  Ultracold Rb atoms were used to demonstrate non-degenerate four-wave mixing
through a Rydberg state. Continuous 5S-5P-nD two-photon excitation to the
Rydberg state was combined with an nD-6P tuned laser in a phase matched
geometry. The angular dependence, spatial profile, and dependence on detuning
were investigated, showing good agreement with theory. Under optimum conditions
50 percent of the radiation was emitted into the phase-matched direction.
