Our experiment is different from the ones usually studied in the resonant FWM in a way that the excited states are not one-photon but three-photon resonant with the input beams. In this experiment we send the near infrared light with three beams which results in getting ultraviolet light as the signal in the fourth beam

The system is sodium atom, and the laser light is tuned to 804 nm which is resonant with the three-photon allowed transitions to the two excited states, 6p_1/2 and 6p_3/2. The one-photon transition from these states to the ground state is at 268 nm.