Origin of the fast photoresponse of epitaxial YBa2Cu3O7-δ thin films

We have measured the photoresponse of current-biased bridge structures of epitaxial YBa2Cu3O7-δ thin films on LaAlO3 using 100 ps, 532 nm laser pulses. Voltage transients with fast and slow components were observed below Tc. The amplitude of the slow component agrees with a resistive bolometric response where the laser pulse heats the bridge into the resistive transition region. The decay time of the slow component is consistent with a thermal time constant for heat loss out of the film governed by the thermal boundary resistance at the interface between the film and the substrate. We show that the fast component can be explained by changes in the kinetic inductance of the bridge due to temperature-induced changes in the superfluid density from heating of the bridge by the laser pulse. Our interpretation of the origin of the fast component, therefore, is bolometric in nature over the time scale of the laser pulse contrary to some of the nonequilibrium or nonbolometric mechanisms that have been proposed. A simulation of this kinetic inductive bolometric effect provides reasonable agreement with the temperature dependence of the observed photoresponse, and a comparison is made between the various postulated temperature dependencies of the superfluid fraction in YBa2Cu3O7-δ. The simulation further shows that the speed of the kinetic inductive bolometric photoresponse is not limited by the thermal escape time out of the film, but is determined by the rate at which the incident laser pulse initially heats the film and is therefore the same duration in time as the laser pulse. Evidence for negative voltage transients observed in the photoresponse data consistent with the simulation results is also presented.