Electromagnetically induced transparency based on magnetic toroidal mode of dielectric reverse-symmetric spiral metasurfaces

Abstract

The intriguing properties of the toroidal mode (TM) resonance can potentially promote a low-loss light–matter interaction. This study proposes an electromagnetically induced transparency (EIT) resonance with a high quality factor, which can reach 7798, and low mode volume can reach 0.009 μm3, high contrast ratio can reach nearly 100%, in the near-infrared region, which is generated by the magnetic TM in a reverse-symmetric coupling spiral metasurface. A two-oscillator model can only explain the influence of near-field coupling at the EIT point for weak coupling. Moreover, a multipole decomposition method shows that the excitation mechanism of EIT resonances originates from the destructive interference between the subradiant modes (magnetic toroidal dipole-electric quadrupole) and magnetic dipole resonance. Consequently, a new general extinction spectrum interference model is applied to fit all coupling conditions for both weak and strong coupling results that perfectly correspond to the multipole decomposition method. The results of this study could be useful in the analysis and understanding of the electromagnetic coupling characteristics of nanoparticles and provide a design approach for novel metasurfaces for low-loss optical applications.