Many-body approaches for simulating coherent nonlinear spectroscopies of electronic and vibrational excitons

The theoretical approaches and key concepts needed for the design and interpretation of multidimensional spectroscopic techniques in systems of localized electronic or vibrational chromophores are surveyed. Two approaches for computing the nonlinear optical response of excitons are considered and related to bath spectral densities. The first is denoted as cumulant expansion of Gaussian fluctuations (CGF) and is based on closed correlation function expressions derived by coupling the dynamical exciton variables to a harmonic bath. The second is based on the nonlinear exciton equations (NEE), which provide a collective oscillator, quasiparticle picture for exciton dynamics.