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arXiv:1410.8683 · Applied Physics Letters

The effect of magnetic field on the intrinsic detection efficiency of superconducting single-photon detectors

J. J. Renema, R. J. Rengelink, Irina Komen, Q. Wang, R. Gaudio, K. P. M. op 't Hoog, Z. Zhou, D. Sahin, A. Fiore, Peter H. Kes, J. Aarts, Martin P. van Exter, M. J. A. de Dood, E. F. C. Driessen

Published Mar 2, 2015• 16 citations
arXivHTMLPDF

Abstract

We experimentally investigate the effect of a magnetic field on photon detection in superconducting single-photon detectors (SSPDs). At low fields, the effect of a magnetic field is through the direct modification of the quasiparticle density of states of the superconductor, and magnetic field and bias current are interchangeable, as is expected for homogeneous dirty-limit superconductors. At the field where a first vortex enters the detector, the effect of the magnetic field is reduced, up until the point where the critical current of the detector starts to be determined by flux flow. From this field on, increasing the magnetic field does not alter the detection of photons anymore, whereas it does still change the rate of dark counts. This result points at an intrinsic difference in dark and photon counts, and also shows that no enhancement of the intrinsic detection efficiency of a straight SSPD wire is achievable in a magnetic field.

DOI
10.1063/1.4914182

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We experimentally investigate the effect of a magnetic field on photon detection in superconducting single-photon detectors (SSPDs). At low fields, the effect of a magnetic field is through the direct modification of the quasiparticle density of states of the superconductor, and magnetic field and bias current are interchangeable, as is expected for homogeneous dirty-limit superconductors. At the field where a first vortex enters the detector, the effect of the magnetic field is reduced, up until the point where the critical current of the detector starts to be determined by flux flow. From this field on, increasing the magnetic field does not alter the detection of photons anymore, whereas it does still change the rate of dark counts. This result points at an intrinsic difference in dark and photon counts, and also shows that no enhancement of the intrinsic detection efficiency of a straight SSPD wire is achievable in a magnetic field.
We investigate the response mechanism of nanowire superconducting single-photon detectors (SSPDs) made of amorphous MoxSi1-x. We study the dependence of photon count and dark count rates on bias current in magnetic fields up to 113 mT at 1.7 K temperature. The observed behavior of photon counts is similar to the one recently observed in NbN SSPDs. Our results show that the detecting mechanism of relatively high-energy photons does not involve the vortex penetration from the edges of the film, and on the contrary, the detecting mechanism of low-energy photons probably involves the vortex penetration from the film edges.
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.
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.
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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.
We theoretically study the dependence of the intrinsic detection efficiency (IDE) of a superconducting nanowire single photon detector on the applied current, I , and magnetic field, H . We find that the current, at which the resistive state appears in the superconducting film, depends on the position of the hot spot (a region with suppressed superconductivity around the place where the photon has been absorbed) with respect to the edges of the film. This circumstance leads to inevitable smooth dependence IDE(I) when IDE , even for a homogenous straight superconducting film and in the absence of fluctuations. For IDE , a much sharper current dependence comes from the fluctuation-assisted vortex entry to the hot spot, which is located near the edge of the film. We find that a weak magnetic field strongly affects IDE when the photon detection is connected with fluctuation-assisted vortex entry to the hot spot (IDE ), and it weakly affects IDE when the photon detection is connected with the current-induced vortex nucleation in the film with the hot spot (IDE ).