Far-infrared transmission of a superconducting NbN film

We report far-infrared optical properties of a thin NbN superconductor in magnetic field, B, up to 10 T. Transmission, Tr (T,B), of monochromatic linearly polarized laser beam with frequency below and above an optical gap is measured both below and above T-c. Tr(T,B=0) is well described by the BCS-based model that approximates the sample as a mixture of superconductors with different T-c. T-r(T,B not equal 0) appears qualitatively different for Voigt and Faraday geometry. To calculate optical properties, we use Bruggeman's approach and present a phenomenological model accounting for both field orientations. The model captures all observed features of the Tr(T,B) data.