Flux-conserving diagrammatic formulation of optical spectroscopy of open quantum systems

We present a theoretical approach to optical spectroscopy of open nonequilibrium systems, which generalizes traditional nonlinear optical spectroscopy tools by imposing charge and energy conservation at all levels of approximation. Both molecular and radiation field degrees of freedom are treated quantum mechanically. The formulation is based on the nonequilibrium Green's function (NEGF) approach and a double sided Feynman diagrammatic representation of the photon flux is developed. Numerical simulations are presented for a model system. Our study bridges the theoretical approaches of quantum transport and optical spectroscopy and establishes a firm basis for applying traditional tools of nonlinear optical spectroscopy in molecular optoelectronics.