Frank A. Hegmann

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.

Picosecond photoresponse of epitaxial YBa2Cu3O7−δ thin films

Photoresponse signals as fast as 16 ps (full width at half‐maximum) have been observed from current‐biased bridge structures of epitaxial YBa2Cu3O7−δ thin films on LaAlO3 using 5 ps, 820 nm laser pulses. Operating at liquid‐nitrogen temperature (77.4 K), the amplitude of the fast response was found to be linear with current at low bias currents. At higher bias currents, a slow component appeared in the signal with a decay over several nanoseconds which could be attributed to a resistive bolometric response. Fast transients about 20 ps wide have been observed in films with thicknesses ranging from 47 to 200 nm. We believe the fast response is primarily due to a kinetic inductive bolometric mechanism associated with heating of the film by the laser pulse. There is some evidence of a nonbolometric contribution but it is not yet conclusive.

Identification of nonbolometric photoresponse in YBa2Cu3O7−δ thin films based on magnetic field dependence

Optically induced transient voltages across a current‐biased bridge structure fabricated from an epitaxial YBa2Cu3O7−δ thin film are reported. A novel technique consisting of applying a small magnetic field perpendicular to the sample was used to identify any bolometric contribution to the photoresponse. Comparison to resistance versus temperature curves with and without the field establishes a slow component in the photoresponse as bolometric and unambiguously identifies a fast component as a nonthermal transient.

High-speed kinetic inductive bolometric photoresponse of epitaxial YBa2Cu3O7-δ thin films

A general trend is observed in the photoresponse of current biased epitaxial YBa2Cu3O7-(delta ) thin film bridge structures exposed to picosecond laser pulses. Both fast and slow components are seen in the photoresponse near the transition region. The slow component, which has a decay over several nanoseconds, is a resistive bolometric response due to heating of the film by the laser pulse in the resistive transition region. At lower temperatures, only the fast component is observed with an amplitude which is linear with bias current. The fast component has been observed in films ranging in thickness from 30 nm to 260 nm. Using 100 ps, 532 nm laser pulses, the origin of the fast component can be explained by a kinetic inductive bolometric response where the superfluid fraction is rapidly decreased by the laser pulse heating the bridge. Recent results using 5 ps, 820 nm laser pulses on samples maintained at liquid nitrogen temperature (77.4 K) in a high speed measurement setup have revealed fast components in the photoresponse as short as 16 ps full width at half maximum. To our knowledge, this is the fastest photoresponse signal observed to date from YBa2Cu3O7-(delta ) thin films. A large portion of this fast response can be attributed to a kinetic inductive bolometric response. The possibility of a nonbolometric component over this short time scale is discussed.