Vol. 1, Issue 7, Part J (2015)

Abstract
The field of experimental nonlinear optics was born with the observation of second harmonic generation by Franken et al. wherein coherent, high-intensity laser light having a center wavelength of 694 nm, impinging on a sample of crystalline quartz produced radiation at twice the frequency (or half the wavelength, i. e., 347 nm) of the incident radiation. Shortly thereafter, observation of a remarkable two-photon process in which radiation incident on an isotropic sample that was seemingly transparent to it produced a fluorescence having a wavelength much shorter than the incident radiation.