Nonlinear optical response of charge-transfer excitons at donor-acceptor interfaces

We discuss resonant and off-resonant optical nonlinearities of a system of charge-transfer excitons (CTEs) at a donor-acceptor (DA) interface. We consider an interface between organic materials for which the lowest electronic excitations are CTEs corresponding to displacement of an electron from donor to acceptor. Such photogenerated CTEs have large static electric dipoles perpendicular to the DA interface plane. This system exhibits strong resonant optical nonlinearities induced by the dipole-dipole repulsion among the CTEs. On account of this long-range interaction, the excitation intensity dependence of the Kerr nonlinearity depends on the two-dimensional density n of CTEs as n32, and the dependence of the CW nonlinear polarization on the laser field is beyond the usual expansion. The static electric field produced by the CTEs modifies the hyperpolarizabilities of nearby molecules, causing the intensity dependence of the nonlinear optical response to be stronger than usual.