太赫兹波前调制超表面器件研究进展

超表面是一种由人工微结构组成的超薄平面器件，能够实现对电磁波振幅、相位以及偏振态的调控，具有体积小、重量轻、集成度高、可灵活操控电磁波等优势，在电磁波谱、波前调制中发挥着巨大的作用。综述了近年来基于超表面的太赫兹波前调制器件的研究进展。总结了基于Pancharatnam-Berry相位、基于局域表面等离子体共振(LSPR)、基于Mie共振的三种超表面单元结构对电磁波的振幅、相位调控机理，并讨论了实现高效率超表面的方法。之后，介绍了用于设计波前调制超表面器件的纯相位调制方法和复振幅调制方法。综述了在太赫兹波段典型的超表面波前调制器，包括单一功能、复合功能以及可调谐功能的超表面波前调制器件。在早期的研究工作中，设计的超表面可实现波束偏转、波束聚焦、全息成像、以及涡旋光束、自聚焦光束、洛伦兹光束等特殊光束产生等功能。为提高太赫兹器件的利用率，波分复用、偏振复用等功能复用的太赫兹超表面器件被提出。随着对太赫兹波前动态调控需求的增长, 一些主动的太赫兹超表面器件被提出并在实验上被验证。共有两种主动的超表面器件。其中一种主动超表面是通过将超表面结构与半导体材料或相变材料结合形成的，另一种是通过光泵浦硅片形成的全光器件。全光超表面在不用重新加工的前提下能够被重复使用。通过调整投影在硅片上的超表面图像即可动态操控太赫兹波前。全光超表面具有动态控制波束扫描和波束聚焦的能力，将来可应用于太赫兹通信、太赫兹雷达等领域。最后，对太赫兹波前调制超表面器件的发展趋势与应用前景进行了展望。

Development of metasurfaces for wavefront modulation in terahertz waveband

Metasurface is an ultrathin planar device composed of artificial microstructures, which can be used to manipulate the amplitude, phase, and polarization of electromagnetic (EM) waves. Metasurface has the advantages of small volume, light weight, highly integrated, flexible manipulation of EM waves, so it plays an important role in the field of EM wave spectrum and wavefront modulation. In this paper, the research progresses of metasurface for wavefront modulation in the terahertz (THz) waveband were reviewed. The amplitude and phase modulation mechanisms of three kinds of microstructure units in the metasurface, including the microstructure based on Pancharatnam-Berry (PB) phase, localized surface plasmon resonance (LSPR) and Mie resonance were summarized, and the methods for realizing metasurface with high efficiency were discussed. After that, the pure phase and complex amplitude modulation methods for designing the wavefront modulation metasurface were introduced. Specifically, the typical functions, including single function and multifunction and tunable function, of the wavefront modulation metasurfaces in the THz waveband were reviewed. In the early research, metasurfaces were used to realize beam focusing, beam deflection, holographic imaging, and special beam generation such as vortex beam, Airy beam, and Lorentz beam in the THz region. In order to improve the utilization of a THz component, multifunctional metasurfaces, such as metasurfaces with polarization and wavelength multiplexing were proposed. With the requirement of dynamic control of the THz wavefront, some active metasurfaces were proposed and demonstrated. There were two kinds of active metasurfaces. One of the active metasurfaces was formed by combining the metasurface with semiconductor or phase transition materials, and the other was the all-optical metasurface formed by a silicon wafer with pump beam. The all-optical metasurfaces can be reused without reprocessing. The THz wavefront can be modulated dynamically by adjusting the image of the metasurface projected on the silicon wafer. Thus, the all-optical metasurface had the ability to dynamically control the beam steering and focusing, and it can be applied in THz communication, THz radar and other fields. At the end, the development trend and application prospects of the metasurfaces for wavefront modulation in the THz waveband were discussed.