Highly efficient four-wave mixing (FWM) and six-wave mixing (SWM) processes can coexist in a four-level $Y$-type atomic system due to atomic coherence. The simultaneously opened dual electromagnetically induced transparency windows in this four-level atomic system allow observation of these two nonlinear optical processes at the same time, which enables detailed studies of the interplay between the FWM and SWM processes. Three-photon and five-photon destructive interferences are also observed.

• Received 25 November 2006

DOI:https://doi.org/10.1103/PhysRevLett.99.123603