  We discuss theoretical approaches to nonlinear optical spectroscopy of
molecular junctions. Optical response functions are derived in the form
convenient for implementation of Green function techniques, and their
expressions in terms of pseudoparticle nonequilibrium Green functions are
proposed. The formulation allows to account for both intra-molecular
interactions and hybridization of molecular states due to coupling to contacts.
Two-dimensional optical spectroscopy in junctions is considered as an example.
