Strong electromagnetically induced transparency effect constructed by quasi-BIC and Brillouin zone folding GMR in composite dielectric metasurfaces

Traditional electromagnetically induced transparency (EIT) typically exhibits low quality (Q) factors and limited tunability. Bound states in the continuum (BIC) offer a promising solution for achieving high-Q EIT. In this work, we engineer a high-Q EIT resonance on a silicon-membrane metasurface by coupling a quasi-BIC with a Brillouin zone folding guided mode resonance (BZF-GMR). In the unit cell of metasurfaces featuring dual rectangular nanoholes, asymmetric perturbation (rotating one nanohole) converts a symmetry-protected BIC, dominated by a toroidal dipole, into a high-Q quasi-BIC that functions as a dark mode. Simultaneously, it tunes a magnetic dipole-dominated BZF-GMR into the bright mode. Synergistic coupling between these modes produces a prominent EIT resonance, characterized by an 8.4 ps group delay and a high-Q of $5.9\times 10^{3}$ (corresponding experimental Q-factor of 250). Furthermore, the EIT resonance wavelength and Q-factor are precisely tuned by varying the asymmetry parameter. The design holds robustness against parameter variations. The fabrication and characterization of silicon metasurface samples validate our design approach. Our results demonstrate a reliable strategy for achieving a high-Q EIT resonance, with promising applications in slow light, high-sensitivity sensing, nonlinear enhancement, and ultrafast optical modulation.