Kinetic equations for a system of interacting electrons and photons

A method is described for deriving kinetic equations for an electron subsystem with a band spectrum interacting with a photon subsystem. Kinetic equations with allowance for the polarization are obtained for the diagonal and nondiagonal (relative to the bands) elements of the density matrix. The polarization leads to a renormalization of the photon frequencies and interaction of the electrons with one another through the exchange of virtual photons. The relaxation of the band populations and the polarization due to an external field are studied in the two-band approximation by means of the collision integrals obtained in the method.