Exponential gain in resonant four-wave mixing via dressed inversions

High-efficiency coherent high-frequency generation via resonant four-wave mixing is investigated in a dressed states analysis of a bichromatically driven three-level system. We propose the existence of exponential gain due to population transfer at the anticrossings of closely spaced inverted dressed states. This occurs when the Rabi frequency of the weaker detuned driving field is much smaller than the leading resonant Rabi frequency and simultaneously comparable to the largest atomic relaxation rate. The dressed population inversion and thus the exponential gain is not affected until the generated field becomes comparable in intensity to one of the two applied fields, when saturation may take place. The analysis is applied to a krypton configuration of interest to current experiments.