Active angular tuning and switching of Brewster quasi bound states in the continuum in magneto‐optic metasurfaces
Diego R. Abujetas, N. de Sousa, Antonio García‐Martín, J. M. Llorens, José A. Sánchez‐Gil
Abstract
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 photonics, and especially in metasurfaces. One of their most outstanding features is their divergent Q‐factors, indeed arbitrarily large upon approaching the BIC condition (quasi‐BICs). Here, we investigate how to tune quasi‐BICs in magneto‐optic (MO) all‐dielectric metasurfaces. The impact of the applied magnetic field in the BIC parameter space is revealed for a metasurface consisting of lossless semiconductor spheres with MO response. Through our coupled electric/magnetic dipole formulation, the MO activity is found to manifest itself through the interference of the out‐of‐plane electric/magnetic dipole resonances with the (MO‐induced) in‐plane magnetic/electric dipole, leading to a rich, magnetically tuned quasi‐BIC phenomenology, resembling the behavior of Brewster quasi‐BICs for tilted vertical‐dipole resonant metasurfaces. Such resemblance underlies our proposed design for a fast MO switch of a Brewster quasi‐BIC by simply reversing the driving magnetic field. This MO‐active BIC behavior is further confirmed in the optical regime for a realistic Bi:YIG nanodisk metasurface through numerical calculations. Our results present various mechanisms to magneto‐optically manipulate BICs and quasi‐BICs, which could be exploited throughout the electromagnetic spectrum with applications in lasing, filtering, and sensing.