Backward saturation in four-wave mixing in neon: Case of parallel pump polarizations

We present in this paper a theoretical and experimental study of the effect of an intense pump beam on four-wave-mixing emission in the geometry of phase conjugation. The theoretical model is developed for Γ (natural width) <${\Omega }_1$ (Rabi frequency of the intense pump) <ku (Doppler width) and uses the dressed-atom approach. We have analyzed degenerate and nearly degenerate four-wave mixing. In this latter case, we have studied three situations according to which beam (intense pump, weak pump, or probe) has a different frequency. The experimental study has been done in a neon cell using cw lasers. Our experimental results are in excellent agreement with theory. This study shows the need to take into account the atomic motion in the theory. However a quantitative analysis cannot be done assuming an infinite Doppler width and we show the importance of the Boltzmann factor. In the case of nearly degenerate four-wave mixing, narrow resonances corresponding to transitions between energy levels of the dressed atom are observed.