Dual‐Band Electromagnetically Induced Transparency Enabled by Quasi‐Bound States in the Continuum

Metasurfaces emerge as exceptional platforms for achieving classical‐analog electromagnetically induced transparency (EIT). In this study, dual‐band EIT is demonstrated by strategically engineering the coupling between a magnetic toroidal dipole (TD) Mie resonance and two quasi‐bound states in the continuum (QBICs) within all‐dielectric metasurfaces. Through deliberate symmetry breaking in the cuboid unit cell—achieved via off‐center holes or U‐shaped configurations—two BICs, predominantly governed by electric TD and magnetic quadrupole modes, are successfully transformed into QBICs with high quality (Q) factors. These QBICs are then coupled to a low‐Q magnetic TD Mie resonance, resulting in the emergence of dual‐band EIT. The corresponding group delays reach up to 9.51 ps (Q = 7,674) and 5.69 ps (Q = 3,631), respectively, and diverge when the Q‐factors approach infinite. Furthermore, the dual‐band EIT with high Q‐factors is experimentally validated by fabricating a series of silicon metasurfaces and characterizing their transmission spectra. Excellent agreement is found between numerical simulation and experimental measurement. Measurement results reveal that both the resonance wavelengths and Q‐factors of the dual‐band EIT are precisely tuned by adjusting the asymmetry parameters. These findings hold significant promise for applications in multi‐wavelength slow light devices and biosensing.