Room-Temperature Exciton Polaritons in a Monolayer\nMolecular Crystal

Strong\ncoupling between excitons and photons in optical microcavities\nleads to the formation of exciton polaritons, which maintain both\nthe coherence of light and the interaction of matter. Recently, atomically\nthin monolayer semiconductors with a large exciton oscillator strength\nand high exciton binding energy have been widely used for realizing\nroom-temperature exciton polaritons. Here, we demonstrated room-temperature\nexciton polaritons with a monolayer molecular crystal. The molecular\nmonolayers behave as J-aggregates with comparable oscillator strength\nand narrow line width as inorganic monolayers, enabling exciton–photon\nstrong coupling at the monolayer limit. Moreover, the coupling strength\ncan be tuned systematically via engineering the in-plane polarization\nor by using a vertical stack of multiple molecular monolayers. Our\nresearch provides a new material platform for realizing strong light–matter\ninteractions inside optical microcavities at room temperature and\nmay motivate the development of molecular-crystal-based exciton-polaritonic\ndevices with novel functions and new possibilities.