The investigation of an inspired type toroidal dipole flexible metasurfaces at terahertz frequencies.

We theoretically and experimentally carried out an inspired type of toroidal dipole (TD) metasurfaces, which composed of a metamolecule of symmetric aluminium semicircles with a bar in the middle fabricated on polyimide substrate in the terahertz (THz) regime. It was found that the three resonances show red-shift tendency due to the increase of inductance and capacitance with the increase of semicircle's outer radius. Meanwhile, both the TD resonances and the current flowing in the metallic bar can generate the head-to-tail magnetic field distribution, which is the most prominent feature of TD phenomenon. The generation of this phenomenon is discussed deeply via the power of the multipoles, which are calculated according to the volume current density distribution data extracted from the simulations. The low frequencies (∼0.5 THz) head-to-tail magnetic field distribution is mainly attributed to TD resonance generated via the mutual effect between the two semicircles, while the same phenomenon at high frequencies (∼0.8 THz) is mainly attributed to the current flowing the middle metallic bar. The enhancement of head-to-tail magnetic field distribution leads to the increase of quality (Q) factor, and the Q factor of fabricated sample is as high as 24.5. Moreover, the electromagnetic properties in TD metasurfaces could be adjusted by the metal bar's width. The optimization of TD resonances provides opportunity to design the high Q-factor metasurfaces, and it opens up potential applications in terahertz high sensitivity devices.