Plasmon-exciton polariton condensation set by quasi-long range order and nonlinearities

Bose-Einstein Condensation of exciton-polaritons, quasi-particles formed by strong coupling of cavity photons and excitons, have been a subject of intensive study. The appearance of the long-range order and extended spatial coherence is one of the underlying properties of Bose-Einstein condensates. We use a lattice of metal nanoparticles supporting propagating modes, which can couple with the electronic transitions of organic molecules. Above the condensation threshold, we observe the formation of extended spatial coherence over distances longer than the excitation spot. This extension can be explained by the presence of the repulsive interactions among the exciton-polaritons within the condensate. We support this hypothesis with time-resolved photoluminescence measurements of the condensate, observing an initial blue-shift consequence of exciton-polariton interactions, followed by a red-shift at later times as a consequence of the reduction of the density of exciton-polaritons.