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arXiv:1505.04689 · Physical Review B

Vortex-assisted mechanism of photon counting in a superconducting nanowire single-photon detector revealed by external magnetic field

Denis Yu. Vodolazov, Yu. P. Korneeva, Alexander V. Semenov, Alexander A. Korneev, Gregory N Gol'tsman

Published Sep 4, 2015• 68 citations
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Abstract

We use an external magnetic field to probe the detection mechanism of a superconducting nanowire single-photon detector. We argue that the hot belt model (which assumes partial suppression of the superconducting order parameter $\mathrm{\ensuremath{\Delta}}$ across the whole width of the superconducting nanowire after absorption of the photon) does not explain observed weak-field dependence of the photon count rate (PCR) for photons with $\ensuremath{\lambda}=450\phantom{\rule{4.pt}{0ex}}\mathrm{nm}$ and noticeable decrease of PCR (with increasing the magnetic field) in a range of the currents for photons with wavelengths $\ensuremath{\lambda}=450--1200\phantom{\rule{4.pt}{0ex}}\mathrm{nm}$. Found experimental results for all studied wavelengths can be explained by the vortex hot spot model (which assumes partial suppression of $\mathrm{\ensuremath{\Delta}}$ in the area with size smaller than the width of the nanowire) if one takes into account nucleation and entrance of the vortices to the photon induced hot spot and their pinning by the hot spot with relatively large size and strongly suppressed $\mathrm{\ensuremath{\Delta}}$.

DOI
10.1103/physrevb.92.104503

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We use an external magnetic field to probe the detection mechanism of a superconducting nanowire single-photon detector. We argue that the hot belt model (which assumes partial suppression of the superconducting order parameter \ensuremathΔ\mathrm{\ensuremath{\Delta}}\ensuremathΔ across the whole width of the superconducting nanowire after absorption of the photon) does not explain observed weak-field dependence of the photon count rate (PCR) for photons with \ensuremathλ=450nm\ensuremath{\lambda}=450\phantom{\rule{4.pt}{0ex}}\mathrm{nm}\ensuremathλ=450nm and noticeable decrease of PCR (with increasing the magnetic field) in a range of the currents for photons with wavelengths \ensuremathλ=450−−1200nm\ensuremath{\lambda}=450--1200\phantom{\rule{4.pt}{0ex}}\mathrm{nm}\ensuremathλ=450−−1200nm. Found experimental results for all studied wavelengths can be explained by the vortex hot spot model (which assumes partial suppression of \ensuremathΔ\mathrm{\ensuremath{\Delta}}\ensuremathΔ in the area with size smaller than the width of the nanowire) if one takes into account nucleation and entrance of the vortices to the photon induced hot spot and their pinning by the hot spot with relatively large size and strongly suppressed \ensuremathΔ\mathrm{\ensuremath{\Delta}}\ensuremathΔ.
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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 ).
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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.