  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.
