Figure 12: (a) Transmission spectra of an asymmetrical Fabry-Perot resonator, formed by a NbN film (  Å) on a sapphire substrate (0.43 mm), measured at two temperatures above and below critical temperature  K. (b) Transmission spectra of a plane parallel sapphire plate (0.41 mm) covered with NbN films on both sides (  Å and  Å), again measured at 300 K and 5 K. The solid lines represent the theoretical fit. (c) For the latter case the temperature dependence of the optical conductivity of NbN film is calculated for two frequencies, 8  and 29  (after [91, 92]).