Photoresponse of the high-temperature superconductor YBa 2Cu 3O 7− δ to ultrashort 10 μm CO 2laser pulses

Abstract

We have measured the photoresponse of the high-temperature superconductor YBa 2Cu 3O 7− δ (YBCO) to pulsed infrared radiation of 10 μm wavelength and analysed its mechanisms. For this purpose, c-axis oriented YBCO films with a thickness between 130 and 280 nm were deposited on single crystal MgO (100) substrates and patterned in a meander structure. The aim of the meander structure was to increase the responsivity at temperatures below the transition temperature. These films were irradiated with 10 μm 35 ps pulses generated by an optical free induction decay (OFID) 10 μm CO 2 laser system developed in our laboratory. We have observed that the form of the transients depends on the temperature. At temperatures below the transition temperature, the photoresponse consists of a 700 ps FWHM peak followed by a slow negative transient. This is the first time that such short transients were observed with 10 μm irradiation. At temperatures above the transition temperature, the transients exhibit a decay in the nanosecond range. We have developed a bolometric model which reproduces well the measured dependence of the peak amplitude of the slow transients on the temperature. We have also investigated the dependence of the fast transients on the bias current and temperature. The observed linear dependence of the peak amplitude of the fast transients on the bias current and the negative transient following the main peak exclude flux-flow and nonequilibrium effects as photoresponse mechanisms, whilst these characteristics agree well with a kinetic-inductance model. The measured dependence of the peak amplitude on the temperature permits one to distinguish between several models for the superfluid fraction ƒ sc. Thus, we have established that the models with superfluid fractions ƒ sc= 1 − ( T/T c) 2 and ƒ sc given by the BCS theory are compatible with our results, whereas the model with ƒ sc = 1 − ( T/T c) 4 corresponding to the Gorter-Casimir two-fluid model does not match our results. The analysis of the amplitude and temporal variation of the negative transient following the fast transient yields a London penetration depth λ L(0 K) ≈ 220 nm in agreement with those determined by other techniques for superconducting films with similar resistivities and critical temperatures.