Tailoring Electromagnetic Responses in Terahertz Metasurface by Breaking the Structural Symmetry in T-Shaped Resonators

Herein, the design of terahertz metasurface with symmetric T-shaped resonators and the corresponding bright and dark modes are reported on. It is demonstrated that the dark mode evoked by breaking the structural symmetry is insensitive to the X-polarized electric field and a typical electromagnetically induced transparency (EIT) effect is also observed in the numerical simulation when introducing a relatively low structural asymmetry where the dark and bright modes are modified to the same frequency. To explain the underlying mechanism, theoretical calculations based on the coupled Lorentz oscillator model are performed. In terms of the desirable fittings with the simulation results, it is confirmed that there exists the dark mode in the symmetric T-shaped resonators and the coupling between the bright and dark modes enhanced by the structural-symmetry breaking is responsible for the observed EIT effect. It is shown in the results further that the structural asymmetry degree has close relation with the coupling strength resonance and can thereby greatly affect the oscillation resonance splitting in frequency that it is reflected by the change of the bandwidth of the transparency window. In addition, the experimental spectra showing the characteristic of broadband transparency window are explained by the excitation of the dark mode whose frequency deviates from the bright mode.