‘Nonlinear’ optical phenomena are not part of our everyday experience. Their discovery and development were possible only after the invention of the laser.

In optics we are concerned with the interaction of light with matter. At the relatively low light intensities that normally occur in nature, the optical properties of materials are quite independent of the intensity of illumination. If light waves are able to penetrate and pass through a medium, this occurs without any interaction between the waves. These are the optical properties of matter that are familiar to us through our visual sense. However, if the illumination is made sufficiently intense, the optical properties begin to depend on the intensity and other characteristics of the light. The light waves may then interact with each other as well as with the medium. This is the realm of nonlinear optics. The intensities necessary to observe these effects can be obtained by using the output from a coherent light source such as a laser. Such behaviour provides insight into the structure and properties of matter. It is also utilised to great effect in nonlinear-optical devices and techniques which have important applications in many branches of science and engineering.

Another effect of light on matter can sometimes be to induce changes in the chemical composition; such ‘photochemical’ processes lie outside the subject of this book.

Origins of optical nonlinearity

We now consider in a simple way how nonlinear-optical behaviour might arise.