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, i.e. planar arrays of sub-wavelength meta-atoms. One of their most outstanding feature is the arbitrarily large Q-factors they induce upon approaching the BIC condition, which we exploit here to achieve a narrow transparency band. We first show 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 our coupled electric/magnetic dipole formulation, we show 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, our experimental measurements in the microwave regime with a large array of high-refractive-index disks confirm the theoretical predictions. Our results reveal a simple mechanism to engineer an ultra-narrow BIC-induced transparency band that could be exploited throughout the electromagnetic spectrum with obvious applications in filtering and sensing.