Diffraction-limited propagation and high gain in optically dense sodium vapor at low pump intensity

Summary form only given. Using coherent population trapping and nondegenerate four-wave mixing, we show that it is possible to simultaneously achieve high gain and diffraction-limited propagation of a weak probe, even though the sodium vapor is opaque to the probe in the absence of the pump beams. This distortion-free propagation is to be compared with what can be achieved using electromagnetically induced transparency (EIT), but the required pump intensity is orders of magnitude smaller, since it is not necessary to saturate the entire inhomogeneous absorption line. This has important implications for low intensity, high gain aberration correction, image correlation, and generation of squeezed light via four wave mixing (FWM), as well as for frequency upconversion.