  We present an experimental study on the ignition and decay of a polariton
optical parametric oscillator (OPO) in a semiconductor microcavity pillar. The
combination of a continuous wave laser pump, under quasi-phase matching
conditions, and a non-resonant, 2 ps-long pulse probe allows us to obtain the
full dynamics of the system. The arrival of the probe induces a blue-shift in
the polariton emission, bringing the OPO process into resonance with the pump,
which triggers the OPO-process. We time-resolve the polariton OPO signal
emission for more than 1 nanosecond in both real and momentum-space. We fully
characterize the emission of the OPO signal with spectral tomography
techniques. Our interpretations are backed up by theoretical simulations based
on the 2D coupled Gross-Pitaevskii equation for excitons and photons.
