Effect of magnetic field on the photon detection in thin superconducting meander structures

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 ±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.

Figure 1

Dependence of the normalized critical current on the applied magnetic field of the NbN meander with 148-nm-wide strips. The solid circles are measured values of $I_c$${}_{,e}$($B$)$/I_c$${}_{,e}$(0) for positive field values, whereas the squares are for negative fields. For clarity, the latter are mirrored at the $y$ axis. The inset shows an AFM picture of an NbN meander and an I-V curve at 4.2 K.

Figure 2

(a) Normalized PCRs vs magnetic field for the NbN meander with a strip width of $w$ = 148 nm at a fixed bias current of 0.57$I_c$${}_{,e}$ for four illumination wavelengths. The solid lines represent fits with Eq. (2). (b) Detailed view of (a) for weak fields between ±50 mT. In the inset, ${\nu }_h$ values vs illumination wavelength are shown. The symbols are the values obtained from fits with Eq. (2). The solid lines are fits to the experimental data, and the dashed and dotted lines are theoretical curves of ${\nu }_h$, which are calculated according to Eq. (3).

Figure 3

PCRs $R_{\mathrm{PCR}}$($B$) normalized to the zero field value $R_{\mathrm{PCR}}$(0) vs the applied magnetic field at a fixed illumination wavelength of 900 nm. The PCRs at four bias currents of the meander with $w$ = 148 nm are plotted. The inset shows the ${\nu }_h$ values obtained by the fit of Eq. (2) to the experimental data of both meanders.

Figure 4

(a) Normalized DCRs vs magnetic field for four ($w$ = 148 nm) and three ($w$ = 104 nm) bias currents. The solid lines represent fits with Eq. (5). (b) ${\nu }_0$ values as a function of the relative bias current obtained by the fit with Eq. (5).