Flat-band polaritons in van der Waals metasurfaces driven by collective coupling of Brillouin-zone-folded Q-BIC, anapoles, and excitons

Bulk transition metal dichalcogenides (TMDs) have emerged as a powerful platform for nanophotonics, condensed-matter physics, and quantum optics, owing to their high refractive indices and robust excitonic and nonlinear responses at room temperature. Here we theoretically propose a WS 2 nanodisk dimer metasurface that simultaneously supports Brillouin-zone-folding-induced bound states in the continuum (BICs) and an intrinsic flat-band anapole. We systematically investigate the mutual coupling and collective optical response arising from the interplay among excitons, anapoles, and quasi-BIC (Q-BIC). We find that the weak coupling between the Q-BIC and the anapole gives rise to a bandwidth-tunable electromagnetically induced transparency-like effect with a group time delay up to 7 ps at a stable wavelength. The hybridization between the Q-BIC and the exciton leads to a typical Fano resonance. In contrast, the anapole–exciton interaction enters the strong-coupling regime, forming self-hybridized exciton–polaritons characterized by a typical anticrossing behavior with a Rabi splitting of 182 meV. Interestingly, these exciton–polaritons inherit the flat-band character of the anapole, remaining nearly dispersionless over a wide-angle range of ±30°. Furthermore, the inclusion of the Q-BIC enables collective three-mode coupling, enhancing the Rabi splitting to 190 meV. All coupling phenomena are quantitatively captured by coupled oscillator models incorporating both two- and three-mode interactions. Our results establish a general framework for engineering strong and ultrastrong light–matter interactions in van der Waals metasurfaces. The resulting self-hybridized exciton–polaritons, combining large Rabi splitting with wide-angle flat-band dispersion, offer promising opportunities for scalable valleytronic functionalities and integrated polaritonic devices.