We report time-resolved characterization of superconducting NbN hot-electron photodetectors using an electro-optic sampling method. Our samples were patterned into micron-size microbridges from 3.5-nm-thick NbN films deposited on sapphire substrates. The devices were illuminated with 100 fs optical pulses, and the photoresponse was measured in the ambient temperature range between 2.15 and 10.6 K ͑superconducting temperature transition T C). The experimental data agreed very well with the nonequilibrium hot-electron, two-temperature model. The quasiparticle thermalization time was ambient temperature independent and was measured to be 6.5 ps. The inelastic electronphonon scattering time e-ph tended to decrease with the temperature increase, although its change remained within the experimental error, while the phonon escape time es decreased almost by a factor of two when the sample was put in direct contact with superfluid helium. Specifically, e-ph and es , fitted by the two-temperature model, were equal to 11.6 and 21 ps at 2.15 K, and 10 (Ϯ2) and 38 ps at 10.5 K, respectively. The obtained value of e-ph shows that the maximum intermediate frequency bandwidth of NbN hot-electron phonon-cooled mixers operating at T C can reach 16(ϩ4/Ϫ3) GHz if one eliminates the bolometric phonon-heating effect.
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