Cooperative nonlinear optical response of molecular aggregates: Crossover to bulk behavior

Equations of motion for the nonlinear optical response of arbitrary-size molecular aggregates are derived. The relative role of intramolecular and intermolecular (two-exciton) nonlinearities and the cooperative enhancement induced by the latter are analyzed. A crossover from {~}N² to {~}N scaling of the nonlinear polarizability is predicted as the aggregate dimension becomes comparable to the optical wavelength, N being the aggregate size. The limitations of the local-field approximation are demonstrated.