NONLINEAR-OPTICAL RESPONSE OF CHARGE-TRANSFER EXCITONS AT DONOR-ACCEPTOR INTERFACES

We discuss resonant and off-resonant optical nonlinearities of a syste m of charge-transfer excitons (CTEs) at a donor-acceptor (D-A) interfa ce. We consider an interface between organic materials for which the l owest electronic excitations are CTEs corresponding to displacement of an electron from donor to acceptor. Such photogenerated CTEs have lar ge static electric dipoles perpendicular to the D-A interface plane. T his system exhibits strong resonant optical nonlinearities induced by the dipole-dipole repulsion among the CTEs. On account of this long-ra nge interaction, the excitation intensity dependence of the Kerr nonli nearity depends on the two-dimensional density n of CTEs as n(3/2), an d the dependence of the CW nonlinear polarization on the laser field i s beyond the usual expansion. The static electric field produced by th e CTEs modifies the hyperpolarizabilities of nearby molecules, causing the intensity dependence of the nonlinear optical response to be stro nger than usual.