Vanadium dioxide-assisted broadband absorption and linear-to-circular polarization conversion based on a single metasurface design for the terahertz wave

Integrating tunable characteristics and multiple functions into a single metasurface has become a new scientific and technological undertaking that needs to deal with huge challenges, especially in the terahertz frequency region. The multifunctional design combining the broadband absorption and broadband polarization conversion using a single switchable metasurface is proposed in this paper. The switchable performance can be realized by treating the insulation to metal phase transition properties of vanadium dioxide (VO₂). At high temperature (74 °C), the proposed metasurface can be used as a broadband absorber which consists of a VO₂ square ring, polyimide (PI) spacer, and VO₂ film. Simulated results show that the terahertz wave absorption can reach above 90% with the bandwidth ratio of 75% in the frequency range of 0.74 THz-1.62 THz. This absorber is insensitive to polarization resulted from the symmetry structure and also shows a good performance at large incident angles. Once the temperature is lower than the cooling phase transition temperature (about 62 °C) and VO₂ is in insulation state, the metasurface can be transformed into a broadband linear-to-circular polarization converter. Numerical simulation depicts that the ellipticity reaches to -1 and the axis ratio is lower than 3 dB from 1.47 THz to 2.27 THz. The designed switchable metasurface provides the potential to be used in the fields of advanced research and intelligent applications in the terahertz frequency region.