Intrinsic picosecond response times of Y-Ba-Cu-O superconducting photodetectors

We report our femtosecond time-resolved measurements on the photoresponse o f an epitaxial YBa2Cu3O7-x (YBCO) thin-film photodetector, patterned into a microbridge geometry. By varying the current-voltage biasing conditions be tween the superconducting and resistive (hot spot) states, we observed tran sients that correspond to the nonequilibrium kinetic-inductance and the non equilibrium electron-heating response mechanisms, respectively. The two-tem perature model and the Rothwarf-Taylor theory have been used to simulate th e measured wave forms and to extract the temporal parameters. The electron thermalization time and the electron-phonon energy relaxation time were det ermined by the electron temperature rise and decay times, which were found to be 0.56 and 1.1 ps, respectively, in the resistive state. We have also m easured the ratio between the phonon and electron specific heats to be 38, which corresponds to a phonon-electron scattering time of 42 ps. No phonon- trapping effect (typical for low-temperature superconductors) was observed in YBCO, in the superconducting state, so the quasiparticle lifetime was gi ven by the quasiparticle recombination time, estimated from the Rothwarf-Ta ylor equations to be below 1 ps. (C) 1999 American Institute of Physics. [S 0003-6951(99)01206- 1].