High-efficiency terahertz beam scanning based on topologically optimized freestanding metasurfaces

Dynamic beam scanning is an indispensable technique in the terahertz band, as the terahertz radiation is highly directive to accommodate the path attenuation. Bi-layer passive metasurfaces offer a promising way to tune the beam direction through in-plane relative twisting. However, the energy efficiency is limited due to the doubled insertion loss from two metasurfaces, and the efficiency is unstable during scanning due to the diffraction sensitivity to the metasurface orientation. Here, we propose a pair of metasurfaces to address the efficiency issue through topological optimization and removal of the substrate for better impedance matching. The freeform and freestanding metasurfaces are rotated to generate various moiré patterns and scan the beam within ±55° field of view without apparent sidelobes. The average energy efficiency is 57% with efficiency fluctuation within 13%. The proposed metasurfaces hold broad application prospects in next-generation wireless communication technologies.