Electric noise and local photon-induced nonequilibrium states in a current-carrying nanostructured superconductor

Abstract

Superconducting thin films and micro- and nanostructured superconducting elements have enormous potential for applications in electronics and as highly sensitive detectors. We present recent studies of the superconductivity of nanostructured Nb and NbN thin films as well as their potential use as very fast, sensitive and energydispersive photon-counters. Using photo- and electron-beam-lithography 5 to 20 nm thick films were patterned into single bridges and meander structures with widths of the conduction paths as narrow as 50 nm. Dependencies of the critical temperature and critical current density on variations in bridge width and thickness can be modelled rather well using existing theories applicable to dirty superconductors with very short coherence lengths. The meander structures have already been shown to be very fast and sensitive single photon detectors in the visible and near-infrared range. Their ultimate sensitivity is hampered however, by electric noise caused at least partly by fluctuations in the superconducting state. The detection mechanism exploits the creation of a localized hot-spot by the absorption of a photon with sufficient energy. Albeit being a relatively small perturbation of the superconducting state, it can be detected as a voltage transient given the right experimental conditions. These conditions require cross-sectional areas of the conduction paths comparable with the cross-section of the hot-spot and an applied bias current only slightly less than the critical current at the operating temperature. Under these conditions fluctuations, like e.g. phase-slip centers, fluctuations in the number density of Cooper-pairs or fluctuations within the vortex system, may also cause voltage pulses very similar in magnitude and shape to those caused by absorbed photons and may be a significant contribution to the dark count rate. Measured dark count rates have been analyzed with respect to these various noise sources. Besides the noise characteristics or noise equivalent power the performance of the single-photon detectors was investigated concerning the quantum efficiencies as a function of the incident photon energy and material parameters of the superconducting meander line.