While some purists might argue that the field of nonlinear optics began with the discovery of the DC Kerr effect in 1875 , for most scientists the field began a year after the invention of the laser with the report in 1961 of the all-optical nonlinear processes of optical harmonic generation by Franken et al.  and two photon absorption by Kaiser and Garrett . Since those early days, nonlinear optics has grown to become a dominant subfield in optics. Indeed, most journal articles in optics today implicitly or explicitly make use of nonlinear optics, whether they deal with fundamental optical phenomena, source development, optical information processing, medical diagnostics, or sensor applications, to name but a few areas. In celebration of the fiftieth anniversary of nonlinear optics, I have invited several prominent individuals and groups, including some of the original pioneers, to give us personal perspectives on how the field has evolved or what its status is today in several important areas from fiber optics to liquid crystals. In reading these articles, the reader will definitely obtain the impression that nonlinear optics continues to be a thriving field 50 years after its birth. Indeed, the number of submissions to this and other optics journals shows no signs of abating.
I thank all the very busy authors for taking the time to offer these substantial contributions, which we hope will serve as an important resource for our readers.
Note added: Shortly after submitting his manuscript, Oleg Aktsipetrov, who I had asked to give a Russian perspective on the history of nonlinear optics as well as his own contributions to the field, passed away suddenly. He will be greatly missed.
1. J. Kerr, “A new relation between electricity and light: dielectrified media birefringent,” Philos. Mag. 50, 337–348 (1875).
2. P. A. Franken, A. E. Hill, C. W. Peters, and G. Weinreich, “Generation of optical harmonics,” Phys. Rev. Lett. 7, 118–119 (1961). [CrossRef]
3. W. Kaiser and C. G. B. Garrett, “2-photon excitation in ,” Phys. Rev. Lett. 7, 229–231 (1961). [CrossRef]