Thermally tunable polarization-insensitive ultra-broadband terahertz metamaterial absorber based on the coupled toroidal dipole modes.

In this paper, we propose a thermally tunable ultra-broadband polarization-insensitive terahertz (THz) metamaterial absorber (MMA) excited by the toroidal dipole moments. Due to the destructive interference resulting from two anti-parallel toroidal dipole moments, which depends on the twelve-fold trapezoidal metallic loops rotated by the axis parallel to the z-axis, the proposed MMA can achieve the absorption over 0.9 in a wide band of 2.38-21.13 THz, whose relative absorption band is 159.5%, at the temperature of 340 K. Meanwhile, by virtue of tuning the conductivity of vanadium dioxide (VO2) controlled by temperature, the tunability of absorption, maximum reaching 0.57, in the above band can be attained. On the other hand, the MMA is insensitive to the polarization angle owing to its symmetric configuration and can simultaneously keep the absorption above 0.9 in the high-frequency band from 15 to 25 THz under the incidence with a large angle of nearly 60°. In this study, a new way to enhance the absorption in a wide band which is based on the toroidal dipole modes is presented. Such a metamaterial can assist in further understanding the underlying mechanism with respect to the toroidal dipole electromagnetic responses.