Observation of Polarization Topology in Magneto-Optical BICs

Polarization topology in momentum space is a fundamental characteristic of photonic bound states in the continuum (BICs), where the winding of far-field polarization defines a quantized topological charge. Although magneto-optical (MO) BICs combine polarization topology with magnetically tunable chirality, experimental characterization remains challenging because time-reversal symmetry (TRS) breaking complicates eigenpolarization extraction. Here, we experimentally resolve the momentum-space polarization topology of an MO BIC by reconstructing its two-dimensional far-field eigenpolarization texture. We extract the resonant output polarization using singular value decomposition of the polarization-resolved angle-resolved reflection matrix without invoking reciprocity. The reconstructed polarization azimuth winds counterclockwise by 2$\pi$ around the $\mathrm{\Gamma }$-point singularity, yielding a topological charge of q = +1. Moreover, the polarization handedness can be reversed solely by varying the magnetic-bias strength, while the field direction and the measured topological charge remain unchanged. A non-Hermitian coupled-mode model traces this reversal to hybridization between the BIC mode and a leaky mode. This work establishes experimental access to momentum-space polarization topology in nonreciprocal photonics, opening a route to explore topological singularities and their evolution under active TRS breaking.