ABSTRACT
The term 'nonlinear optics' was first introduced by the Russian scientist S. I. Vavilov in 1925, who managed to observe a decrease in the absorption of light by uranium glasses when optical radiation passes through them with high intensity. In the early 60s of the same century, after the creation of lasers, nonlinear optical phenomena became not only observable, but also turned into a serious tool for studying matter, and became the basis for the creation of completely new laser devices. Nonlinear optics significantly expands our understanding of the interaction of light with matter. For example, in the earliest experiments on nonlinear optics carried out by S. I. Vavilov and V. L. Levshin, a slight decrease of the uranium glass absorption coefficient with increasing light intensity was observed. Nonlinear incoherent effects include the multiphoton process of radiation absorption. For example, in the act of interaction of light with the medium, it is possible to simultaneously absorb three photons or more.
