metasurface include diode switches, micro-electro-mechanical system, and the use of tunable materials such as graphene. Graphene, an atomically thin layer of carbon atoms arranged in a honeycomb lattice, has aroused the enormous interest due to its outstanding mechanical, thermal, and electrical properties. With the capability of being electrically tunable, graphene has manifested itself as a promising candidate for designing the tunable metasurfaces. Although the reflection angle can be changed by electrically reconfiguring the graphene Fermi level distribution of the metasurface, the steering angle is still limited. In this paper, we propose and design a tunable graphene metasurface with the capability of dynamically steering the reflection angle in a wide range,which is achieved based on the new pattern shift theory. The theoretical results and the numerically simulated results both show that the reflection angle can be steered from 5° to 70° with an interval less than 10°, implying the promising potential in the design of tunable antenna.