Thermalizationkinetics of light: Observingthe lasingto photonBose-Einsteincondensationcrossover Julian Schmitt, Tobias Damm, David Dung, Frank Vewinger, Jan Klaers, and Martin Weitz Institut für AngewandtePhysik, Universität Bonn, Wegelerstraße 8, 53115 Bonn, Germany Thermal equilibrium, the state of minimum free energy for a system coupledtoa thermal bath, is a cornerstone of statistical mechanics, with its predictions includingself-organisationprocesses at a phase transition 1 . One intriguingexample is Bose-Einsteincondensation, where a gas of bosons macroscopically occupies the groundstate belowa critical temperature. This phenomenonhas beenobservedwith coldatomic gases 2 , quasiparticles insolidstate systems as polaritons 3,4 , andmore recently alsowithphotons 5 . Ingeneral however, drivingand loss canforce a system away from thermal equilibrium 6-10 . The perhaps most prominent example for the emergence of anorderedstate in nonequilibrium physics is the laser, withmacroscopically occupied modes of arbitrary energy 11 . Here we report the observationof a crossover betweenBose-Einsteincondensationandlasingoperation, by insitumonitoringof the photonkineticsina dye- flledmicrocavity. Whenthe thermalizationof the light tothe dye temperature by repeated absorptionandre-emissionis faster thanphotonloss inthe cavity, photons accumulate at low-energy states near the cavity low-frequency cutofand form a photonBose-Einsteincondensate. The thermalization of the photongas andthe evolutionfrom nonequilibrium initial distributions tocondensationis monitoredinreal-time. Incontrast, if photons leave the cavity before they thermalize, the device operates as a