HARMONIC-GENERATION DUE TO PLASMA EFFECTS IN A GAS UNDERGOING MULTIPHOTON IONIZATION IN THE HIGH-INTENSITY LIMIT

In cases in which the ponderomotive energy is much larger than the ionization energy, which correspond to the highintensity limit or to the low-frequency limit, harmonic generation in gases near the ionization threshold can be understood from a plasma-physics point of view. Multiphoton ionization, which can be described by using the dc tunneling limit, takes place in a time interval localized around the maximum of the electric field. The plasma current thus generated varies on the same fast time scale and is responsible for harmonic generation. A model using those properties that has been developed shows the presence of a large number of odd harmonics. © 1990 Optical Society of America.