Sixty years of nonlinear optics

Apparently, the concept of 'nonlinear optics' was first introduced by S.I. Vavilov when analysing the results of absorption saturation in uranium glasses obtained in 1926. In particular, he noted that “In an absorbing medium 'nonlinearity' should be observed not only in relation to absorption. The latter is related to dispersion, so the speed of light propagation in a medium, generally speaking, must also depend on the light power”. One of the defining criteria for nonlinear optics, according to S.I. Vavilov, is the violation of the superposition principle. Sixteen years later, in 1942, E. Schrödinger, considering the problem of light scattering by electrons, also defined the processes as nonlinearly optical.