High‐Q Transparency Band in All‐Dielectric Metasurfaces Induced by a Quasi Bound State in the Continuum

Abstract Bound states in the continuum (BICs) emerge throughout physics as leaky/resonant modes that remain, however, highly localized. They have attracted much attention in optics and photonics, and especially in metasurfaces, that is, planar arrays of sub‐wavelength meta‐atoms. One of their most outstanding features is the arbitrarily large Q‐factors they induce upon approaching the BIC condition, which is exploited here to achieve a narrow transparency band. It is first shown how to shift a canonical BIC in an all‐dielectric metasurface, consisting of high‐refractive disks exhibiting in‐ and out‐of‐plane magnetic dipole (MD) resonances, by tuning the periodicity of the array. By means of the coupled electric/magnetic dipole formulation, it is shown analytically that when the quasi‐BIC overlaps with the broad (in‐plane MD) resonance, a full transparency band emerges with diverging Q‐factor upon approaching the BIC condition in parameter space. Finally, the experimental measurements in the microwave regime with a large array of high‐refractive‐index disks confirm the theoretical predictions. The results reveal a simple mechanism to engineer an ultra‐narrow BIC‐induced transparency band that can be exploited throughout the electromagnetic spectrum with obvious applications in filtering and sensing.