K. Il’in

Tantalum nitride superconducting single-photon detectors with low cut-off energy

Materials with a small superconducting energy gap are expected to favor a high detection efficiency of low-energy photons in superconducting nanowire single-photon detectors. We developed a TaN detector with smaller gap and lower density of states at the Fermi energy than in comparable NbN devices, while other relevant parameters remain essentially unchanged. The observed reduction of the minimum photon energy required for direct detection is in line with model predictions of ≈1/3 as compared to NbN.

Fluctuation effects in superconducting nanostrips

Superconducting fluctuations in long and narrow strips made from ultrathin NbN films, have been investigated. For large bias currents close to the critical current fluctuations led to localized, temporary transitions into the normal conducting state, which were detected as voltage transients developing between the strip ends. We present models based on fluctuations in the Cooper pair density and current-assisted thermal-unbinding of vortex-antivortex pairs, which explain the current and temperature dependence of the experimental fluctuation rates.

An energy-resolving superconducting nanowire photon counter

We report on the energy-resolving capability of a superconducting NbN nanowire photon counter, which is read out by a superconducting quantum interference device. For counters operated at 6.5 K, a resolution of 0.55 eV was measured in the wavelength range from 1000 to 1500 nm (photon energies 1.2–0.8 eV) along with a counting rate of 2 MHz. The best energy resolution occurred in the spectral range where the quantum efficiency of the counter began to decrease with the wavelength. The results are explained by the change of the detection scenario from the hot-spot formation to unbinding of vortex–antivortex pairs.

Detection mechanism of superconducting nanowire single-photon detectors

In this paper we intend to give a comprehensive description of the current understanding of the detection mechanism in superconducting nanowire single-photon detectors. We will review key experimental results related to the detection mechanism, e.g. the variations of the detection probability as a function of bias current, temperature or magnetic field. Commonly used detection models will be introduced and we will analyze their predictions in view of the experimental observations. Although none of the proposed detection models is able to describe all experimental data, it is becoming increasingly clear that vortices are essential for the formation of the initial normal-conducting domain that triggers a detection event.

Evidence of non-bolometric mixing in the bandwidth of a hot-electron bolometer

The gain bandwidth of a superconducting NbN hot-electron mixer was measured at local oscillator (LO) frequencies 2.5, 0.6 and 0.3 THz and compared to values from presently known bolometric mixer models. At 2.5 THz variations of the bandwidth with the LO power agree rather with the hot-spot mixer model than with any of the homogeneous bolometric models, whereas an increase of the bandwidth at low LO frequencies plausibly evidences non-bolometric direct interaction of magnetic vortices with the radiation field.

Adjustment of self-heating in long superconducting thin film NbN microbridges

The self-heating in long superconducting microbridges made from thin NbN films deposited on top of high silicon mesa structures was studied by analyzing the hysteresis current density j(H). We observed a more than twofold decrease of j(H) with increase in the ratio of the height of the Si mesa, h, to the width of the microbridge, W, from 0 to 24. We describe our experimental results using one-dimensional thermal balance equations taking into account disordered matter in our thin NbN films and limitations imposed on the phonon mean free path by the width of the Si mesa. In the framework of this model we obtain a good agreement between theory and experiment over a wide temperature range from 4.2 K up to the critical temperature T-C for all h/W ratios.