Far infrared response of thin film Bi2Sr2CaCu2O8 using a free electron laser

Abstract

The far infrared response of granular thin-film Bi2Sr~.CaCu208 superconductor has been investigated using long (~ 5/ts) but sharply truncated free electron laser pulses in the fi'equency range between 50 cm -1 and 125cm -1. Under constant current bias, a fast response and a slow bolometric signal component could be identified in this energy range, which is below the BCS energy gap (~ 200 cm-1). Measurements of the power dependences of the signal voltages showed that both the fast and the thermal responses are consistent with the predictions of the resistively shunted Josephson junction model. 1660 Prettl and Lengfellner IntroductionThere has been considerable interest in understanding the optical and infrared response of high-To superconductors.In previous investigations of thin granular films a wide range of response times have been observed. Slow signals, with time constants in the range of microseconds to milliseconds, were identified as a bolometric response arising from heating of the film. Fast signals in the far infrared (FIR), with time constants of nanoseconds, were attributed to several non-thermal mechanisms: an optical destruction of wave function coherence [1], the depairing of vortex-antivortex pairs [2], an optically induced charge imbalance in neighbouring superconducting grains [3] or infrared radiation generated currents in Josephson junctions inherent in granular films [4,5]. A drastic variation of the response time with frequency were found in T12Ba2CalCu208 films using FIR molecular lasers pumped by TEA CO2 lasers [6]. Below a critical frequency uc ~ 100cm -1 only fast signals, with a time constant in the order of lns, were observed. In contrast, above uc a slow response of bolometric character occurred. The transition frequency uc was identified with the energy gap value at the surface of superconducting grains. Recently non-thermal processes have been questioned by an investigation of the thermal boundary resistance of YBa2CuaOT-x films and their substrate materials [7]. It was shown that a bolometric response may occur on a nanosecond time scale: thus thermal and nonthermal effects cannot be distinguished by the time constant alone.In the present paper we report on measurements of the FIR response of thin granular film Bi2Sr2CaCu2Os using the University of California, Santa Barbara,(UCSB) Free Electron Laser (FEL). The sample was irradiated by long laser pulses truncated at the trailing edge by a rapid optical switch. Our results demonstrate that in the FIR both fast and slow signals of very different time constants are present at high powers, indicating two distinct signal generation mechanisms. The slow signal is readily identified as a bolometric response by the non -exponential decay which follows from one dimensional heat flow through the substrate to the cold finger [8]. Both types of signals show a nonlinear dependence on intensity which may be explained by the model of an effective resistively shunted Josephson junction which has the properties of a random array of junctions [9,...