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(Color online) Detection efficiency (DE) as a function of Ib /Ic measured at 2.0 K and photon wavelengths as indicated. The shift in threshold current with different photon energies is roughly linear as suggested by Eq. (1). Open symbols show dark-count rates (right axes) as a function of bias current for 2.0 K. Inset (a) shows a comparison between TaN and NbN detectors for 300 nm photons and similar operating temperatures. Inset (b): averaged and normalized traces of single-photon detection events in TaN and NbN.
(Color online) Detection efficiency (DE) as a function of λ at different bias currents and T = 2.0 K. The averaged maximum DE is indicated by the horizontal dashed line. Inset (a): ratio of DE (DETaN/DENbN) vs. λ. DETaN is significantly increased for long wavelengths and λ cutoff is easily identified. λ cutoff for NbN is less clear due to large systematic errors in this wavelength range. Inset (b): relation between minimum photon energy E photon = hν and the threshold current Ith , error bars are estimates of the accuracy in determining the cut-off criterium. The red line is a least-square fit according to Eq. (1).
Material and device parameters of the TaN SNSPD of this study and a reference NbN detector. w is the width, d the thickness, L the total length of the meander, and ρ □ is the square resistance of the superconducting film just above Tc . The superconducting energy gap has been calculated from the BCS-relation Δ = 1.76kBTc (with the Boltzmann constant kB ).
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