Stimulated Cooling of a Frenkel Exciton-Polariton Bose-Einstein Condensate at Room Temperature.

Exciton-polaritons have been widely studied for their ability to form a lasing state at sufficiently high excitation density. These states are typically described in terms of Bose-Einstein condensation, but in driven-dissipative systems, they achieve thermal equilibrium only in exceptional cases. Explicit consideration of the nonequilibrium nature of polariton condensates suggests that under incoherent pumping, they can achieve thermalization below the bath temperature. We provide experimental support for this effect in a simple Fabry-Pérot architecture exploiting the unique physical characteristics of organic semiconductors. We find that the effective temperature and chemical potential of the polariton gas can be tuned by pumping rate, enabling significantly sub-bath cooling of the polariton population. This effect occurs spontaneously, with no need for external potentials, and arises from the rapid thermalization and confined Frenkel excitons of the organic active layer. Furthermore, we show that exciton leakage through the cavity arising from the large-bandwidth emission of conventional organic semiconductors provides a unique probe of the underlying exciton photophysics, such as the efficiency of bosonic stimulation, inaccessible to semiconductors with narrow emission line widths.