Evidence for parametric memory and correlation in semiconductor microcavities

We measured the dynamics of polariton parametric stimulated scattering in semiconductor microcavities, by time-resolving the amplified signal with 250 fs resolution. Our experiments demonstrate that the stimulation process is considerably delayed with respect to the arrival time of both probe and pump pulses. This effect is clearly observable in our high quality sample due to the long lifetime of the microcavity polaritons (about 10 ps), and it is in excellent agreement with the model describing the coherent conversion of two pump polaritons into a signal-idler pair. We demonstrate that the non-instantaneous character of the polariton stimulation occurs because the polariton-wave amplification requires the build up of correlated signal-idler polariton fields.