  An abnormal increase in the SDE was observed for superconducting nanowire
single-photon detectors (SNSPDs) when the bias current (Ib) was close to the
switching current (Isw). By introducing the time-correlated single-photon
counting technique, we investigated the temporal histogram of the detection
counts of an SNSPD under illumination. The temporal information helps us to
distinguish photon counts from dark counts in the time domain. In this manner,
the dark count rate (DCR) under illumination and the accurate SDE can be
determined. The DCR under moderate illumination may be significantly larger
than the conventional DCR measured without illumination under a high Ib, which
causes the abnormal increase in the SDE. The increased DCR may be explained by
the suppression of Isw under illumination.
