Spectral dependence of ultra-low dark count superconducting single photon detector for the evaluation of broadband parametric fluorescence

Superconducting nanowire single photon detectors (SNSPD) have unique characteristics of ultra low dark counts and wide spectrum sensitivity. These natures are indispensable for the evaluation of ultra-broadband parametric fluorescence, which are used for the quantum optical coherence tomography and novel optical non-linear experiments. Here we report the spectral dependence of the detection efficiency of a meander type SNSPD device, having reduced strip width of 50 nm, over a wide spectrum range up to near infra-red wavelength. The fiber coupled, meander type device was fabricated using 6 nm thick Niobium nitride (NbN) nanowires of reduced strip width, 50 nm, patterned over a MgO substrate with active area of 10 x 10 μm2. A maximum efficiency of 32% at 500 nm, 30% at 600 nm, 16% at 800 nm, 10% at 1000 nm, and 1% at 1550 nm with the normalized bias current of 0.95 (bias 37 μA ) was observed at 4.2 K. The salient feature of the device is, it exhibits a very low dark count rate (DCR) of only 2 Hz at the standard operating bias of 37 μA and ultra low DCR of 0.01Hz at 34 μA. Moreover, at this reduced bias with 0.01Hz DCR, the detection efficiency is not appreciably decreased in the visible region (32% at 500 nm and 30% at 600 nm) and an order decrease is observed (0.1%) at 1550 nm. The noise equivalent power (NEP) is of the order 10-19 WHz-1/2 in the visible region and 10-17 WHz- 1/2 in the near IR region. Ultra-broad band parametric fluorescence of band width from 791 nm to 1610 nm generated by a quasi-phase matched (QPM) device was successfully detected with this SSPD.