A novel material deposited by molecular beam epitaxy at low substrate temperatures using Ga and As4 beam fluxes has been used as the active layer for a high-speed photoconductive optoelectronic switch. The high-speed photoconductive performance of the material was assessed by fabricating two devices: an Auston switch and a photoconductive-gap switch with a coplanar transmission line. In a coplanar transmission line configuration, the speed of response is 1.6 ps (full width at half maximum) and the response is 10 to 100 times greater than that of conventional photoconductive switches. Since the material is compatible with GaAs discrete device and integrated circuit technologies, this photoconductive switch may find extensive applications for high-speed device and circuit testing.
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.
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