Saturation-induced distortion of four-wave-mixing spectra in a two-level system

It is shown that saturation causes the three-peaked four-wave-mixing (FWM) signal at frequency 21-2, obtained from the interaction of a two-level system with a probe laser of varying frequency 2 and a pump laser of fixed frequency 1, to become asymmetrical on increasing the probe-laser intensity. Whereas saturation peaks occur in the population-inversion spectrum at one-photon and multiphoton resonances, this is not so for the FWM spectrum, where often the peaks correspond to nothing more than minima in the population-inversion spectrum.