Anomalous Refraction and Nondiffractive Bessel-Beam Generation of Terahertz Waves through Transmission-Type Coding Metasurfaces

Coding metasurfaces, composed of an array of coding particles with discrete phase responses, are encoded with predesigned coding sequences to manipulate wavefronts of electromagnetic (EM) waves and realize novel functionalities such as anomalous beam deflection, broadband diffusion, and polarization conversion. Such a new concept can be viewed as a bridge linking metamaterial and digital codes, yielding the investigation of metamaterials from a digital perspective and eventually the realization of real-time control of EM waves. Here, we propose and experimentally demonstrate a transmission-type coding metasurface to bend normally incident terahertz beams in anomalous directions and generate nondiffractive Bessel beams in normal and oblique directions. To overcome the larger reflection and strong Fabry–Perot resonance that usually originate from a thick silicon substrate, a free-standing design is presented for the coding particle, which is formed by stacking three metallic layers with four polyimide spacers alternately. Experimental results show that the fabricated sample could bend the normally incident terahertz wave to anomalous refraction angles of 26° and 58° with 58% and 40% efficiencies, respectively. Owing to the excellent mechanical and chemical properties of polyimide, the fabricated sample is extremely flexible and stable, implying promising applications in terahertz imaging and communication.