Picosecond hot-electron energy relaxation in NbN superconducting photodetectors

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 $TC).$ 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 electron–phonon scattering time $τe–ph$ tended to decrease with the temperature increase, although its change remained within the experimental error, while the phonon escape time $τes$ decreased almost by a factor of two when the sample was put in direct contact with superfluid helium. Specifically, $τe–ph$ and $τes,$ fitted by the two-temperature model, were equal to 11.6 and 21 ps at 2.15 K, and $10(±2)$ and 38 ps at 10.5 K, respectively. The obtained value of $τe–ph$ shows that the maximum intermediate frequency bandwidth of NbN hot-electron phonon-cooled mixers operating at $TC$ can reach $16(+4/−3)$ GHz if one eliminates the bolometric phonon-heating effect.