Far-infrared optical absorption and reflectivity of a superconducting NbN film

We report measurements of bolometric absorption and absolute reflectivity of a thin film of NbN on a silicon substrate, above and below the superconducting transition temperature of 13.3 K. The opening of the gap at an energy of 3.9 kB{T}c$ is seen clearly in both types of measurement. The data are compared with the predictions of the Mattis-Bardeen theory. The change in the reflectivity and the absorption at energies above 2{Δ} is larger than that calculated from the theory. By an analysis of the reflectivity data, it is shown that the theoretical values of the complex superconducting conductivity are too high.