Efficiency of degenerate four-wave mixing in a two-level saturable absorbing medium

Degenerate four-wave mixing in a two-level saturable absorbing medium is calculated on and off resonance for arbitrary input laser intensities and absorption coefficients. The maximum efficiency occurs at laser intensities higher than saturation in an optically thick medium. On resonance, the maximum efficiency can approach ∼4%, and it can be as large as 25% when the laser is tuned off resonance.