LAUSR

The theory of third-harmonic generation in optical gas breakdown by a high power laser field is developed. The theory treats third-harmonic generation in the presence of a self-generated quasi-static magnetic field. The distribution function of originated electrons and nonlinear third-harmonic current density are evaluated based on the kinetic theory. We have shown that the third-harmonic is generated by beating of electron's longitudinal velocity component vz and laser magnetic field B(r, t). The influences of the laser field polarization and magnetic field strength on the third-harmonic efficiency are discussed, separately. The results show that the third-harmonic exists even in the absence of the self-generated magnetic field; however, the magnetic field smoothly enhances the third-harmonic power with polarization near to the circular polarization. Furthermore, calculations revealed that the third-harmonic is never generated by a circularly polarized laser field, while it is more effectively generated ...