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

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.