Terahertz Beam Steering Technologies: From Phased Arrays to Field-Programmable Metasurfaces

In past decades, terahertz radiation, which locates between microwave and far-infrared light in the electromagnetic spectrum, has attracted more and more attention. Various passive and active components such as absorbers, filters, polarizers, and focusing lenses have been developed for manipulating terahertz radiation. With the further evolution of metamaterials and metasurfaces, unprecedented freedom is gained for flexibly controlling terahertz waves, including focusing, deflection, beam steering, polarization conversion, and generation of orbit angular momentum. Among them, terahertz beam steering has become the focus of considerable interest owing to its significance for wireless communication, high-resolution imaging, and radar applications. In this paper, first the conventional terahertz beam steering technologies are reviewed, including mechanical scanning, phased array, frequency scanning antennas, and multibeam switching technology. Then, the reconfigurable metasurface routes based on semiconductor active components, phase transition materials, electrically tunable materials, and micro-electromechanical technology are summarized and the respective performances for terahertz beam steering are discussed. Moreover, programmable metasurfaces with digitalized description of metasurfaces and real-time manipulations of radiation patterns are also illustrated in detail. Finally, a summary of the present terahertz beam steering technologies is provided and an outlook for the future is discussed.