Theoretical design of tunable terahertz metasurfaces with dual-mode polarization conversion and ultra-broadband absorption functionality

In this paper, we propose a vanadium dioxide and germanium telluride composite metasurface. The conductivity of vanadium dioxide and germanium telluride is varied by changing the temperature, which enables the switching of functions such as ultra-broadband absorption, reflective-type polarization, and transmissive-type polarization. When vanadium dioxide is metallic and germanium telluride is crystalline, the terahertz wave is incident along the –<i>z</i> direction, and the metasurface can be used as a broadband absorber, with an absorption rate greater than 90% in a frequency range of 7.96–17.76 THz, and the absorption bandwidth reaches 9.8 THz, with a relative bandwidth of 76.2%. In addition, the designed metasurface absorber is polarization-insensitive and exhibits good absorption performance at large incidence angles. Terahertz waves are incident along the +<i>z</i> direction, and this metasurface can be used as a reflective polarization converter with a polarization conversion ratio greater than 0.9 for <i>x</i>– and <i>y</i>–polarized waves in the frequency band from 2.04 to 4.44 THz. The effects of incidence angle and structural parameters on polarization conversion performance are also investigated. When vanadium dioxide is in the dielectric state and germanium telluride is in the amorphous state, the metasurface can be used as a transmissive polarization converter, with a polarization conversion rate of greater than 0.9 in a frequency band of 0.65–5.07 THz. And the high polarization conversion performance can be maintained in a wide range of incidence angles. Finally, the physical mechanism of polarization conversion is analyzed using surface currents. The results show that the metasurface structure has bi-directional, switchable and multi-functional characteristics for terahertz wave manipulation, and has broad application prospects in terahertz wave sensing, imaging and communication.