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.