We experimentally demonstrate a supercurrent-assisted, hotspot-formation mechanism for ultrafast detection and counting of visible and infrared photons. A photon-induced hotspot leads to a temporary formation of a resistive barrier across the superconducting sensor strip and results in an easily measurable voltage pulse. Subsequent hotspot healing in ∼30 ps time frame, restores the superconductivity (zero-voltage state), and the detector is ready to register another photon. Our device consists of an ultrathin, very narrow NbN strip, maintained at 4.2 K and current-biased close to the critical current. It exhibits an experimentally measured quantum efficiency of ∼20% for 0.81 μm wavelength photons and negligible dark counts.

We report our studies on spectral sensitivity of meander-type, superconducting NbN thin-film single-photon detectors (SPDs), characterized by GHz counting rates of visible and near-infrared photons and negligible dark counts. Our SPDs exhibit experimentally determined quantum efficiencies ranging from ∼0.2% at the 1.55 μm wavelength to ∼70% at 0.4 μm. Spectral dependences of the detection efficiency (DE) at the 0.4 to 3.0-μm-wavelength range are presented. The exponential character of the DE dependence on wavelength, as well as its dependence versus bias current, is qualitatively explained in terms of superconducting fluctuations in our ultrathin, submicron-width superconducting stripes. The DE values of large-active-area NbN SPDs in the visible range are high enough for modern quantum communications.

The paper reviews the main aspects of nonequilibrium hot-electron phenomena in superconductors and various theoretical models developed to describe the hot-electron effect. We discuss implementation of the hot-electron avalanche mechanism in superconducting radiation sensors and present the most successful practical devices, such as terahertz mixers and direct intensity detectors, for far-infrared radiation. Our presentation also includes the novel approach to hot-electron quantum detection implemented in superconducting x-ray to optical photon counters.

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.

We report our femtosecond time-resolved measurements on the photoresponse of an epitaxial YBa 2 Cu 3 O 7Ϫx ͑YBCO͒ thin-film photodetector, patterned into a microbridge geometry. By varying the current-voltage biasing conditions between the superconducting and resistive ͑hot spot͒ states, we observed transients that correspond to the nonequilibrium kinetic-inductance and the nonequilibrium electron-heating response mechanisms, respectively. The two-temperature model and the Rothwarf-Taylor theory have been used to simulate the measured wave forms and to extract the temporal parameters. The electron thermalization time and the electron-phonon energy relaxation time were determined by the electron temperature rise and decay times, which were found to be 0.56 and 1.1 ps, respectively, in the resistive state. We have also measured the ratio between the phonon and electron specific heats to be 38, which corresponds to a phonon-electron scattering time of 42 ps. No phonon-trapping effect ͑typical for low-temperature superconductors͒ was observed in YBCO, in the superconducting state, so the quasiparticle lifetime was given by the quasiparticle recombination time, estimated from the Rothwarf-Taylor equations to be below 1 ps.

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PACS. 78.47.+p -Time-resolved optical spectroscopies and other ultrafast optical measurements in condensed matter. PACS. 74.72.-h -High-Tc compounds. PACS. 74.76.Bz -High-Tc films.Abstract. -Using a femtosecond pump-probe optical technique, we measured the time-resolved change in reflectivity of three single-layer cuprate samples, including Bi2Sr2−xLaxCuO6+z, La2CuO4+y and La2−xSrxCuO4 grown on SrTiO3. We analyze the data in terms of a nonthermal electron relaxation model, and obtain quantitative agreement between the model and the data. The initial electron-electron thermalization process is much longer in the superconducting samples as opposed to the overdoped nonsuperconducting sample, suggesting a bottleneck in electron thermalization process due to the presence of the superconducting gap.c EDP Sciences