Ilya Charaev

Characteristics of superconducting tungsten silicide WxSi1-x for single photon detection

Superconducting properties of three series of amorphous WxSi1−x films with different thickness and stoichiometry were investigated by dc transport measurements in a magnetic field up to 9 T. These amorphous WxSi1−x films were deposited by magnetron cosputtering of the elemental source targets onto silicon substrates at room temperature and patterned in the form of bridges by optical lithography and reactive ion etching. Analysis of the data on magnetoconductivity allowed us to extract the critical temperatures, superconducting coherence lengths, magnetic penetration depths, and diffusion constants of electrons in the normal state as functions of film thickness for each stoichiometry. Two basic time constants were derived from transport and time-resolving measurements. A dynamic process of the formation of a hotspot was analyzed in the framework of a diffusion-based vortex-entry model. We used a two-stage diffusion approach and defined a hotspot size by assuming that the quasiparticles and normal-state electrons have the same diffusion constant.With this definition and these measured material parameters, the hotspot in the 5-nm-thickW0.85Si0.15 film had a diameter of 107 nm at the peak of the number of nonequilibrium quasiparticles.

Physical mechanisms of timing jitter in photon detection by current-carrying superconducting nanowires

We studied timing jitter in the appearance of photon counts in meandering nanowires with different fractional amount of bends. Timing jitter, which is the probability density of the random time delay between photon absorption in current-carrying superconducting nanowire and appearance of the normal domain, reveals two different underlying physical scenarios. In the deterministic regime, which is realized at large currents and photon energies, jitter is controlled by position dependent detection threshold in straight parts of meanders and decreases with the current. At small photon energies, jitter increases and its current dependence disappears. In this probabilistic regime jitter is controlled by Poisson process in that magnetic vortices jump randomly across the wire in areas adjacent to the bends.

Frequency-multiplexed bias and readout of a 16-pixel superconducting nanowire single-photon detector array

We demonstrate a 16-pixel array of microwave-current driven superconducting nanowire single-photon detectors with an integrated and scalable frequency-division multiplexing architecture, which reduces the required number of bias and readout lines to a single microwave feed line. The electrical behavior of the photon-sensitive nanowires, embedded in a resonant circuit, as well as the optical performance and timing jitter of the single detectors is discussed. Besides the single pixel measurements, we also demonstrate the operation of a 16-pixel array with a temporal, spatial, and photon-number resolution.

Asymmetry in the effect of magnetic field on photon detection and dark counts in bended nanostrips

Current crowding in the bends of superconducting nanostructures not only restricts measurable critical current in such structures, but also redistributes local probabilities for the appearance of dark and light counts. Using structures in the form of a square spiral, where all bends have the same symmetry with respect to the directions of the bias current and external magnetic field, we have shown that areas around the bends largely contribute to the rate of dark counts and to the rate of light counts at small photon energies. The minimum in the rate of dark counts reproduces the asymmetry of the maximum in the critical current as a function of the magnetic field. Contrarily, the minimum in the rate of light counts demonstrates opposite asymmetry. The rate of light counts becomes symmetric at large currents and fields. Comparison of the computed local absorption probabilities for photons and the simulated local threshold detection current reveal the areas near bends that deliver the asymmetric rate of light counts. Asymmetry in count rates is absent in circular spirals without bends.

Current dependence of the hot-spot response spectrum of superconducting single-photon detectors with different layouts

We show that avoiding bends in a current-carrying superconducting nanowire enhances the probability for low energy photons to be detected and that this enhancement is entirely due to the increase in the experimentally achievable critical current. We studied nanowires shaped as either meander or spiral. The spirals had different layouts, a double-spiral layout with an S-turn in the middle and a single-spiral layout without such turn. Nanowires were prepared from films of niobium nitride with a thickness of 5 nm. For specimens with each layout we measured the spectra of the single-photon response in the wavelength range from 400 nm to 1600 nm and defined the cut-off wavelength

Enhancement of superconductivity in NbN nanowires by negative electron-beam lithography with positive resist

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Proximity effect model of ultranarrow NbN strips

beyond which the response rolls off. The largest and the smallest

Enhancement of Critical Currents and Photon Count Rates by Magnetic Field in Spiral Superconducting Nanowire Single-Photon Detectors

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Operation of Multipixel Radio-Frequency Superconducting Nanowire Single-Photon Detector Arrays

were found for the single-spiral layout and for the meander, respectively. For all three layouts the relationship between

Magnetic-field enhancement of performance of superconducting nanowire single-photon detector.

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