High-Harmonic Generation
A high-power femtosecond infrared laser, focused into a pressurized argon jet in a high-vacuum chamber, creates a glowing plasma. In the plasma, the highly nonlinear process of high-harmonic generation (HHG) generates coherent harmonics from the infrared all the way to the extreme-UV and soft-X-ray regimes, which can be used for coherent imaging.
—Sven Weerdenburg, Delft University of Technology, Delft, Netherlands