Dynamics of triple quantum beats retrieved from coupled four-wave mixing signals

A theoretical model of coupled four-wave mixing (FWM) processes comprising five atomic energy levels was proposed to investigate the coherent characteristics of atomic wave packets using perturbation theory. As several states in atoms are coherently excited by a two-photon transition, triple coupled FWM can be produced, and triple quantum beats associated with the atomic energy levels are embedded in the FWM signals. Triple coupled FWM processes were achieved experimentally in Rb vapor by two-photon excitation using broadband optical pulses. A pump–probe scheme was utilized to record the time-varying FWM signals, and Fourier transform was applied to retrieve the quantum beats. Furthermore, short-time Fourier transform was implemented to investigate the dynamics of quantum beats. The results show that the coherent characteristics of atomic wave packets, which are intimately related to the dynamics of quantum beats, can be probed by coupled nonlinear optical wave mixing.