[Paper Review] Arbitrary Polarization Generation in Magneto-optical Metasurfaces Enabled by Bound States in the Continuum

Siyuan Gao, Guangtai Lu+2 more|arXiv (Cornell University)|Mar 11, 2026

Metamaterials and Metasurfaces Applications0 citations

TL;DR

The paper demonstrates a magneto-optical all-dielectric metasurface at a BIC resonance that enables continuous, full Stokes polarization control of normal radiation by adjusting the external magnetic field orientation, without changing the structure.

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.

Motivation & Objective

• Motivate generation of arbitrary light polarization states for optical communications and photonic processing.
• Leverage bound states in the continuum (BICs) in metasurfaces for polarization engineering without static symmetry breaking.
• Demonstrate active, magnetic-field-based tuning of polarization state at the Γ point to span the Poincaré sphere.

Proposed method

• All-dielectric magneto-optical metasurface composed of a square array of MO nanorods in air.
• Model the MO permittivity tensor with off-diagonal components g_i parameterized by g0, θ (azimuth) and φ (elevation) of the magnetic field.
• Use COMSOL finite-element simulations to compute eigenmodes and momentum-resolved far-field radiation.
• Project Bloch eigenmodes onto outgoing plane waves to obtain far-field polarization and Stokes parameters.
• Characterize polarization state by orientation angle ψ and ellipticity χ from Stokes parameters.
• Investigate both in-plane (φ=0) and out-of-plane (φ≠0) MO effects on BICs to induce quasi-BIC with tunable polarization and Q.

Experimental results

Research questions

• RQ1 Can an external magnetic field reconfigure a symmetry-protected BIC into a radiative quasi-BIC with continuously tunable polarization at normal incidence?
• RQ2 How do the azimuthal and elevation angles of the applied magnetic field independently control polarization orientation and ellipticity, and how does g0 affect the Q factor?
• RQ3 Is it possible to achieve full coverage of the Poincaré sphere for the emitted polarization without altering the metasurface geometry?
• RQ4 What are the topological features (C points, L-lines) of polarization in momentum space under MO perturbations and how do they evolve with field orientation?

Key findings

• An in-plane MO effect (φ=0) transforms the BIC into a quasi-BIC with finite Q, enabling continuous, linear polarization control at Γ by varying θ.
• Polarization orientation ψ at Γ sweeps continuously along the equator of the Poincaré sphere as θ is varied, independent of g0; Q decreases with increasing g0.
• Out-of-plane MO effect (φ ≠ 0) enables emergence of circular polarization at Γ when g0 is sufficiently large, with χ transitioning from linear to circular; Q generally decreases with larger g0 or smaller φ.
• Combining θ and φ under fixed g0 yields arbitrary polarization states, achieving full coverage of the Poincaré sphere in momentum space at Γ.
• The movement of C points in momentum space and the presence of L-lines corroborate a topological interpretation of the polarization states and their evolution under MO perturbation.
• The design provides a reconfigurable, high-Q platform for deterministic polarization control at a single-frequency BIC resonance without structural changes.