Modulation of Time-Delayed Four-Wave Mixing Signals: Theory and Experiments Performed in Sodium Vapor.

Two transient time-delayed four-wave mixing experiments have been performed on the Na D doublet and modulations of 1.9 picoseconds and 980 attoseconds have been observed. These periods correspond to the difference and sum frequencies of the two Na D lines. Both experiments were performed using 7 ns pulses of light with frequency components at each of the D-line transitions. The effects of superposition -state modulations are observed in the integrated TDFWM signal as a function of the time delay. As the time delay is varied, the lowest diffraction order mixing signal is modulated. Higher diffraction order mixing signals contain modulation components at integral multiples of the beat frequency. In the case of the difference frequency beats this modulation exists even for delays greater than the excitation pulse width. This work thus extends photon -echo modulation spectroscopy into a new regime to study coherences between states that are widely separated in frequency compared with the bandwidth of the exciting laser pulses.