Arbitrary Polarization Generation in Magneto-optical Metasurfaces Enabled by Bound States in the Continuum

Abstract

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 through 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 magnetooptical metasurface that generates arbitrary polarization states of light at normal radiation. By applying an external magnetic field with variable rientation, 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 polarizationencoded photonic devices.

Figures (4)

• Figure 1: (a) Schematic illustration of the MO metasurface and direction of external magnetic field. (b) Band diagram of the investigated structure, inset: Mode profile of the investigated BIC mode. (c) The map of polarization state, orientation angle $\psi$ and $Q$ factor of this TE band in momentum space without external magnetic field.
• Figure 2: In-plane MO effect and manipulation of SoP and $Q$ of TE-mode at $\Gamma$ point. (a) SoP at the $\Gamma$ point plotted on the Poincaré sphere for $g_0 = 0.03$ and $\phi = 0^\circ$ as $\theta$ varying from $-90^\circ$ to $90^\circ$, showing a continuous sweep along the equator that covers all linear-polarization orientations; (b) Orientation angle $\psi$ as a function of $\theta$ with $g_0 = 0.03, 0.08$ and $0.13$. (c) $Q$ factor distribution with varying $\theta$ and $g_0$. Polarization plot, $\chi$ and $\psi$ distribution of TE quasi-BIC mode in momentum space with (d) $g_0 = 0.08$, $\phi = 0^\circ$ and $\theta = 0^\circ$ and (e) $g_0 = 0.08$, $\phi = 0^\circ$ and $\theta = 45^\circ$; (f) The movement trajectory of LCP and RCP point with increasing $\theta$ with $g_0 = 0.03, 0.08$ and $0.13$ and $\phi = 0^\circ$.
• Figure 3: Out-of-plane MO effect and manipulation of SoP and $Q$ of TE-mode at $\Gamma$ point. (a) SoP at the $\Gamma$ point plotted on the Poincaré sphere for $g_0 = 0.03, 0.08$ and $0.13$ as $\phi$ varying from $-89^\circ$ to $89^\circ$; (b) Ellipticity $\chi$ distribution with varying $\phi$ ($0^\circ - 89^\circ$) and $g_0$, the white dashed line is the iso-valued line presenting $\chi = \frac{\pi}{4}$; (c) $Q$ factor distribution with varying $\phi$ ($0^\circ -89^\circ$) and $g_0$; (d) Polarization plot, $\chi$ and $\psi$ distribution in momentum space with $g_0 = 0.08$, $\phi = 66.1^\circ$ and $\theta = 0^\circ$; (e) The trajectory of LCP and RCP point with increasing $\phi$ with $g_0 = 0.03, 0.08, 0.13$ under $\theta = 0^\circ$.
• Figure 4: Arbitrary polarization states control. The SoP at $\Gamma$ point under different $\theta$ and $\phi$, where polarization plot and colormap of ellipticity $\chi$ (rad) (a), the SOP plot on Poincaré sphere and the corresponding $Q$ factor (c).