Room-Temperature Exciton Polaritons in a Monolayer Molecular Crystal

Strong coupling between excitons and photons in optical microcavities leads to the formation of exciton polaritons, which maintain both the coherence of light and the interaction of matter. Recently, atomically thin monolayer semiconductors with a large exciton oscillator strength and high exciton binding energy have been widely used for realizing room-temperature exciton polaritons. Here, we demonstrated room-temperature exciton polaritons with a monolayer molecular crystal. The molecular monolayers behave as J-aggregates with comparable oscillator strength and narrow line width as inorganic monolayers, enabling exciton–photon strong coupling at the monolayer limit. Moreover, the coupling strength can be tuned systematically via engineering the in-plane polarization or by using a vertical stack of multiple molecular monolayers. Our research provides a new material platform for realizing strong light–matter interactions inside optical microcavities at room temperature and may motivate the development of molecular-crystal-based exciton-polaritonic devices with novel functions and new possibilities.