Abstract Body

In the field of high-sensitivity single-photon detectors, superconducting nanowire single-photon detectors (SNSPDs) are widely used due to their excellent performance. The quasi-2D structure of superconducting nanowires introduces inherent nanoscale inhomogeneities that significantly impact device performance. For example, the dark count phenomenon in SNSPDs is believed to be closely related to these inhomogeneities. Traditional characterization methods, such as AFM and SEM, can visualize geometric shapes, surface morphology, composition, or superconducting energy gaps, but have not yet revealed the direct relationship between these inhomogeneities and electrical transport performance.

This study proposes a method for investigating random inhomogeneities in superconducting nanowires by probing and mapping self-heating hotspots. We mounted a fiber focuser on a 3-axis cryogenic nano-scanner to focus and scan a laser spot on a superconducting NbN nanowire. Localized hotspot can be generated within a maximum distance of ~300 nm triggered by the laser. The associated self-heating IV curve that revealed the shrink of the hotspot was collected and then be used for extracting information of inhomogeneities, such as geometry variation and spatial distribution. Furthermore, this method allowed us to locate where dark counts were generated along the nanowire. The number of dark count locations increased as temperature rose. A total number of eight dark count locations were observed at a temperature of 4 K. This method not only reveals the intrinsic inhomogeneities hidden in a superconducting nanowire, but also serves as a useful tool for investigating nanowire’s electrical and optical performance associated with inhomogeneities.

References

[1] Dane, A., Allmaras, J., Zhu, D. et al. Self-heating hotspots in superconducting nanowires cooled by phonon black-body radiation. Nat Commun 13, 5429 (2022).

[2] Skocpol, W. J., Beasley, M. R. & Tinkham, M. Self-heating hotspots in superconducting thin-film microbridges*. J. Appl. Phys. 45,4054–4066 (1974).

[3] Andreev, V. et al. Dark Counts in SNSPD Studied With Spatial Resolution. IEEE Trans. Appl. Supercond. PP, 1–5 (2024).

Figure 1. A diagram of focusing and scanning a laser spot on a superconducting NbN nanowire. The optical fiber was mounted on a 3-axis cryogenic nano-scanner to achieve precise scanning.

Keywords: Superconducting nanowire, Self-heating hotspots, Self-heating current, Nanoscale inhomogeneities, Dark counts