Thermal properties of NbN single-photon detectors

TL;DR

This paper measures the thermal resistance of NbN single-photon detectors using optical calibration and resistive thermometry, providing insights into energy relaxation mechanisms and heat capacity ratios.

Contribution

It introduces a method to determine the thermal resistance and analyze energy relaxation in NbN detectors, linking experimental data with theoretical bounds.

Findings

Determined the absolute optical power absorbed by NbN film.

Measured the thermal resistance Z(T) as a function of temperature.

Established an upper bound on the electron-to-phonon heat capacity ratio.

Abstract

We investigate thermal properties of a NbN single-photon detector capable of unit internal detection efficiency. Using an independent calibration of the coupling losses we determine the absolute optical power absorbed by the NbN film and, via a resistive superconductor thermometry, the thermal resistance Z(T) of the NbN film in dependence of temperature. In principle, this approach permits a simultaneous measurement of the electron-phonon and phonon-escape contributions to the energy relaxation, which in our case is ambiguous for their similar temperature dependencies. We analyze the Z(T) within the two-temperature model and impose an upper bound on the ratio of electron and phonon heat capacities in NbN, which is surprisingly close to a recent theoretical lower bound for the same quantity in similar devices.