Enhanced light-matter coupling and nonlinearities of moiré excitons from a bound state in the continuum

Nonlinear optical phenomena are crucial to both quantum science and emerging photonic and polaritonic technologies, especially when nonlinear responses can be accessed at the level of individual or a few quanta. However, realizing strong optical interactions in solid-state systems at low excitation densities remains a major challenge. Moiré excitons offer a promising route forward, benefiting from their spatially-localized and engineerable trap potential. In these van der Waals heterostructures, the twist angle or lattice mismatch between layers introduces an additional degree of freedom that reshapes the exciton band structure into an equivalent lattice of quantum dots. In parallel, recent nanotechnology advances have established bound states in the continuum (BIC) photonic modes, with exceptionally large quality factors, as robust photonic building blocks for polaritons. Here, by combining moiré heterostructure excitons with BIC photonic modes, we observe the achievement therein of the strong light-matter coupling regime. Under resonant excitation, we observe pronounced and reversible density-dependent energy shifts larger than that on a monolayer, illustrating unambiguously higher nonlinearities. This hybrid lithographically-defined architecture is compact and compatible with integrated photonics, providing a practical route to low-power switches, modulators, and nonlinear polaritonic circuits. Our results establish BIC-enabled moiré polaritons as a general strategy for boosting light–matter interactions and advancing polariton-based information processing.