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The generation of arbitrary polarization states of light is essential for optical communication and photonic information processing. Photonic crystal and metasurface platforms supporting bound states in the continuum (BICs) provide a powerful route for polarization engineering by tailoring the radiation from the resonant modes. However, existing approaches typically rely on static structural symmetry breaking or off-normal radiation, which limits continuous polarization tuning of vertical radiation. Here, we demonstrate a magneto-optical metasurface that generates arbitrary polarization states of light at normal radiation. By applying an external magnetic field with variable orientation, a symmetry-protected BIC is transformed into a quasi-BIC whose radiation polarization can be continuously tuned. The magneto-optical perturbation drives the controlled migration of polarization singularities in momentum space, allowing the emitted states to continuously span the entire Poincaré sphere without structural modification. This approach establishes a compact platform for actively tunable polarization sources and polarization-encoded photonic devices.
Gao et al. (Fri,) studied this question.