Efficient Nonlinear Frequency Conversion with Maximal Atomic Coherence

We demonstrate efficient nonlinear frequency conversion in atomic Pb vapor under conditions where the linear susceptibility and the effective nonlinear susceptibility are of the same order. This is accomplished by using electromagnetically induced transparency to prepare a near-maximal atomic coherence on a Raman transition. This strongly driven transition is used to convert an intense laser beam from 425 to 293 nm with an efficiency of $\sim 40\%$.