We report our femtosecond time-resolved measurements on the photoresponse of an epitaxial YBa 2 Cu 3 O 7Ϫx ͑YBCO͒ thin-film photodetector, patterned into a microbridge geometry. By varying the current-voltage biasing conditions between the superconducting and resistive ͑hot spot͒ states, we observed transients that correspond to the nonequilibrium kinetic-inductance and the nonequilibrium electron-heating response mechanisms, respectively. The two-temperature model and the Rothwarf-Taylor theory have been used to simulate the measured wave forms and to extract the temporal parameters. The electron thermalization time and the electron-phonon energy relaxation time were determined 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 measured 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 superconductors͒ was observed in YBCO, in the superconducting state, so the quasiparticle lifetime was given by the quasiparticle recombination time, estimated from the Rothwarf-Taylor equations to be below 1 ps.

Photoresponse signals with widths as short as 1.5 ps are observed from epitaxial YBa2Cu3O7−δ  thin films using electro-optic sampling techniques. Voltage transients less than 2 ps wide are seen in 100- and 200-nm films exposed to 150-fs laser pulses and cooled to 79 K. At low bias currents, the amplitude of the fast response varies linearly with the bias current, suggesting a kinetic inductive mechanism. A negative transient about 15-ps long is also seen that may provide evidence for nonequilibrium recombination of excited quasiparticles into Cooper pairs. At high bias currents or large laser fluences, a fast tail with a decay time of about 10 ps appears in the response followed by a slow, resistive bolometric component due to sample heating. Nonequilibrium aspects of the photoresponse and the origin of the fast tail are discussed.

We have investigated the dc current–voltage characteristic of high temperature superconducting microbridges. When a dc voltage is applied to a microbridge, it switches to a lossy state due to the formation of a hotspot in the bridge. We have measured the length and temperature of the hotspot as a function of the applied voltage, and have developed a thermal model to explain its steady state behavior. The hotspot has a flat-topped temperature profile, with the maximum temperature independent of the applied voltage. The length of the hotspot, and hence the bridge resistance, increases linearly with the applied bias, so the current is independent of the applied voltage once switching has occurred.

We report our femtosecond time-resolved measurements of the photoresponse of microbridges in YBa2Cu3O70x (YBCO) thin films, performed using an electrooptic sampling technique. Our test structures consisted of 5-m-wide, 7-m-long microbridges, incorporated in 4-mm-long coplanar waveguides, fabricated in 100-nm-thick, high-quality epitaxial YBCO films grown on LaAlO3 substrates by laser deposition. When varying the biasing conditions between the superconducting and switched states, we observed transients of single-picosecond duration that corresponded to the nonequilibrium kinetic-inductance and the electron-heating response mechanisms, respectively. In both cases, experimental waveforms could be accurately simulated using a nonequilibrium (two-temperature) electron-heating model. From the fits, the YBCO intrinsic temporal parameters associated with the nonequilibrium conditions were extracted. The electron thermalization time was found to be 0.56 ps in the state above the material's critical temperature (T c = 89 K) and 0.9 6 0.1 ps in the superconducting state at temperatures ranging from 20 to 80 K. The electron-phonon energy relaxation time was found to be 1.1 ps. The single-picosecond pulse distortion due to propagation on a YBCO coplanar waveguide was also studied. Our results show that a YBCO microbridge can intrinsically operate as a photodetector at rates exceeding 100 Gb/s, making it useful as an optical-to-electrical transducer for optoelectronic interfaces in YBCO digital electronics. Simultaneously, YBCO mixers, based on hot-electron effects, should exhibit an intrinsic bandwidth exceeding 100 GHz.