Polarization Beats in a Cascade Three-Level System

Based on the interference between one-photon and two-photon processes, we report polarization beats in a cascade three-level system (PBCTS). We performed PBCTS experiments in Na vapor, which showed a modulation of the four-wave mixing signal intensity with period 55fs, corresponding to the beating between the resonant frequencies of the transitions from 3S_1/2 to 3P_3/2 and from 3P_3/2 to 4D_3/2,5/2.