Green's function approach to the dynamics-controlled truncation formalism:: Derivation of the χ(3) equations of motion

The dynamics-controlled truncation (DCT) formalism is a successful microscopic approach that describes coherent correlations in optically excited semiconductors. For practical reasons (including numerical evaluations), its application is limited to lowest-order nonlinearities, such as the chi((3)) regime. Therefore, it is not convenient to use this formalism to examine the role played by incoherent many-body effects, such as carrier-carrier scattering and screening. Traditionally, the most powerful approach to study incoherent effects and correlations in highly excited semiconductors is that of nonequilibrium Green's functions (NGF). A combination of the insights and technical advantages provided by the two (NGF and DCT) approaches will lead to a comprehensive microscopic theory for nonlinear optical phenomena in semiconductors. In this paper, we take a first step in this direction by presenting detailed one-to-one relations between the two formalisms within the chi((3)) approximation. Starting from the standard perturbation theory of nonequilibrium Green's functions, we derive the essential minimal order factorization theorems, to arbitrary order, of DCT and the equations of motions for the interband polarization and the "biexcitonic" correlation function. This lays the foundation for future diagrammatic high-intensity generalizations of the DCT formalism.