In the earlier days of nonlinear optics, the materials used for experiments and devices were mainly inorganic dielectric crystals, vapours, liquids and bulk semiconductors. The search for ‘good’ nonlinear-optics media was made amongst the known materials. However in more recent years, with the growing interest in optical devices and applications, attention has focused increasingly on new artificial solid-state materials which may offer higher nonlinearity; in particular, those that will allow nonlinearoptical devices to operate efficiently at relatively low power levels, such as the outputs from semiconductor-diode lasers. Organic materials offer great scope since modern methods of synthesis allow considerable flexibility in the design of materials at the molecular level. As mentioned in §4.4.3, the macroscopic nonlinear-optical properties of many organic crystals are given by the tensor sum of the properties of the constituent molecules, with due regard to local-field factors and molecular orientation. It is this feature of organic materials that allows a ‘molecular-engineering’ approach to the optimisation of macroscopic properties. Several materials with large second-order nonlinearity have been successfully fabricated (Chemla and Zyss, 1, 1987). Some of these newer organic optical materials also exhibit other desirable properties, such as a greater resistance to optical damage. These have applications in devices such as compact optical-frequency doublers and parametric- amplifiers and -oscillators. However, materials with large third-order nonlinearity are perhaps of greater interest currently, since the nonlinear refractive-index effect can be exploited for switching, optical bistability, phase conjugation and other types of signal processing (Gibbs, 1985).