Intrinsic strong coupling mediated by toroidal-dipole bound states in the continuum in a split-disk WSe2 metasurface.

Photonic bound states in the continuum (BICs) offer a robust platform for realizing intrinsic strong light-matter coupling in transition-metal dichalcogenides (TMDCs). Here, we demonstrate intrinsic exciton-photon strong coupling in a structurally optimized bulk WSe2 split-disk metasurface supporting a toroidal-dipole bound state in the continuum (TD-BIC). Multipolar decomposition and eigenmode analyses confirm the toroidal dipole response and high-Q characteristics of the TD-BIC resonance. The optimized metasurface exhibits pronounced anticrossing and a large Rabi splitting, enabled by enhanced field confinement and increased excitonic oscillator strength. Furthermore, tailoring the split-gap geometry provides precise control over the resonant linewidths and coupling strength. These results establish TD-BIC WSe2 metasurfaces as a compact and versatile platform for intrinsic strong-coupling nanophotonics.