Effects of strong driving fields in resonant four-wave mixing schemes withdown-conversion

An explicit solution is obtained for the four-wave frequency mixing omega(d )= omega(a) - omega(b) + omega(c) Of two strong fields a and c and two weak fields b and d in a four-level system with large Doppler broadening in col linear geometry: where the frequencies of weak fields are nearly equal, ome ga(b) similar or equal to omega(d), and the medium is optically thin. Witho ut weak fields there are two independent two-level systems. A pair of weak fields probes two other allowed transitions. A peak of the mixing coefficie nt as a function of intensity is found around an equal Rabi splitting of bo th two-level systems. The effect is based on a resonance between two closed cycles of four-wave mixing via different dressed states. Three, four, or s ix peaks are predicted in the dependence of the mixing coefficient on the f requency of the weak field; two of them are a consequence of averaging over velocities. The model allows an interpretation of the dependence of the ou tput wave power on the intensity and detuning in recent experiments on freq uency mixing in sodium vapor. [S1050-2947(99)06401-X].