Optical nonlinearities of hybrid pairs of organic molecules

The excited electronic states of nearby molecules are in general coupled via Coulomb interaction even in the absence of wavefunction overlap. When two different organic molecules having a nearly resonant excited state are close enough, the dipole-dipole interaction can significantly affect their optical response. Even though they do not chemically interact, concerning their coupling to light, such molecules do not act independently, but rather as a 'virtual heterodimer' the response of which stems from, but is different from that of each molecule alone. We discuss here a simple and general model to estimate their resonant nonlinear susceptibilities.