High Harmonic Generation (HHG)
The first High Harmonic Generation (HHG) was observed in 1977 in interaction of intense CO2 laser pulses with plasma generated from solid targets. HHG in gases, far more widespread in application today, was first observed by McPherson and colleagues in 1987, and later by Ferray et al. in 1988, with surprising results: the high harmonics were found to decrease in intensity at low orders, as expected, but then were observed to form a plateau, with the intensity of the harmonics remaining approximately constant over many orders. Plateau harmonics spanning hundreds of eV have been measured which extend into the soft x-ray regime. This plateau ends abruptly at a position called the High Harmonic Cut-off.
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