  Recent approximate analytical work has suggested that, at certain values of
the external pump, the optical parametric oscillator (OPO) regime of
microcavity polaritons may provide a long sought realisation of
Kardar-Parisi-Zhang (KPZ) physics in 2D. Here, by solving the full microscopic
model numerically using the truncated Wigner method, we prove that this
predicted KPZ phase for OPO is robust against the appearance of vortices or
other effects. For those pump strengths, spatial correlations in the direction
perpendicular to the pump, and the distribution of phase fluctuations, match
closely to the forms characteristic of the KPZ universality. This strongly
indicates the viability of observing KPZ behaviour in future polariton OPO
experiments.
