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 ${\mathit{k}}_{\mathit{B}}$${\mathit{T}}_{\mathit{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 2Δ 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.