  Multidimensional coherent spectroscopy is a powerful tool to characterize
nonlinear optical response functions. Typically, multidimensional spectra are
interpreted via a perturbative framework that straightforwardly provides
intuition into the density matrix dynamics that give rise to specific spectral
features. When the goal is to characterize system coupling to a thermal bath
however, the perturbative formalism becomes unwieldy and yields less intuition.
Here, we extend an approach developed by Vagov et al. to provide an exact
expression for multidimensional spectra of a spin-boson Hamiltonian up to
arbitrary order of electric field interaction. We demonstrate the utility of
this expression by modeling polaron formation and coherent exciton-phonon
coupling in quantum dots, which strongly agree with experiment.
