ABSTRACT

All third order nonlinear optical phenomena are fundamentally four wave mixing (FWM) events, meaning that the sum of the number of incoming and outgoing photons is four. This includes the generalization to quadratic electro optic phenomena in which two of the photons are at zero frequency. In other chapters of this volume, specific four wave mixing phenomena such as third harmonic generation, self-focusing, and the quadratic electro optic effect have been considered. Here we cover all other FWM techniques and measurements of organic materials and composites using them. A general FWM quantum electrodynamic diagram is given in Fig. 1, where we depict the virtual excitations that occur within a molecular system as it interacts with an electromagnetic field containing frequency components at w 1, w2 , w3 , and w4 • As is conventional, each vertex corresponds to a transition dipole matrix element with a corresponding resonant energy denominator.