Nonlinear mixing in the presence of transparency induced by an additional strong field

It is shown that the use of additional strong laser radiation may result in a resonant increase of the nonlinear susceptibility in the generation of the sum frequency and harmonics, and it may also reduce the absorption of the generated radiation near the centres of the Stark-split components. The phase-matching conditions are also satisfied near these centres. The conversion efficiency is two or three orders of magnitude higher than in the absence of an additional strong radiation field.