High-order harmonic generation (HHG) is a strongly nonlinear process in which femtosecond (1 fs = 10-15s) laser pulses are disturbed by a medium, e.g. gas, in such a way so that very high-order harmonics of fs laser wavelength are created. Typically, the harmonics orders could as big as 27th and higher. In physical explanation the phenomenon involves outer most electrons of the (for example) gas atom. The atom is ionized by very strong laser field. The resuming energy of the laser field is then transferred into kinetic energy of the ionized electron.
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