The scientific subfield of nonlinear optics celebrates its 50th birthday this year. The publication in Physical Review Letters in 1961 by Peter Franken and coworkers at the University of Michigan announcing the second harmonic generation of light by a ruby laser pulse in a quartz crystal is generally regarded as the birth of this subfield. It was commemorated by a symposium at the University of Michigan on October 26, 2011. It has also been celebrated at the NLO meeting in Lihue, Hawaii, in July 2011. Apart from reviews of the early historical development at that meeting, many exciting new topics were explored, especially of nonlinear optical phenomena in media of nanometer dimensions and with pulses of attosecond duration. It is a privilege for some of the early pioneers to witness that NLO at age 50 is alive and well and actively pursued in many countries.

The rapid development of the field in its first two years, 1961 and 1962, was made possible by the existence of the ruby laser, first demonstrated by Theodore Maiman in 1960. The achievement of obtaining shorter pulses by the Q-switching technique, introduced by R. W. Hellwarth and coworkers, and the nonlinear phenomenon by self-focusing by the index profile caused by the laser intensity variation in the beam cross section, contributed to achieve the high beam intensity required to observe many nonlinear optical phenomena. These include two- and three-photon absorption, stimulated Raman and Brillouin scattering, pulse-matching and quasi-phase-matching, etc.

It should be noted that the pumping mechanism used in the realization of the ruby laser is itself an example of nonlinear optics. The imaginary part of the index of refraction at the ruby fluorescence frequency is made negative by intensity of the light modes of the xenon flash lamp at the strong absorption bands in the blue and green region. So perhaps the 50th anniversary of nonlinear optics should have been celebrated one year earlier, together with the 50th anniversary of the ruby laser. Nonlinear optics was indeed included in the laserfest manifestations last year.

It is sad that neither Theodore Maiman nor Peter Franken lived long enough to celebrate these 50th birthdays. Neither did Boris Stoicheff, who first demonstrated stimulated Brillouin scattering. We should also commemorate the contributions of our Russian colleagues Rem Khokhlov and Sergei Akhmanov and their successor Nikolai Koroteev. These three pioneers all died at an age well below the average life expectancy. Franken’s early paper on second harmonic generations inspired Khokhlov and Akhmanov, just as it inspired me. The Soviet state was quick to recognize the importance of nonlinear optics by awarding the Lenin Prize to Khokhlov and Akhmanov. I remember with pleasure my cordial reception in the Soviet Union during my visits in 1967 and 1971.

The main purpose of this focus issue, which contains contributions of pioneers who are still active in the 50-year-old field, is to inspire a younger generation in building the future of nonlinear optics.