Giant Enhancement of the Anomalous Transverse Magneto-Optical Kerr Effect in a Magneto-Optical Grating.

The anomalous transverse magneto-optical Kerr effect (TMOKE) under s-polarization is typically suppressed by symmetry constraints, leading to negligible signals. Here, we propose an indirect magneto-optical (MO) coupling mechanism that originates from the spatially asymmetric near-field distribution of resonant modes, rather than from the direct coupling between the incident field and the MO tensor. Guided by this principle, we design a quasi-two-dimensional MO grating that strategically coordinates resonances along two orthogonal directions. Specifically, modes perpendicular to the grating ridges establish a wide-angle broadband near-zero reflection background to satisfy the rigorous mathematical condition for maximum TMOKE, whereas the orthogonal high-Q resonances are responsible for inducing maximal near-field asymmetry. This allows us to push the s-polarized TMOKE to approach its theoretical limit (∼2)-a performance leap of over an order of magnitude compared to previous reports. The proposed structure exhibits exceptional robustness against material losses and fabrication tolerances, alongside a broad angular tolerance (up to 40°), offering a practical blueprint for high-performance nonreciprocal photonic devices.