Nanolayered VO₂-Based Switchable Terahertz Metasurfaces as Near-Perfect Absorbers and Antireflection Coatings

Achieving absorption–transmission switchable devices is still a great challenge due to the complex competitions among absorption, reflection, and transmission. Here, we propose a nanolayered terahertz (THz) metasurface exhibiting switchable functionality from quad-band near-perfect absorption to strongly enhanced transmission. The switchable performances are attributed to the electrical tunability of graphene and insulator–metal transition in vanadium dioxide (VO2). When VO2 functions as a metal, the proposed metasurface performs as a quad-band perfect absorber. The four absorption peaks can be effectively adjusted by altering the related graphene Fermi energy levels. When VO2 is in an insulating state, the proposed device functions as a THz antireflection coating, which diminishes reflection and substantially enhances transmission. In addition, a theoretical interference model is employed to clarify the physical mechanisms behind the near-perfect absorber and antireflection coating. The obtained theoretical results significantly agree with the numerical simulations. Furthermore, owing to the rotationally symmetric patterns, the performances of both the quad-band perfect absorber and antireflection coating are rather insensitive to light polarization and angle of incidence. Hence, our results can facilitate the progress of nanoscale switchable metadevices in the THz regime.