Actively tunable toroidal dipole resonance in hybrid terahertz metal-graphene metamaterials

The toroidal dipole is a localized electromagnetic excitation induced by a reversely distributed circulating current. The magnetic fields with opposite directions are connected end to end to form a closed circular magnetic field, and a toroidal dipole with non-radiative electric field characteristics is obtained. However, active manipulation of the toroidal resonance has remained largely unexplored due to the scarcity of compact tunable elements so far. Here, we propose a planar terahertz metamaterial composed of an asymmetric metallic double splits ring resonator, which can excite a toroidal dipole resonance. To active control the toroidal dipole resonance, the monolayer graphene is integrated with the metamaterial. By changing Fermi energy of graphene, the transmission amplitude of the toroidal dipole resonance is actively modulated and finally switched into a electric resonance or magnetic resonance. Therefore, switching between the non-radiation and the high-radiation, as well as active modulation of the toroidal dipole have potential application prospects in the design of filters, high-sensitivity sensors, and modulators.