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.