Efficient low-intensity optical phase conjugation based on coherent population trapping in sodium

We have observed optical phase-conjugate gain (>50) in sodium vapor, using low-intensity pump lasers (1 W/cm2), with a response time of the order of 1 μs. Coherent population trapping is experimentally identified as the phase-conjugate mechanism. A theoretical model is presented that supports these observations by showing that coherent population trapping can write large-amplitude nonlinear-optical gratings at laser intensities well below those needed to saturate the optical transitions.