Expanding the scope of optical nonlinearity

Optical nonlinearity is nothing new. In 1961 – within a single year of the laser age dawning – the first such observations (of two-photon induced fluorescence) were recorded [1], fulfilling theoretical predictions that had been made by Göppert-Mayer [2] some thirty years before. At much the same time, observations of the first known parametric nonlinear phenomenon, second harmonic generation [3], proved that optical nonlinearity need not necessarily equate to extremely weak signals. These discoveries paved the way for the rapid ensuing development of frequency conversion tools, now so familiar, in which Bloembergen’s pioneering role was recognized by his award of a Nobel Prize in 1981.