ORIGIN OF THE FAST PHOTORESPONSE OF EPITAXIAL YBA2CU3O7-DELTA THIN-FILMS

We have measured the photoresponse of current-biased bridge structures of epitaxial YBa2CU307-delta thin films on LaAlO3 using 100 ps, 532 n m laser pulses. Voltage transients with fast and slow components were observed below T(c). 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 compo nent is consistent with a thermal time constant for heat loss out of t he film governed by the thermal boundary resistance at the interface b etween the film and the substrate. We show that the fast component can be explained by changes in the kinetic inductance of the bridge due t o temperature-induced changes in the superfluid density from heating o f the bridge by the laser pulse. Our interpretation of the origin of t he fast component, therefore, is bolometric in nature over the time sc ale of the laser pulse contrary to some of the nonequilibrium or nonbo lometric mechanisms that have been proposed. A simulation of this kine tic inductive bolometric effect provides reasonable agreement with the temperature dependence of the observed photoresponse, and a compariso n is made between the various postulated temperature dependencies of t he superfluid fraction in YBa2Cu307-delta. The simulation further show s that the speed of the kinetic inductive bolometric photoresponse is not limited by the thermal escape time out of the film, but is determi ned 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. Ev idence for negative voltage transients observed in the photoresponse d ata consistent with the simulation results is also present