Tunable Weak‐to‐Strong Coupling of Dual‐Band Quasi‐Bound States in the Continuum in Metasurfaces

ABSTRACT This work reports that the coupling of dual‐band quasi‐bound states in the continuum (qBICs) in an all‐dielectric metasurface composed of periodic silicon nanodisks can be tuned from the collective electromagnetically induced transparency (EIT) like effect to the Rabi splitting by adjusting the lattice period or the nanodisk diameter. These two qBIC modes are out‐of‐plane toroidal dipole resonance and electric dipole surface lattice resonance, which are not allowed to emit at point. When the lattice period decreases or the nanodisk diameter increases, these two qBICs experience different spectral shifts and spectrally overlap when they have larger quality factors, resulting in enhanced coupling strength that transits from the weak‐ to the strong‐coupling regime. Specially, in the weak coupling regime, the slow‐light effect accompanied with the EIT‐like phenomenon can be tuned by the quality factor. Our findings open a new paradigm for regulating the coupling of qBICs, which could find applications in lasing, nonlinear optics, and biochemical sensing.