Detailed velocity-dependent line shapes for degenerate four-wave mixing spectra in a two-level atomic system

We study phase-conjugate degenerate four-wave mixing in an optically pumped, two-level atomic system (the 3s(2)S(1/2), F=2, m(F)=2-->3p(2)P(3/2), F=3, m(F)=3 transition in a diffuse, collision-free, thermal beam of sodium). In this investigation, we study the detailed line shape and strength of the four-wave mixing signal as a function of the intensity of the cw pump waves, and of the velocity of the atoms. Our technique allows us to examine the four-wave mixing interaction under conditions in which the Doppler shift, the natural linewidth of the transition, and the Rabi frequency for the interaction are each comparable with one another. From our experimental measurements and computational results, we show that the line shapes are very complex in this regime, and that they depend sensitively upon the atomic velocity, especially in the case when the intensity of the forward and backward pump beams exceeds the saturation intensity for the transition.