Observation of long-lived polariton states in semiconductor   microcavities across the parametric threshold
  The excitation spectrum around the pump-only stationary state of a polariton optical parametric oscillator (OPO) in semiconductor microcavities is investigated by time-resolved photoluminescence. The response to a weak pulsed perturbation in the vicinity of the idler mode is directly related to the lifetime of the elementary excitations. A dramatic increase of the lifetime is observed for a pump intensity approaching and exceeding the OPO threshold. The observations can be explained in terms of a critical slowing down of the dynamics upon approaching the threshold and the following onset of the soft Goldstone mode. 
D. Ballarini, D. Sanvitto, A. Amo, L. Vina, M. Wouters, I. Carusotto,   A. Lemaitre, J. Bloch