EXCITON COHERENCE-SIZE AND PHONON-MEDIATED OPTICAL NONLINEARITIES IN RESTRICTED GEOMETRIES

The magnitude of optical nonlinearities of molecular nanostructures is determined by a characteristic coherence-length which controls the cooperativity of the optical response. Equations of motion describing the evolution of the optical polarization coupled to two-exciton variables are derived, and used to calculate the third order optical response (chi(3)) of a one-dimensional molecular crystal or a polymer. We show that the coherence-length is determined by the interplay between intramolecular and intermolecular (nonlocal) nonlinearities and explore the limitations of the local-field approximation. Phonon-mediated exciton-exciton attractive interaction may result in the formation of bound exciton pairs (biexcitons). We show how unbound (two-exciton) as well as biexciton states could readily be observed as resonances in two-photon absorption and third harmonic generation.