Reversible switching of ultrastrong light-molecule coupling

Strong coupling of light and matter can give rise to a multitude of exciting physical effects [1]. Following the first observation of strong coupling between a cavity and dye molecules [2], organic molecules have been increasingly used for the study of strong coupling, since their large transition dipole moment permits the observation of Rabi splitting in the range of a few hundreds of meV at room temperature. However, organic molecules, as we show here, can also be used for combining light-matter interaction with molecular functionality [3], and for the study of strong coupling at the interface between physics and chemistry [4]. Remarkable, the light-molecule interaction in our system enters the regime of ultrastrong coupling [5], where the energy splitting is substantial fraction of the coupled transition energies, and new phenomena such squeezed vacuum state and generation of entangled photon pairs are predicted.