Sub-Doppler resolution in strongly saturated copropagating degenerate four-wave mixing

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Abstract
The observation of a new type of saturation effects in degenerate four-wave mixing in Na vapor is reported. The experimental configuration is neither counterpropagating nor velocity selective, and yet sub-Doppler high-resolution lines are observed at the atomic-transition frequencies, and at the weighted center of a Doppler-broadened distribution. These new features are the result of an interplay between absorption and saturation, and the inhomogeneous broadening is essential for their existence.