Angle-robust chiral exciton-polaritons in symmetry-broken WS2 metasurfaces.

Chiral light-matter interaction in metasurfaces has drawn considerable interest for chiral sensing and spin-selective photonics. While quasi-bound states in the continuum(quasi-BIC) metasurfaces and transition metal dichalcogenides (TMDs) exciton-polariton systems have been widely explored, their circular dichroism typically deteriorates under oblique incidence, limiting practical applications that require wide-angle operation. In this study, we propose a symmetry-broken double-cross WS2 metasurface that maintains a stable chiroptical response over a broad angular range through two geometric parameters that can be optimized quasi-independently. The internal crossing angle α controls the quasi-BIC linewidth, while the inter-rod rotation angle β tunes the handedness-dependent coupling via the combined phase α + β. The resulting circular dichroism remains robust over ±0.1 rad, with a peak value of 0.96, while the quasi-BIC resonance simultaneously couples to the WS2 A-exciton to form exciton-polaritons with a Rabi splitting of 46 meV. Coupled-mode theory and multipole decomposition reveal that the chiral response originates from electric-magnetic dipole interference. These findings establish a quasi-independent tuning strategy for angle-robust chiral exciton-polaritons, paving the way toward wide-angle chiroptical devices.