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.

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We propose an all-graphene-dielectric tunable terahertz absorber composed of a graphene microstructure layer, a dielectric spacer layer and a metallic ground plane. The absorption performance and internal mechanism of the absorber have been comprehensively analyzed. When E f changes from 0 eV to 0.6 eV, the absorption peak changes from 0.48 THz to 1.579 THz. In addition, we present a novel equivalent circuit model to analyze the mechanism of the absorber. The numerical calculation results are basically consistent with the result of the equivalent circuit model. Our work provides a reference for the design and theoretical study of all-graphene terahertz devices.

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