Attosecond nonlinear optics open the door to coherent x-rays.

The article discusses the history of nonlinear optics which resulted to the development of coherent x-ray lasers. The first experiment in nonlinear optics was done by Peter Franken of the University of Arizona using a ruby laser shortly after the laser's first introduction. On the other hand, the first successful study in high-order harmonic generation (HHG) happened in 2003 with the demonstration of quasi-phase-matching (QPM) of HHG using periodically modulated waveguides.