Picosecond hot-electron energy relaxation in NbN superconducting photodetectors

We report time-resolved characterization of superconducting NbN hot-electro n photodetectors using an electro-optic sampling method. Our samples were p atterned into micron-size microbridges from 3.5-nm-thick NbN films deposite d 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, tw o-temperature model. The quasiparticle thermalization time was ambient temp erature independent and was measured to be 6.5 ps. The inelastic electron-p honon scattering time tau(e-ph) tended to decrease with the temperature inc rease, although its change remained within the experimental error, while th e phonon escape time tau(es) decreased almost by a factor of two when the s ample was put in direct contact with superfluid helium. Specifically, tau(e -ph) and tau(es), fitted by the two-temperature model, were equal to 11.6 a nd 21 ps at 2.15 K, and 10(+/- 2) and 38 ps at 10.5 K, respectively. The ob tained value of tau(e-ph) shows that the maximum intermediate frequency ban dwidth of NbN hot-electron phonon-cooled mixers operating at T-C can reach 16(+4/-3) GHz if one eliminates the bolometric phonon-heating effect. (C) 2 000 American Institute of Physics. [S0003-6951(00)02419-0].