Far-infrared transmission of a superconducting NbN film

Abstract

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 ${T}_{c}$. $Tr(T,B\ensuremath{\ne}0)$ 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.