Toroidal Response of Asymmetric Metasurfaces with Multiple High Q-Factor Resonances*

Recently, toroidal resonance-based metamaterials (MMs) have been used to demonstrate fascinating THz applications including, but not limited to, electromagnetically induced transparency (EIT), optical filters, bio-chemical sensing, etc. Here, we report numberical simulations and experimental investigations of an array of split ring resonators (SRRs) deposited periodically on the top side of high-resistivity silicon substrate. The investigated SRRs exhibits a toroidal dipolar resonance at around 1.6 THz for TM-polarization and 1.5 THz for TE-polarization. Numerical simulations were performed using a finite element method (FEM) based software to analyze the electromagnetic responses of the proposed metasurfaces. In addition, by breaking the symmetry of the resonators, we achieved a high Q-factor ~ 43. We employed a high-speed asynchronous sampling terahertz spectroscopy system (HASSP-THz) to experimentally study the toroidal geometry. We present electric, magnetic and surface current of each high Q factor Fano resonance within the 0.8-2.4 frequency range.