  We experimentally explore the dynamic optical hysteresis of a semiconductor
microcavity as a function of the sweep time. The hysteresis area exhibits a
double power law decay due to the shot noise of the driving laser, which
triggers switching between metastable states. Upon increasing the average
photon number and approaching the thermodynamic limit, the double power law
evolves into a single power law. This algebraic behavior characterizes a
dissipative phase transition. Our findings are in good agreement with
theoretical predictions, and the present experimental approach is promising for
the exploration of critical phenomena in photonic lattices.
