Overview

This thesis is a study of superconducting niobium and niobium nitride nanowires used as single optical and near-infrared photon detectors. The nanowires are biased in the zero-voltage state with a current just below their critical current and at a temperature well below their critical temperature. In this state, an absorbed photon induces localized heating at the point of absorption. This suppresses the critical current in that location, creating a resistive region in the nanowire. The resistive region can grow under Joule heating and can self-reset to the zero-voltage state without the dc bias current being reduced.This study is twofold. First, niobium is investigated as an alternate detector material to niobium nitride. This study compares the performance niobium nanowire detectors of several geometries and fabricated in two different ways to the performance of niobium nitride nanowire detectors. Niobium detectors are found to have longer reset times and are more difficult to bias in a regime where they self-reset to the zero voltage state after detecting a photon. This makes niobium a less suitable material than niobium nitride for these detectors.In the second part of this study, the reset dynamics of these detectors are studied. Thermal relaxation is studied using a combination of experiments and numerical simulations. It is found that the thermal relaxation time for a niobium nanowire depends significantly on the amount of energy dissipated into the hotspot during the detection event. This energy depends on the bias current and on the kinetic inductance of the nanowire. The kinetic inductance is proportional to the length; thus a shorter nanowire will have a shorter thermal relaxation time, and a shorter reset time. Using this theoretical framework, the difference in reset time between niobium and niobium nitride nanowire detectors is explained. The temperature and current dependence of the kinetic inductance of niobium and niobium nitride nanowires is also investigated.

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