Photoresponse of Nb films; observation of biexponential recovery times of the superconducting state

The authors present a systematic study of the photoresponse of superconducting Nb films over ranges of fluence and bias current and for temperatures of 6.5 K<T<300 K. The response of thin niobium films to fast pulses of 665-mn laser light was studied over a temperature range that spans the superconducting transition temperature. Below T_c, the results are consistent with those of L.R. Testardi (1971), but the improved time resolution obtained in the present work (10 ps compared with 0.1 μs) has made it possible to resolve more subtle features of the response. At very low fluences, the quasi-particle recombination time τ_r is measured in the range of reduced temperature T/T_c from 0.75 to 0.95; τ_r is so short in Nb that it has previously been measured only once, by M. Johnson and R.H. Silsbee (1987). That experiment used a branch imbalance diffusion length measurement and unexpectedly observed a biexponential decay. The present measurement agrees with that result, and the fast decay time is used to obtain a value for τ_r, winch agrees with theory. At higher fluences, the response scales in a manner consistent with a T* model. Above T_c, a small signal of unknown origin is observed and is compared with the photoresponses of a gold and nickel film. The results on a single YBa₂Cu₃O₇ sample are briefly compared with those of Nb