Unusual Behavior of two-photon absorption from three-level molecules in a one-dimensional lattice

We derive an analytical expression for the two-photon absorption spectrum of a one-dimensional lattice of three-level molecules. This result enables us to investigate the interference between collective and intramolecular effects in the nonlinear optical response of molecular aggregates. We find that, depending on the system parameters, the third molecular level gives rise to zero, one, or two additional peaks in the two-photon absorption spectrum