Steven H. Moffat

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

We report our femtosecond time-resolved measurements on the photoresponse of an epitaxial YBa2Cu3O7−x (YBCO) thin-film photodetector, patterned into a microbridge geometry. By varying the current–voltage biasing conditions between the superconducting and resistive (hot spot) 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 ...

Ultrafast photoresponse in microbridges and pulse propagation in transmission lines made from high-T/sub c/ superconducting Y-Ba-Cu-O thin films

We report our femtosecond time-resolved measurements of the photoresponse of microbridges in YBa/sub 2/Cu/sub 3/O/sub 7-x/ (YBCO) thin films, performed using an electrooptic sampling technique. Our test structures consisted of 5-/spl mu/m-wide, 7-/spl mu/m-long microbridges, incorporated in 4-mm-long coplanar waveguides, fabricated in 100-nm-thick, high-quality epitaxial YBCO films grown on LaAlO/sub 3/ 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 materials critical temperature (T/sub c/=89 K) and 0.9/spl plusmn/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.

Electro‐optic sampling of 1.5‐ps photoresponse signal from YBa2Cu3O7−δ thin films

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.

Current–voltage characteristics of dc voltage biased high temperature superconducting microbridges

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.

YBa/sub 2/Cu/sub 3/O/sub 7-x/ thin-film picosecond photoresponse in the resistive state

Using a subpicosecond electro-optic sampling technique, we have characterized the photoresponse of current-biased YBa/sub 2/Cu/sub 3/O/sub 7-x/ (YBCO) photodetectors, designed as 5-/spl mu/m-wide and 7-/spl mu/m-long microbridges patterned in 100-nm-thick, high-quality epitaxial films grown on LaAlO/sub 3/ substrates by pulsed laser deposition. The bridges were centered in a coplanar waveguide structure, allowing the photogenerated pulses to be measured 20 /spl mu/m from the detector. The experiments were conducted in the temperature range between 20 and 80 K; however, the bridges were biased in the switched (resistive) state, which corresponded to a hot-spot formation at the center of the microbridge. The photoresponse from 100-fs laser pulses (395-nm wavelength) was measured to be in the form of a single spike with the width as short as 1.3 ps. The physical origin of this ultrafast response is attributed to nonequilibrium electron heating, We extracted the intrinsic temporal parameters of the YBCO response-the electron thermalization time equal to 0.56 ps and electron-phonon energy relaxation time equal to 1.1 ps, Our measurements demonstrate that a simple YBCO microbridge can operate as a >100-GHz bandwidth photodetector, e.g., as an optical-to-electrical transducer for optoelectronic interface in YBCO digital electronics.

Switching speed for controlled damping using thin film YBa2Cu3O7-δ

Abstract The yttrium barium copper oxide (YBCO) high- T c superconductor has a sharp dissipation peak at the magnetic flux phase transformation temperature, whether in the form of a single crystal, ceramic, or thin film. This peak has been proposed as the basis for a controlled damping system: if a magnet is attached to a mechanical oscillator and a fixed YBCO element is placed close to the magnet, then switching the temperature of the YBCO element from above the phase transformation temperature to the phase transformation temperature can be used to reduce the resonance Q of the mechanical oscillator by two or more orders of magnitude. It is demonstrated that, for a thin-film element, the damping can be switched on in less than 1 s; this time is limited by the thermal time constant of the YBCO element. The time to switch off the damping is governed by both the thermal time constant and the mechanical time constant of the undamped oscillator, τ = Q /πν 0 .

Correlations between critical current density and penetration depth in ion irradiated YBa/sub 2/Cu/sub 3/O/sub 7/ thin films

Point defects have been introduced into YBa/sub 2/Cu/sub 3/O/sub 7/ through low energy helium ion irradiation in order to probe the origin of dissipation in a current-carrying superconductor. Resistivity, infrared reflectance and x-ray diffraction measurements indicate that the films are not chemically altered and that the induced point defects act as scattering centres. Measured electric field-current density characteristics are found to be well described by a model based on quantum current fluctuations. This description is used to extract the change in the superconducting carrier density with ion damage which agrees well with direct measurements of the same quantity by infrared reflectance. The implications of the relation between dissipation and the superconducting carrier density, or alternatively the magnetic penetration depth, are discussed.

Picosecond photoresponse of YBa2Cu3O7−x thin films

The photoresponse of current-biased YBa2Cu3O7−x (YBCO) microbridges, patterned in 100-nm-thick, high-quality epitaxial films on LaAlO3, has been characterized by means of an electro-optic sampling technique. The photoresponse from 100-fs laser pulses (390-nm wavelength) was measured to be in the form of Gaussian-shaped pulses with the width as short as 0.8 ps. The physical origin of the photoresponse signal can be attributed to a nonequilibrium electron heating mechanism. The measured signal represents, in our opinion, the intrinsic photoresponse of YBCO.

Flux dynamics in thin films of YBa2Cu3O7−δ

An oscillator study was used to study flux dynamics in 5 laser-ablated thin films of YBa2Cu3O7−δ. The orientation was B | ab for B=1 and 3T. The low temperature response to rotation in the field was a restoring torque α B2 and independent of thickness as expected for full pinning. On increasing the temperature, the restoring torque decreased and the dissipation increased due to depinning. All samples showed a single well defined dissipation peak a few Kelvin below Tc.