SEM image of the meander structure: the bright lines are reflections from the surface of the sapphire substrate. The gray area is the NbN film.
Schematics of the experimental setup. Inset shows the typical voltage transient which appears at the input of the counter in response to a single photon.
Computed ABS of the meander structures with different thicknesses for incident light polarized parallel (P-pol) and orthogonal (S-pol) with respect to the meander lines.
Total count rate vs relative bias current at the wavelength 500 nm for the meander NbN412. At the count rate increases quickly due to contribution of the dark counts (dashed circle).
Detection efficiency of the meander NbN344 at different bias currents at 6 K. Arrow marks the cutoff wavelength for . Solid lines show computed DE due to thermal excitation of magnetic vortices.
Wavelength dependences of the DE normalized to its value at for the meanders with different thicknesses. The DE for all meanders was measured at and 6 K. The arrows mark the cutoff wavelengths. Solid lines represent DE due to vortex excitation.
The experimental critical current (symbols) and the Bardeen’s depairing critical current (solid line) for the 4 nm thick meander.
Intrinsic DE for the meanders with different thicknesses at the wavelengths 500 and 1500 nm. The IDE is plotted vs reciprocal square resistance. Solid line is the computed IDE for vortex assisted photon detection.
Parameters of the studied meanders: is the film thickness corresponding to the sputtering time, is the nominal width of the meander line, is the zero-resistance critical temperature of the meander, is the experimental critical current at 4.2 K, is the square resistance at 20 K, and is the wavelength corresponding to the model cutoff of the hot-spot detection mechanism. Meanders with equal thicknesses belong to different detector series.
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