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.
@article{arxiv.1810.03134,
title = {Thermal properties of NbN single-photon detectors},
author = {E. M. Baeva and M. V. Sidorova and A. A. Korneev and K. V. Smirnov and A. V. Divochy and P. V. Morozov and P. I. Zolotov and Yu. B. Vakhtomin and A. V. Semenov and T. M. Klapwijk and V. S. Khrapai and G. N. Goltsman},
journal= {arXiv preprint arXiv:1810.03134},
year = {2019}
}