  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.
