Nonthermal kinetic inductance photoresponse of thin superconducting films

We report on the photoresponse of structured superconducting YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// and NbN films to femtosecond visible light pulses. The photoresponse signals obtained at temperatures below the transition temperature consist of a positive voltage peak of 25 ps width for the YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// sample and 40 ps for the NbN sample, each limited by the temporal resolution of the different setups, followed by a longer negative voltage transient. We attribute the photoresponse signals to a nonthermal change of the kinetic inductance of the superconducting films. We also present a model describing the dynamics of nonequilibrium quasiparticles (QP) and phonons, allowing to simulate our results. Our model provides the physical background for optimization of fast detectors based on the kinetic inductance photoresponse of thin superconducting films.<>