The role of multiphoton ionization in harmonic generation by wave-mixing

We analyze the correlation between harmonic generation and multiphoton ionization produced by the mixing of the fundamental and second harmonic of a picosecond Nd-YAG laser in gas. The ion and harmonic signals are recorded versus the relative delay between both fields by a self-correlation technique and versus the focus position of both beams through the gas jet. Our results suggest, at least in our conditions, a positive influence of ionization on the harmonic conversion efficiency. We show that the numerical solution of the propagation equation, which includes electron-ion scattering as the basic mechanism of harmonic generation, gives a good description of the experimental results.