Characterization of nonlinear optical materials

We discuss the characterization of nonlinear optical processes that give rise to changes in the absorption coefficient and refractive index. We primarily concentrate on methods for determining the dominant nonlinearities present in condensed matter and the responsible physical mechanisms. In extensive studies of a wide variety of materials we have found that there is seldom a single nonlinear process occuring. Often several processes occur simultaneously, sometimes in unison, sometimes competing. It is neseccary to experimentally distinguish and separate these processes in order to understand and model the interaction. There are a variety of methods and techniques for determining the nonlinear optical response, each with its own weaknesses and advantages. In general, it is advisable to use as many complementary techniques as possible over a broad spectral range in order to unambiguously determinine the active nonlinearities. Here we concentrate on the use of nonlinear transmittance, Z-scan and degenerate four-wave mixing experiments as applied to polycrystalline and single crystal semiconductors and dielectric materials