Robust Ultrahigh‐Q Quasi‐Bound States in the Continuum in Metasurfaces Enabled by Lattice Hybridization

Abstract Bound states in the continuum (BICs) have been of growing interest in photonics for their infinite quality factors and strong field localization. The last decade has witnessed great progress on the highest achievable quality factors of quasi‐BICs. However, it remains challenging to realize robust high‐Q quasi‐BICs that are tolerant to the parameter perturbation. Here, it is demonstrated that silicon metasurfaces formed by the hybridization of two lattices can support robust quasi‐BICs with the highest measured quality factor reaching 4130 and the minimum one larger than 1024, even when the relative displacement varies over the whole range of all the possible values. Taking advantage of the two in‐plane mirror‐symmetry points, for the first time, the transition between a toroidal dipole BIC and an electric quadrupole BIC, which guarantees robust ultrahigh‐Q quasi‐BICs at Γ point, is reported. The minimum quality factors of quasi‐BICs can be further improved significantly by shrinking the meta‐atom size and are immune from the intrinsic absorption loss of silicon in the visible regime. It is envisioned that this study will pave the way for achieving robust ultrahigh‐Q metasurfaces and boost their applications in lasing, nonlinear optics, and biosensing.