Quasiparticle recombination in hotspots in superconducting current-carrying nanowires

We describe a kinetic model of recombination of nonequilibrium quasiparticles generated by single photon absorption in superconducting current-carrying nanowires. The model is developed to interpret two-photon detection experiments in which a single photon does not possess sufficient energy for breaking superconductivity at a fixed low bias current. We show that quasiparticle self-recombination in relaxing hotspots dominates diffusion expansion effects and explains the observed strong bias current, wavelength, and temperature dependencies of hotspot relaxation in tungsten silicide superconducting nanowire single-photon detectors.