Tunable high-Q electromagnetically induced transparency enabled by quasi-bound states in the continuum in all-dielectric metasurfaces

Achieving highly tunable, high-quality-factor (high-Q) electromagnetically induced transparency (EIT) via quasi-bound states in the continuum (q-BICs) is crucial for advanced nanophotonic applications. In this work, we propose an all-dielectric metasurface composed of a pair of T-shaped silicon resonators arranged symmetrically on a SiO 2 substrate, capable of supporting multiple symmetry-protected bound states in the continuum. The q-BICs can be accessed by breaking either the illumination symmetry or the in-plane structural symmetry. Specifically, introducing in-plane asymmetry enables the metasurface to simultaneously excite a high-Q EIT resonance and q-BIC modes. Multipole decomposition reveals that the EIT resonance arises from destructive interference between a toroidal dipole mode oriented along the y -axis and a q-BIC mode predominantly associated with a toroidal dipole along the z -axis. Furthermore, by integrating a graphene layer with the metasurface, dynamic modulation of the EIT transmission window is achieved. A maximum modulation depth of up to 95% can be obtained by tuning the Fermi level of graphene, and the maximum group index reaches as high as 3760, indicating a pronounced slow-light effect. This research holds promising potential for applications in optical switches, modulators, and slow-light devices.