Non-linear X-ray Science.

The advent of the laser in the early 1960s represented a revolution in Optics and Spectroscopy that is still impacting our lives to this day. The availability of coherent and intense beams of laser light enabled the birth of non-linear optics as first demonstrated by the historic experiments in 1961 on second harmonic generation, and the first observation of two-photon absorption. The rapid discovery of diverse non-linear (NL) optical methods had a deep impact in Science that were recognised by Physics (1981, 2018) and Chemistry (1999) Nobel Prizes. [ABSTRACT FROM AUTHOR]