Brewster quasi bound states in the continuum in all-dielectric metasurfaces from single magnetic-dipole resonance meta-atoms

Bound states in the continuum (BICs) are ubiquitous in many areas of physics, attracting especial interest for their ability to confine waves with infinite lifetimes. Metasurfaces provide a suitable platform to realize them in photonics; such BICs are remarkably robust, being however complex to tune in frequency-wavevector space. Here we propose a scheme to engineer angularly-selective BICs with single magnetic-dipole resonance meta-atoms. Upon changing the orientation of the magnetic-dipole resonances, we show that the resulting (Brewster-like) BICs can be tuned across the continuum at will. While yielding infinite Q-factors at normal incidence, these are termed quasi-BICs since a transmission channel is allowed that slightly widens resonances at oblique incidences. This is demonstrated experimentally through reflectance measurements in the microwave regime with high-refractive-index mm-disk metasurfaces. This Brewster-inspired configuration is a plausible scenario to achieve tunable quasi-BICs throughout the electromagnetic spectrum, and it could be extrapolated to other kind of waves.