Figure 10: (a) Ratio of the infrared transmission through a NbN film in the superconducting state and the normal state for several temperatures; the critical temperature of this film is K (after ). (b) The bolometric absorption spectra of a superconducting NbN film at different temperatures below K ratioed with a 14 K spectrum above . The curves have been scaled individually to agree at 110 (for the purpose of comparison) and overall to give a 5.3 K spectrum, which equals 1.0 at large wave numbers. The resolution is 2 . (c) Optical reflectivity of a 250 nm NbN film on silicon ( mm) above and below the superconducting transition temperature. The resolution was 0.5 below 100 and 2 above. The inset gives the low-frequency part on a magnified scale in order to demonstrate the shift of the Fabry-Perot resonance pattern: At high frequencies the reflection maxima are in phase, but, for low frequency, the reflectivity in the superconducting state exhibits minima where maxima are observed for . (d) Real and imaginary parts of the conductivity at K normalized to the normal state value, as extracted from the reflection data shown above. The dashed lines correspond to the calculations based on Mattis-Bardeen theory, yielding an energy gap (after ).