Saturation behavior of resonant degenerate four-wave and multiwave mixing in the Doppler-broadened regime: Experimental analysis on a low-pressure Ne discharge

In this paper, we present a detailed analysis of saturation effects in resonant degenerate four-wave mixing (phase conjugation) or multiwave mixing (high-order diffraction on an optically induced grating). The experiments are performed in a low-pressure Ne discharge in the Doppler-broadened regime, both on an "ideal" two- or three-level system (1s_{4}(J = 1) - 2p_{3}(J = 0)) or on a degenerate multi-level system (1s_{5}(J = 2) - 2p_{9}(J = 3)). The saturation effects appearing when both pump beams are intense are studied, and a very good agreement is found with the previously developed theory 8]. It is demonstrated that there is an absolute maximum to the phase-conjugate reflectivity, and various effects on the lineshape (splitting, linewidth, etc.) are investigated. The effects produced by the Gaussian nature of the incident beams are analyzed in order to interpret some of the discrepancies between theory and experiment. Several new theoretical results for high-order diffraction are also presented, and compared with the experiments. The possibility of dealing with the saturation effects in four-wave mixing when all three incident fields are saturating appears as a consequence of these studies on multiwave mixing. Notably discussed are the case of an intense diffracting field, and of intense grating fields, whose intensity may be unbalanced. Finally, the polarization selection rules for high-order diffraction experiments are established.