Suppression of Amplified Spontaneous Emission by the Four-Wave Mixing Process

Two-photon-resonant excitation of the sodium $3d$ level can lead to the generation of new frequencies either by amplified spontaneous emission at the $3d\to 3p$ transition frequency or by a resonantly enhanced four-wave mixing process. Competition between these two processes has been observed. The four-wave mixing process can suppress amplified spontaneous emission by preventing the excitation of the $3d$ level due to an interference between two different pathways connecting the ground ($3s$) and $3d$ states.