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IEEE Transactions on Applied Superconductivity

Enhancement of critical currents and photon-count rates by magnetic field in spiral superconducting nanowire single-photon detectors

Ilya Charaev, Alexei D. Semenov, R. Lusche, K. Ilin, Heinz‐Wilhelm Hübers, M. Siegel

Published Jan 1, 2016• 7 citations
Publisher

Abstract

We investigated the influence of magnetic fields on critical currents and photon count rates for optical and near-infrared photons in spiral-shaped nanowire structures of different types. Nanowires with a width of about 100 nm were made in forms of square and circular spirals with a pitch of 150 nm. Magnetic field dependencies of the critical current in square spirals were asymmetric with respect to the field direction. In the current maximum, we achieved a more than 6% increase of the critical current. On the contrary, circular spirals showed fully symmetric dependencies with the current maximum at zero magnetic field. Photon count rates of spirals have been studied in the wavelength range from 500 to 1400 nm at magnetic fields up to 500 mT. In circular spirals, the rates of photon and dark counts were symmetric in magnetic fields at all achievable experimental conditions. In square spirals, dark and photon count rates are asymmetric with the minima occurring at opposite directions of the field. Dark count rates are lower than 10 cps at bias currents up to 90% of the critical value in the whole range of applied magnetic fields. Based on our results, we propose a way to decrease the minimum detectable photon flux by applying external magnetic field to the detector.

DOI
10.1109/tasc.2016.2528545

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We investigated the influence of magnetic fields on critical currents and photon count rates for optical and near-infrared photons in spiral-shaped nanowire structures of different types. Nanowires with a width of about 100 nm were made in forms of square and circular spirals with a pitch of 150 nm. Magnetic field dependencies of the critical current in square spirals were asymmetric with respect to the field direction. In the current maximum, we achieved a more than 6% increase of the critical current. On the contrary, circular spirals showed fully symmetric dependencies with the current maximum at zero magnetic field. Photon count rates of spirals have been studied in the wavelength range from 500 to 1400 nm at magnetic fields up to 500 mT. In circular spirals, the rates of photon and dark counts were symmetric in magnetic fields at all achievable experimental conditions. In square spirals, dark and photon count rates are asymmetric with the minima occurring at opposite directions of the field. Dark count rates are lower than 10 cps at bias currents up to 90% of the critical value in the whole range of applied magnetic fields. Based on our results, we propose a way to decrease the minimum detectable photon flux by applying external magnetic field to the detector.
We present SNSPDs from NbN nanowires shaped after square-spiral, that allows an increase not only in critical currents, but also an extension of spectral detection efficiencies by just applying an external magnetic field. Using negative electron-beam lithography with the positive resist for shaping nanowires made it possible to reduce the inner bend radius. Consequently, the effect of critical-current enhancement in the magnetic field becomes stronger than it was demonstrated earlier. Here we achieved a 13% increase of the critical current in the magnetic field. We measured the spectra of the single-photon detection efficiency in the wavelength ranging from 400 to 1100 nm in the magnetic field. At zero field, the square-spiral has the spectrum similar to that of a meander. At the field providing the maximum of the critical current, the detection efficiency and the cutoff wavelength in the spectrum increase by 20% and by 54%, respectively. The magnetic-field dependence of the dark count rate is well described by the proposed analytical model.
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.
We have studied the influence of an externally applied magnetic field on the photon and dark count rates of meander-type niobium nitride superconducting nanowire single-photon detectors. Measurements have been performed at a temperature of 4.2 K, and magnetic fields up to 250 mT have been applied perpendicularly to the meander plane. While photon count rates are field independent at weak applied fields, they show a strong dependence at fields starting from approximately \ifmmode\pm\else\textpm\fi{}25 mT. This behavior, as well as the magnetic field dependence of the dark count rates, is in good agreement with the recent theoretical model of vortex-assisted photon detection and spontaneous vortex crossing in narrow superconducting lines. However, the local reduction of the superconducting free energy due to photon absorption, which is the fitting parameter in the model, increases much slower with the photon energy than the model predicts. Furthermore, changes in the free-energy during photon counts and dark counts depend differently on the current that flows through the meander. This indicates that photon counts and dark counts occur in different parts of the meander.
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We investigate the detection efficiency of a spiral layout of a superconducting nanowire single-photon detector. The design is less susceptible to the critical current reduction in sharp turns of the nanowire than the conventional meander design. Detector samples with different nanowire widths from 100 to 300 nm are patterned from a 4-nm-thick NbN film deposited on sapphire substrates. The critical current ICat 4.2 K for spiral, meander, and single-bridge structures is measured and compared. On the 100-nm-wide samples, the detection efficiency is measured in the wavelength range 400-1700 nm and the spectral bandwidth of the intrinsic detection efficiency is determined. In the optical range, the spiral detector reaches a detection efficiency of 27.6%, which is ~ 1.5 times the value of the meander. In the infrared range the detection efficiency of the spiral superconducting nanowire single-photon detector is more than doubled.
Abstract Superconducting nanowire single-photon detectors (SNSPDs) are widely used for the detection of visible and near-infrared single photons. Due to the nature of the polarization sensitive absorption of the nanowire, the detection efficiency (DE) of the meander SNSPD is polarization sensitive. In order to obtain a polarization-insensitive device with high DE, we fabricated NbN SNSPDs with spiral structure, which were embedded into an optical cavity. No meaningful current crowding effect was observed in the spiral SNSPDs. The spiral SNSPD showed a maximal system detection efficiency of 52.5% at 1550 nm with a dark count rate of 100 Hz and a polarization extinction ratio of 1.04, due to the combination of a spiral geometry and an optical cavity.