The optical conductivity is obtained exactly within BCS theory as a function of frequency, temperature, and disorder parameter, by simplifying the general impurity scattering mechanism to backward scattering. The familiar features of infrared measurements are recovered. In the dirty limit, the ratio of superconducting to normal absorption rate approaches the Mattis-Bardeen result.

• Received 11 February 1993

DOI:https://doi.org/10.1103/PhysRevLett.71.2304