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The piston problem, i.e. the dynamics in a uniform gas at rest under theaction of a moving piston is fundamental problem of physics and a canonicalcase study in shock wave physics. We investigate theoretically andexperimentally the analogous problem for a photon fluid, which turns out to bestrongly influenced by the dispersive character of the problem. The experimentmakes use of a fiber optics setup where an all-optically controlled quasiinstantaneous change of frequency of the input light mimics the piston action.We show that the flow exhibits phase transitions which involve cross-over fromregimes characterized by 2-shocks (pushing piston) to 2-rarefaction waves(retracting piston), with the appearance of cavitating states at criticalamplitudes of the jump. Importantly, the appearance of vacuum points into the2-shock marks the transition to a regime which is unique to the photon fluid.Our observations allow for the extensive and quantitative test of the state ofthe art description of the dispersive Riemann problem, i.e. Whitham modulationtheory, applied to the universal defocusing nonlinear Schr\"odinger equation.

Phase transitions of photon fluid flows driven by a virtual all-optical piston