When a light beam propagates through an ensemble of two-level quantum systems, the resonant interaction between these systems and the radiation field may be described by a complex susceptibility. Even if the medium is of a type ordinarily considered isotropic, the susceptibility will be a tensor. In this paper, explicit expressions for the elements of this tensor are derived from quantum-statistical mechanics. A physical explanation of the induced anisotropy is given, and the experimental conditions under which it can be neglected are described.
J. D. Macomber, "Optical Anisotropy Induced by Resonant Absorption or Amplification of Radiation," Appl. Opt. 10, 2506-2512 (1971)