Wide-angle and polarization-insensitive magneto-optical Kerr effect enhancement empowered by Brillouin zone folding driven toroidal dipole resonances.

The magneto-optical Kerr effects play a crucial role in photonic devices. We propose Brillouin zone folding (BZF) based on lattice perturbations to achieve multiple ultrahigh-Q toroidal dipole (TD) resonances, enabling polarization- and incident angle-independent enhancement of the magneto-optical Kerr effect at multiple wavelengths. The metasurface achieves perfect 180° Kerr rotation near TD resonances, with the polarization direction designable at any angle. The enhancement arises from prolonged light-magneto-optical medium interaction, which strengthens electric field confinement and magneto-optic coupling. The BZF-TD mode exhibits a tunable quality factor, offering an approach for multi-band nonlinear enhancement. We also demonstrate a refractive index sensor based on enhanced Kerr rotation with a sensitivity of 224 nm/RIU.