Nonlinear optics: the first 50 years

In the summer of 1961, a landmark experiment was performed at the University of Michigan in which optical second harmonic generation was observed for the first time. This event 50 years ago marked the birth of modern nonlinear optics, and this article celebrates the first half century of what is now a vast and vibrant field at the cutting edge of laser technology. The focus is mainly on nonlinear optics in the 1960s partly because it is appropriate in this anniversary year to remember the genesis of the field, but also because such remarkable progress was made in the first few years. However, a brief review of where things stand at present is included, and one aspect of the field today (high harmonic generation) is taken as a representative example of an area of nonlinear optics that lies at the current frontier of knowledge.