THE METHOD OF GREEN-FUNCTIONS FOR A TIME-DEPENDENT HAMILTONIAN IN SYSTEMS FAR OFF EQUILIBRIUM

A Green' function method is outlined for open systems being used if the Hamiltonian depends sensitively on time. The method is applied to the quantum-mechanical treatment of the photon field for the problem of highly excited semiconductors. As a result, we obtain the creation of laser-induced n-dependent gaps, where n = 0, 1, 2,.... are the eigenvalues of the photon number operator of the laser field.