Tailoring Accidental Double Bound States in the Continuum in All‐Dielectric Metasurfaces

Abstract Bound states in the continuum (BICs) have been thoroughly investigated due to their formally divergent Q‐factor, especially those emerging in all‐dielectric, nanostructured metasurfaces from symmetry protection at the Γ point (in‐plane wavevector k_|| = 0). Less attention has been paid to accidental BICs that may appear at any other in the band structure of supported modes, being in turn difficult to predict. Here, a coupled electric/magnetic dipole model is used to determine analytical conditions for the emergence of accidental BICs, valid for any planar array of meta‐atoms that can be described by dipolar resonances, which is the case of many nanostructures in the optical domain. This is explored for all‐dielectric nanospheres through explicit analytical conditions that allow in turn to predict accidental BIC positions in the parameter space (ω, k_||). Finally, such conditions are exploited to determine not only single, but also double (for both linear polarizations) accidental BICs occurring at the same position in the dispersion relation ω − k_|| for realistic semiconductor nanodisk meta‐atoms. This might pave the way to a variety of BIC‐enhanced light–matter interaction phenomena at the nanoscale such as lasing or nonlinear conversion, that benefit from emerging at wavevectors away from the Γ point (off‐normal incidence) overlapping for both linear polarizations.