Light scattering from three-level systems: The T-matrix of a   point-dipole with gain
  We present an extension of the T-matrix approach to scattering of light by a three-level system, using a description based on a Master equation. More particularly, we apply our formalism to calculate the T-matrix of a pumped three-level atom, providing an exact and analytical expression describing the influence of a pump on the light scattering properties of an atomic three-level system. 
Tom Savels, Allard Mosk, and Ad Lagendijk