ABSTRACT
The nonlinear-optical quantities, defined at the microscopic level as hyperpolarizabilities and at the macroscopic level as nonlinear susceptibilities, have played a key role in determining the suitability of substances for practical use, for example, in electro-optical switching and frequency mixing. With few exceptions, a useful nonlinear optical material will be in the solid phase, for example, a single crystal or a poled polymer embedded in a film. In the physics arena the theoretical determination of nonlinear optical (NLO) properties of solids has been more advanced though not to the degree that has been achieved for simple gas-phase molecules using modem quantum-chemical practices. An important break from the conventional molecular calculations and one that is likely to see a great deal more use in the NLO field in the near future. Refinement of the oriented-gas model, including more and more precise accounting for inter-species interactions, is another likely avenue for progress.
