In recent years there has been a rapid expansion of activity in the field of nonlinear optics. Judging by the proliferation of published papers, conferences, international collaborations and enterprises, more people than ever before are now involved in research and applications of nonlinear optics. This intense activity has been stimulated largely by the increasing interest in applying optics and laser technology in tele-communications and information processing, and has been propelled by significant advances in nonlinear-optical materials.

The origins of these recent developments can be traced through three decades of work since the invention of the laser and the first observations of nonlinear-optical phenomena by Franken et al (1961). From the earliest days it was recognised that such phenomena can have useful practical applications; for example, effects such as optical-frequency doubling allow the generation of coherent radiation at wavelengths different from those of the available lasers. In the 1960s many of the most fundamental discoveries and investigations were made. Work was then mainly concerned with the interaction of high-power lasers with inorganic dielectric crystals, gases and liquids. Effects such as parametric wave-mixing, stimulated Raman scattering and self-focusing of laser beams were investigated intensively. The invention of the wavelength-tunable dye laser paved the way for the development during the 1970s of many tunable sources utilising nonlinear effects, such as harmonic generation, sum- and difference-frequency mixing, and stimulated Raman scattering. In this way tunable coherent radiation became available in a wide spectral range from the far infrared to vacuum ultraviolet.