Terahertz coding metasurfaces for beam scanning and tunable focusing based on Dirac semimetals

Metasurfaces are artificially constructed structures that mimic the lattices of natural materials, they have made many contributions to the development of conventional optical devices. In this study, we numerically explore a tuneable terahertz (THz) coding metasurface based on the Pancharatnan-Berry (PB) phase principle by varying the rotation angle of the Dirac semimetals (DSMs) pattern, which leads to a relative phase delay of the incident THz wave. The coding metasurface has the advantages of broadband and high reflectivity, which can provide excellent characteristics for THz multifunctional devices. Based on the fractional phase-coding strategy, the period spacing scale Nx of the super-unit cells was controlled to change the 1-bit and 3-bit coding sequences. The wide-angle quasi-continuous scanning function was realised for both single and multiple beams, with a scanning range of (5.9 degrees, 56.0 degrees). In addition, the DSMs is also utilized to design tunable metalens, enabling dynamic tuning of the reflection focusing by varying the EF of the DSMs. The metalens can handle multichannel synchronous super-resolution imaging and single-channel asynchronous focusing. To further expand the application potential of DSMs-based metalens, combining them with vortex beams carrying orbital angular momentum (OAM) enables the focusing of vortices with different topological charges, and allows focal length adjustment through frequency variation. The results of this study are relevant for applications in THz communication, imaging and sensing.