Picosecond Photoresponse of High-Tc Superconductor Thin Films

Abstract

<p>The origin of the fast photoresponse observed below Tc from current-biased, epitaxial YBa₂CU₃O₇_₈ (YBCO) thin films has been a source of controversy for several years now. The photoresponse signals have been attributed to either bolometric (thermal or equilibrium) or nonbolometric (nonthermal or nonequilibrium) mechanisms. A variety of nonbolometric mechanisms have been proposed to explain the fast photoresponse transients, such as nonequilibrium electron heating and transient flux dynamics.</p> <p>We have studied the fast photoresponse of epitaxial YBCO thin film bridge structures using 100 ps and 5 ps laser pulses. Both fast and slow voltage transients were observed. In the resistive transition region, a slow component was seen in the photoresponse wave form with a decay time of several nanoseconds which could be attributed to a resistive bolometric response. However, below the resistive transition region and well into the superconducting state, we have observed for the first time fast voltage transients with subnanosecond widths that were not followed by a slow resistive component. The fast transients cannot be explained by a simple resistive bolometric response.</p> <p>We have developed what is called the kinetic inductive bolometric (KIB) model to explain the origin of the fast photoresponse signals. The KIB model assumes equilibrium heating of the film by the laser pulse, contrary to some of the nonbolometric mechanisms that have been proposed. We show that the KIB model provides excellent qualitative and reasonable quantitative agreement with the observed photoresponse using 100 ps and 5 ps laser pulses. With 5 ps laser pulses, we have observed voltage transients as fast as 16 ps wide from a 200 nm YBCO film. To our knowledge, this is the fastest photoresponse signal reported to date (September, 1994) from epitaxial YBCO thin films. The implications that the success of the KIB model has on the nature of superconductivity in the high-Tc materials is also discussed.</p>