Tunable and switchable multi-functional terahertz metamaterials based on a hybrid vanadium dioxide–graphene integrated configuration

In this paper, an actively tunable and switchable multi-functional terahertz metamaterial device based on a hybrid vanadium dioxide (VO2)-graphene integrated configuration is proposed. By transiting the phase of VO2, the functions of the proposed device can be reversibly switched between asymmetric transmission (AT) and two different polarization conversions in the terahertz region. When VO2 operates at the isolating state, the AT effect can be achieved with a maximum value of 0.34 for linearly polarized lights due to the excitation of enantiomerically sensitive plasmons in patterned graphene nanostructures. Furthermore, when VO2 is transited from the isolating state to the conducting state, the metamaterial does not only exhibit a linear dichroism response but also perform linear-to-linear and linear-to-circular polarization conversions simultaneously. Specifically, the designed device behaves like a half-wave plate, where a linear polarization conversion ratio exceeds 96.5% at a frequency of 9.17 THz. Meanwhile, it acts as a quarter-wave plate which can convert the linear polarization light into left-handed and right-handed circularly polarized lights with high efficiencies at frequencies of 9.04 and 9.3 THz, respectively. Moreover, the performance of the designed structure can be actively controlled by adjusting the geometrical parameters and Fermi energy of graphene. This work provides a new avenue in developing multi-functional terahertz metamaterial devices.