Rayleigh Bound States in the Continuum

We predict a class of bound states in the continuum (BICs) in non-subwavelength periodic metasurfaces -- Rayleigh BICs -- that emerge precisely at Rayleigh anomalies, where diffraction channels open.

In contrast to conventional symmetry-protected and accidental BICs, which are typically engineered in the subwavelength regime with only a few radiation channels, Rayleigh BICs remain nonradiating far beyond this limit, even when multiple diffraction channels are open.

Their formation originates from the interplay between collective lattice resonances, Rayleigh anomalies, and anapole states of the constituent metaatoms.

We show that Rayleigh BICs are characterized by an unusual cubic scaling of the quality factor in reciprocal space, ($Q \propto 1/k^3$), an asymptotic behavior that is not characteristic of either symmetry-protected or accidental BICs.

Our results establish a constructive route for engineering nonradiating states in metasurfaces beyond the subwavelength regime.