Figure 14: Raw data of transmission and relative phase shift spectra of the Nb film on the sapphire substrate. The critical temperature is  K. The phase shift is divided by the frequency for the better perception. The dots represent the measured values; the red lines for 9 K are the fits with the Drude model. The blue lines correspond to the fit using the Mattis-Bardeen equation. In the insets the temperature dependence of and is displayed for two particular frequencies corresponding to the first and last transmission maxima in measured spectral range (after [97]).