Crossover from stretched to compressed exponential recovery in a photoinduced phase transition

Upon intense femtosecond photoexcitation, a many-body system can undergo a phase transition through a nonequilibrium route, but understanding these pathways remains an outstanding challenge. In many strongly correlated systems, the photoexcited dynamics is known to occur across multiple microscopic and mesoscopic length scales, which can give rise to non-exponential behavior. Here, we use second harmonic generation to study a photo-induced phase transition in Ca3Ru2O7 and find striking non-exponential behavior of the system's recovery back to equilibrium. In particular, we observe a crossover from stretched- to compressed-exponential recovery as the photoexcitation fluence is increrased, which is suggestive of a crossover from indpendent local relaxations to a correlated recovery. We describe the possible mechanisms that may give rise to this behavior.