Bidirectional vortex beam and reflective polarization conversion terahertz metasurfaces

We design a terahertz metasurface with the functions of bidirectional vortex beams and reflection polarization conversion. The unit cell of the proposed metasurface consists of a vanadium dioxide metal mixed layer, a dielectric layer, a vanadium dioxide thin film, a dielectric layer, and a metal open ring. At room temperature, when a left-handed circularly polarized wave is incident, the metasurface generates a reflected mode vortex beam with topological charges of l =−1 and l =−2 at frequencies of 0.33 and 0.34 THz. Simultaneously, the metasurface produces transmitted mode vortex beams with topological charges of l =−1 and l =−2 at frequencies of 0.33 and 0.36 THz. When the temperature changes to 68°C and y -polarized waves are incident, the metasurface generates a reflected left-handed circularly polarized wave in the frequency bands of 0.6–0.65 THz and 1.75–1.93 THz, and a right-handed circularly polarized wave in the frequency band of 0.86–0.9 THz, achieving the function of converting linearly polarized waves into circularly polarized waves. This metasurface provides a new method for designing a single metasurface structure to achieve bidirectional multifunctional terahertz devices for transmission and reflection modes. In addition, the designed metasurface achieves a perfect polarization conversion function that can convert linearly polarized waves into circularly polarized waves.