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    Degenerate toroidal quasibound states in the continuum enabling polarization-independent exciton-polaritons

    Xianyan Wei1, Chaomian Wu1, Yawei Chen1, Jianhua Zeng2, Weiyi Hong1,*, and Fu Deng1,†

    • 1Guangdong Provincial Key Laboratory of Nanophotonic Functional Materials and Devices, School of Optoelectronic Science and Engineering, South China Normal University, Guangzhou 510006, China
    • 2Jiangxi Province Key Laboratory of Applied Optical Technology, School of Physical Science and Intelligent Education, Shangrao Normal University, Shangrao 334001, China

    • *Contact author: hongwy@m.scnu.edu.cn
    • †Contact author: dengfu@m.scnu.edu.cn

    Phys. Rev. B 114, 045401 – Published 1 July, 2026

    DOI: https://doi.org/10.1103/rkn1-pyhg

    Abstract

    Achieving polarization-independent strong light-matter coupling remains a central challenge for practical integrated photonic platforms. Conventional metasurfaces based on bound states in the continuum (BICs) can sustain ultrahigh quality (Q) factors, yet their intrinsic polarization sensitivity limits their applicability in polarization-insensitive systems. Here, we propose a C4v symmetric WSe2 metasurface that supports degenerate toroidal quasi-BIC resonances and enables polarization-independent exciton-polaritons. Electromagnetic field analyses and multipole decomposition reveal that the dual degenerate modes originate from electric and magnetic toroidal dipole excitations, whose resonance wavelengths and Q factors can be precisely tuned through a structural asymmetry parameter. Leveraging these polarization-independent resonances, we demonstrate strong coupling between the quasi-BIC modes and WSe2 excitons, evidenced by persistent anticrossing behavior for all polarization angles and coupling strengths that can be flexibly controlled via geometric perturbation. These results establish a viable route toward polarization-independent exciton-polaritons in van der Waals metasurfaces and provide a promising platform for robust light-matter interaction, sensing, and integrated nanophotonic devices.

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