Direct photoluminescence observation of the negative Bogoliubov branch in an exciton-polariton condensate

Bose-Einstein condensates exhibit fascinating behavior such as superfluidity and vortex formation, effects owing their presence to self-interactions between the underlying bosons [1]. In exciton-polariton condensates another peculiar effect has been theoretically predicted, where the negative energy Bogoliubov dispersion becomes observable in the photoluminescence (PL) spectrum [2-5]. The effect originates from the anomalous mixing of creation and destruction operators of polaritons, which results in the creation of a Bogoliubov particle when the PL is observed. Here we directly observe the PL of the negative Bogoliubov dispersion branch for the first time in a high density regime far above the exciton-polariton condensation threshold. The observation is made possible by strong coupling being maintained at densities corresponding to ~ 100 times the threshold density for condensation. The presence of strong coupling at such densities is evidenced by second-order coherence and PL measurements showing clear differences to photon lasing, a weak coupling phenomenon. Comparison of the negative dispersion PL spectrum shows good agreement to theoretical predictions.