Bifunctional metasurface for high-efficiency terahertz absorption and polarization conversion

A reconfigurable metasurface with a switchable function, broad band, high efficiency, and ultra-compact size is crucial for the development of efficient and compact devices. We propose a bifunctional metasurface that utilizes vanadium dioxide (VO 2 ) and graphene to achieve high-efficiency absorption and polarization conversion (PC) in the terahertz (THz) range. In our design, an extra dielectric layer is added on the top of VO 2 and graphene. It is worth pointing out that the presence of the additional dielectric layer greatly enhances the coupling of the wave in the Fabry–Perot cavity, resulting in remarkable improvement in absorption and PC efficiency. Furthermore, by controlling the working state of VO 2 and graphene, the functionality of the metasurface can be flexibly switched among absorption, cross-polarized conversion, and linear-to-circular PC (LTC). Simulation results indicate that the metasurface works in the absorption mode when VO 2 is in a metal state, and it can efficiently absorb THz waves at 2.0–7.0 THz with a remarkable relative bandwidth of 111.1%. Furthermore, the absorption is over 98.4% under a normal incident case and still maintains over 90% with an incident angle of 50° at 2.8–7.0 THz. Importantly, by changing the conductivity of VO 2 , the absorption can be flexibly adjusted, allowing for tuning the absorption between 10% and 98.4%. When VO 2 is in an insulator state, the function of the designed metasurface is altered to PC mode, and it can efficiently convert incident linearly polarized (LP) waves into cross-polarized waves with a PC ratio exceeding 95% at 1.8–3.4 THz when the Fermi level of graphene is 1 eV. When switched to the LTC mode, it can convert incident LP waves into right-circularly polarized waves with ellipticity less than −0.95 at 1.7–2.1 THz and into left-circularly polarized waves with ellipticity greater than 0.90 at 2.7–3.0 THz when the Fermi level of graphene is 0.55 eV.