Overview

Chapter 10 has been reviewed by Dr. Ludovic Brass from Teemphotonics.

A problem often encountered in Physics is to find the response r( t) of a system to some excitation e( t) that is time dependent. In many cases the relationship (excitation ? response) is linear, the simplest case being proportionality. If the excitation is harmonic, which means sinusoidal time variation, the linear response is also harmonic, with the same frequency. In many cases, the linear approach is a simplification of more general systems that are said to be nonlinear. The response to a harmonic signal (frequency ?) is still periodic with the same frequency, but is no longer harmonic: the frequency spectrum has many more components, among which the double frequency (2 ?) component is often the most important.

As a light beam propagates inside a piece of material (see Annex 8.B of the chapter on the Index of Refraction), the electric field of the incident wave can be considered as an excitation, while the polarization taken by the material is the response. As long as the light intensity is not very powerful, the interaction remains linear and the light that is transmitted has only one component at the same frequency. The nonlinear aspect of the light/material interaction could not be revealed before the appearance of the powerful beams delivered by lasers. The first demonstration was made in 1961: the incident red light (0.6943 ?m) of a ruby...