High efficiency four-wave mixing at low power densities and low concentrations: The role of coherence

Two transform limited nanosecond lasers axe used in a delayed pulse mode to prepare a highly coherent mixture of the ground state and the upper state of a two-photon transition. A picosecond laser co-propagates through the prepared medium at a delayed time and generates a four-wave mixing field. With a carefully chosen time delay, the wave mixing process is phase matched for all detunings and concentrations, and all of the third laser photons can be converted to the mixing field, independent of the intensity of the short-pulse laser. The condition for optimum conversion efficiency does not usually correspond to maximum atomic coherence. In the case of a near resonance short-pulse laser, the condition for phase matching leads to efficient conversion, with the mixing field propagating at the speed of light in vacuum with no distortion.