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Dark count rates and spectral sensitivity of superconducting single-photon detectors

Andreas Engel, Alexei Semenov, Heinz‐Wilhelm Hübers, K. Ill'in, M. Siegel

Published Jan 1, 2004• 0 citations
OpenAlex

Abstract

Recent results for a superconducting single-photon counter are presented. The detection mechanism of the photon counter is based on the combined action of the hot-spot formation after photon absorption and the subsequent current-assisted formation of a normal domain in a thin and narrow superconducting strip. Dark count rates caused by fluctuations of the superconducting order parameter as a function of the applied current are discussed as well as the dependence of the spectral sensitivity on detector dimensions and material parameters.

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Recent results for a superconducting single-photon counter are presented. The detection mechanism of the photon counter is based on the combined action of the hot-spot formation after photon absorption and the subsequent current-assisted formation of a normal domain in a thin and narrow superconducting strip. Dark count rates caused by fluctuations of the superconducting order parameter as a function of the applied current are discussed as well as the dependence of the spectral sensitivity on detector dimensions and material parameters.
We present results on dark count rates and spectral sensitivities of superconducting single-photon detectors in the visible and near-infrared spectral range. The active detector element is a nanometre-sized (a few nanometres thick and less than 100nm wide) meander line carrying a supercurrent. The superconducting materials are NbN and Nb, respectively. The NbN detector exhibited a flat spectral sensitivity up to about 2.4μm. Fluctuations of the superconducting order parameter are considered as a major source of dark count events. A simple model and its limitations to explain the observed dark count rates is discussed.
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We measured the temperature- and current-dependence of dark count rates of a superconducting singlephoton detector. The detector's key element is a 84 nm wide meander strip line fabricated from a 5 nm thick NbN film. Due to its reduced dimensions various types of fluctuations can cause temporal and localized transitions into a resistive state leading to dark count events. Adopting a recent refinement of the hotspot model we achieve a satisfying description of the experimental dark count rates taking into account fluctuations of the Cooper-pair density and current-assisted unbinding of vortex-antivortex pairs. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
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 $\mathrm{\ensuremath{\Delta}}$.
The spectral detection efficiency and the dark count rate of superconducting nanowire single-photon detectors (SNSPD) have been studied systematically on detectors made from thin NbN films with different chemical compositions. Reduction of the nitrogen content in the 4 nm thick NbN films results in a decrease of the dark count rates more than two orders of magnitude and in a red shift of the cut-off wavelength of the hot-spot SNSPD response. The observed phenomena are explained by an improvement of uniformity of NbN films that has been confirmed by a decrease of resistivity and an increase of the ratio of the measured critical current to the depairing current. The latter factor is considered as the most crucial for both the cut-off wavelength and the dark count rates of SNSPD. Based on our results we propose a set of criteria for material properties to optimize SNSPD in the infrared spectral region.
We study photon count statistics in superconducting single-photon detectors consisting of up to 70 narrow superconducting strips connected in parallel. Using interarrival time analysis, we demonstrate that our samples are operated in the “arm-trigger” regime and require up to seven subsequently absorbed photons to form a resistive state in the whole sample. We also performed numerical simulation of the light and dark count rates versus detector bias current, which are in good agreement with the experimental results.