  Fluorescent proteins (FPs) have recently emerged as a serious contender for
realizing ultralow threshold room temperature exciton-polariton condensation
and lasing. Our contribution investigates the thermalization of FP microcavity
exciton-polaritons upon optical pumping under ambient conditions. We realize
polariton cooling using a new FP molecule, called mScarlet, coupled strongly to
the optical modes in a Fabry Perot cavity. Interestingly, at the threshold
excitation energy (fluence) of ~ 9 nJ/pulse (15.6 mJ/cm2), we observe an
effective temperature, Teff ~ 350 +/- 35 K close to the lattice temperature
indicative of strongly thermalized exciton-polaritons at equilibrium. This
efficient thermalization results from the interplay of radiative pumping
facilitated by the energetics of the lower polariton branch and the cavity Q
factor. Direct evidence for dramatic switching from an equilibrium state into a
metastable state is observed for the organic cavity polariton device at room
temperature via deviation from the Maxwell-Boltzmann statistics at k = 0 above
the threshold. Thermalized polariton gases in organic systems at equilibrium
hold substantial promise for designing room temperature polaritonic circuits,
switches, and lattices for analog simulation.
