Exciton polaritons based on strong exciton-photon coupling hold great promise for developing novel polaritonic devices and studying quantum many-body physics. However, due to the relatively small exciton binding energy of most semiconductor materials, exciton polaritons can typically only be realized at low temperatures (~4 K), which limits their room-temperature applications. Therefore, there is an urgent need to develop a platform capable of generating exciton polaritons at room temperature to facilitate both fundamental research and practical applications.

In this work, leveraging the room-temperature stability of excitons in monolayer WS₂ and the ultrahigh quality factor of bound states in the continuum (BICs) in photonic crystals, we successfully designed and fabricated a composite device consisting of a substrate-free SiN_x one-dimensional grating and a monolayer WS₂, demonstrating room-temperature generation of exciton polaritons. By measuring the momentum-resolved photoluminescence spectrum of the device, we obtained the exciton-polariton dispersion curve and observed a clear mode anticrossing between the dispersionless WS₂ excitons and the highly dispersive cavity photons. This unambiguously confirms the formation of distinct upper and lower polariton branches, evidencing strong coupling between monolayer WS₂ excitons and cavity photons. By applying a coupled-oscillator model to the experimental dispersion, we extracted an exciton–photon coupling strength of ~17.8 meV and a Rabi splitting as high as ~34.6 meV, which rigorously satisfy the strong coupling criteria.

In conclusion, we successfully achieved room-temperature strong coupling between WS₂ excitons and cavity photons by harnessing the ultrahigh quality factor of bound states in the continuum within a suspended microcavity. This novel suspended microcavity design with an ultrahigh quality factor effectively eliminates substrate-induced energy dissipation, thereby fully preserving the intrinsically large oscillator strength of the excitons. Ultimately, this architecture provides a robust foundation for integrating van der Waals materials into room-temperature quantum and polaritonic devices, and paves the way for room-temperature observation of macroscopic quantum phenomena such as polariton Bose-Einstein condensation as well as the development of ultra-low-threshold coherent light sources.