Guided-mode enabled electromagnetically induced transparency with quasi-BICs in all-dielectric metasurfaces

In this work, we propose and numerically demonstrate an all-dielectric metasurface that supports ultrahigh-Q quasi-bound states in the continuum (q-BIC). The metasurface unit cell consists of two silicon cuboids deposited on a silicon waveguide layer, which is supported by a Si₃N₄ substrate. By breaking the structural symmetry of the metasurface via turning one of the cuboids sideways, guided modes within the waveguide layer are selectively excited, and destructive interference between the guided modes and the grating modes gives rise to an electromagnetically induced transparency (EIT)-like effect along with the q-BIC. Strong phase dispersion near the transparency window leads to a group index as high as 330, indicating pronounced slow-light effects. These findings provide a feasible platform for slow-light devices and integrated photonic systems in the near-infrared region.