Application of Kubo response functions to radiative transport

Abstract

The semi-classical radiative transport equation is derived, and the absorption coefficient is determined from the dielectric loss by using Kubo theory. The fluctuation-dissipation theorem is applied to obtain the field fluctuations in terms of the imaginary part of the retarded Maxwell Green's function. The generalized quantum action principle developed by Schwinger is utilized to derive the equations of motion for the quantum Maxwell Green's functions. To decouple matter and radiation, the current response functions, or polarization tensors, are approximated by current-current correlation functions. The retarded Green's function is found to be identical with the classical expression. Possible uses of this theory to describe masers and non-equilibrium radiative transfer are discussed.