Laboratory searches for DM particles have played a key role in constraining DM candidates at the electro weak scale. But for energies in the sub-MeV region, we require experimental thresholds in the sub-eV regimes. New detectors with much lower thresholds are needed. Superconducting Nanowire Single Photon Detectors (SNSPDs) are one of these new promising detectors.
Experiment
Superconducting Nanowires Single-Photon Detectors (SNSPDs)
In principle, an SNSPD consists of a superconducting nanowire placed on a substrate. In the case of QROCODILE, the QROCODILE SNSPD is a ∼2 nm-thick, 1µm-wide WSi meander encapsulated between two layers of SiO₂ on a Si substrate of size 10mm × 10mm, optimized both optically and thermally to reach a 0.11 eV threshold. This stacking and engineering not only enables single‐photon sensitivity in the near-infrared, but also makes the device a dual‐use target and sensor for sub-MeV dark‐matter searches.
Experimental Setup
Before the Dark Matter run starts, the internal detection efficiency of the detector has to be calibrated using MIR radiation at wavelengths of 5 and 11µm. The sketch shows the experimental setup used.
One critical parameter is the bias current Ib. It can be defined as the optimal current, where the internal detection efficiency is close to 100%. The photon count rate (PCR) reaching a plateau for high bias currents is a crucial indicator for the saturation of the detector. Simultaneously, for high Ib the dark count rate (DCR) increases, leveraging the level of noise. The DCR is a parameter that denotes the rate of false counts over time in relation to the detection events.
The picture on the left shows the ideal case of a PCR and DCR measurement. The optimal bais current has to be chosen by decreasing the noise level as much as possible while maintaining high intrinsic detection efficiency. The bias current was then set to 12.2 µA.