Time-resolved infrared spectroscopy of superconducting NbTiN films*

Time-resolved, pump-probe measurements of superconducting thin NbTiN films were performed at the National Synchrotron Light Source, Brookhaven National Laboratory. Near-infrared Ti:sapphire laser pulses break Cooper pairs, producing an excess of non-thermal quasiparticles. The recombinations of these excess quasiparticles are probed by time-synchronized, far-infrared, synchrotron pulses, with a time resolution of order 200 picoseconds. The main process probed is the bottleneck between gap-edge quasiparticles and excess 2$\Delta $ phonons. (The phonons, generated by recombination of quasiparticles into Cooper pairs, are pairbreaking, producing gap-edge quasiparticles.) We will report the temperature, magnetic field, and laser fluence dependence of the spectrum-averaged far-infrared photoinduced transmission and reflection. We will also report the changes in the photoinduced far-infrared transmission spectrum.