Basics of Nonlinear Processes

Abstract

It is well known from acoustics that when it comes to oscillations, nonlinearitynonlinearityleads to the appearance of overtones. The same phenomenon also exists in optics. A first experimental demonstration succeeded in the early 1960s [14] when the generation of twice the irradiated frequency was shown in a nonlinear crystal. The mechanism relied on the anharmonicity of the oscillation of the medium's polarization as produced by an intense light wave. Shortly thereafter, the third harmonic was also demonstrated. Since then, nonlinear optics has evolved into a field of research in its own right. Processes under study are optical rectification, parametric amplification, self-focusing, and self-phase modulationself phase modulation, to name just a few. Optical nonlinearitynonlinearityis responsible when optical properties of some material show intensity-dependent modifications, when light waves with frequencies are generated that are not present in the irradiated light, or when---speaking in more general terms---power is redistributed between different Fourier components of a light field. As a rule, nonlinear effects get more pronounced as the light intensity is increased. The reverse is also true: When the light intensity is sufficiently weak, nonlinear processes may safely be neglected. All of classical optics is therefore linear optics.