Dual-Band Kerr Sensing Empowered by Quasi-Bound States in the Continuum Strong Coupling in a Magneto-Optical Metamaterial

The magneto-optical (MO) effect was usually enhanced by the excitation of surface plasmons, surface lattice resonance, or coupling between them. Recently, bound states in the continuum (BIC) have been of great potential to boost the magneto-optical Kerr effect (MOKE) due to its high Q-factor and small mode volume. However, it is always limited to the transverse MOKE. Here, we realized the strong coupling between two Q-BICs with enhanced polar MOKE in a MO metamaterial composed of Ag double-step gratings and a magnetic dielectric layer. The origin of the two Q-BIC modes is fully investigated. One BIC is generated by the waveguide (WG) mode, and another BIC is formed by the interference between the WG mode and FP mode. In addition, two Kerr reversals are observed in the Kerr rotation angle spectrum due to the Q-BIC strong coupling, offering a viable strategy for developing dual-band magneto-plasmon sensors, with large FoM values of 2467/RIU and 5568/RIU. The FoM value achieved in this work is at least two times as large as the previously reported results of the same kind (FoM = 1823/RIU and 755/RIU). These findings offer a promising approach for achieving strong coupling induced by Q-BICs and designing novel MP nanostructures with the desired optical and MO responses.