Bound states in the continuum (BICs) represent a unique class of physical phenomena characterized by resonant or leaky modes that remain perfectly confined, despite residing within the continuum. The high-quality $Q$-factor guided-mode resonances associated with BICs have garnered considerable attention in photonics research. In this study, we achieve electromagnetically induced transparency (EIT) with perfect chirality by coupling a high-$Q$ chiral quasi-BIC with a broad achiral resonance. We demonstrate the utilization of the topological properties of BICs to realize a high-$Q$ transparent band with maximum chirality in an all-dielectric planar metasurface. Additionally, we observe a spin-dependent slow-light effect, where the group delay difference between the two circularly polarized states exceeds 150 ps. These results reveal a straightforward mechanism for engineering ultranarrow spin-dependent EIT, which holds broad applicability across the electromagnetic spectrum, particularly in filtering and sensing.

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• Electromagnetically induced transparency
• Metasurfaces
• Nanophotonics