Degenerate toroidal quasibound states in the continuum enabling polarization-independent exciton-polaritons
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 () factors, yet their intrinsic polarization sensitivity limits their applicability in polarization-insensitive systems. Here, we propose a symmetric 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 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 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.