Extreme Nonreciprocity in Metasurfaces Based on Bound States in the Continuum

Nonreciprocal devices, including optical isolators, phase shifters, and amplifiers, are pivotal for advanced optical systems. However, exploiting natural materials is challenging due to their weak magneto‐optical (MO) effects, requiring substantial thickness to construct effective optical devices. In this study, it is demonstrated that subwavelength metasurfaces supporting bound states in the continuum (BICs) and made of conventional ferrimagnetic material can exhibit strong nonreciprocity in the Faraday configuration and near‐unity magnetic circular dichroism (MCD). These metasurfaces enhance the MO effect by 3–4 orders of magnitude compared to a continuous film of the same material. This significant enhancement is achieved by leveraging Huygens' condition in the metasurface whose structural units support paired electric and magnetic dipole resonances. The multi‐mode temporal coupled mode theory (CMT) is developed for the observed enhancement of the MO effect, and the findings with the full‐wave simulations are confirmed. Nonreciprocity is a crucial property of many optical systems. Magneto‐optical materials are usually used to implement these devices. However, this class of materials presents weak effects at optical frequencies. In this work, a metasurface supporting bound states in the continuum enhances magneto‐optical effects and its nonreciprocal response. In addition, a coupled‐mode‐theory model is developed to explain the physical mechanisms at play.