Terahertz Metagrating Accordion for Dynamic Beam Steering

Metasurfaces can be used to manipulate the beam direction beyond the Snell's law by the constitutive gradient elements. Independent control of the phase response of each element is usually required for real-time beam steering, which is very challenging at terahertz (THz) frequencies. Here a type of accordion-shaped single-element metagratings for THz beam scanning and angle-adaptive retroreflection is proposed. The metagrating is carefully designed to enhance the +1st order diffraction while suppressing other orders for efficient beam steering. Benefiting from the periodic arrangement, dynamic beam steering is realized by imparting desired momentum via mechanical compression and rotation, instead of the complicated element-by-element modulation. Beam scanning in a plane and over the 3D space is experimentally validated with elevation angles from 45 degrees to 75 degrees and azimuthal angles over 360 degrees. An angle-adaptive retroreflector by adding a metallic plate to the metagrating is studied with near perfect theoretical diffraction efficiency for incident angles within 40 degrees-85 degrees. The physical mechanism of wide-angle high efficiency is detailed by the mode analysis, and can be generalized into other frequencies. The study here may find applications in THz communications and radar detections, and open a new avenue to facilitate reconfigurable metadevices for wavefront modulation.