Magneto-Optic Control of Brewster Quasi Bound States in the Continuum

Bound states in the continuum (BICs) emerge throughout Physics as leaky/resonant modes that remain, however, highly localized[1]. They have attracted much attention in Photonics, and especially in metasurfaces[2,3]. One of their most outstanding features is their divergent Q-factors, indeed arbitrarily large upon approaching the BIC condition (quasi-BICs) [4,5]. 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 Si spheres with MO response. Through our coupled electric/magnetic dipole formulation, the MO activity is found to manifest itself through the interference of the (MO-induced) out-of-plane electric/magnetic dipole resonances with the in-plane magnetic/electric (directly induced) 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 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 [6].<br/><br/><b>References</b><br/><br/>[1] Chia Wei Hsu, et al., Nat. Rev. Mater. <b>1</b> 16048, (2016).<br/>[2] D.C. Marinica, et al., Phys. Rev. Lett., <b>100, </b>183902 (2008).<br/>[3] Chia Wei Hsu, et al., Nature <b>499</b>, 188 (2013).<br/>[4] Kirill Koshelev, et al., Phys. Rev. Lett. <b>121</b>, 193903 (2018).<br/>[5] Diego R. Abujetas, et al., Sci. Rep. <b>9</b>, 16048 (2019).<br/>[6] Diego.R. Abujetas, et al., Nanophotonics 10, 42232 (2021).