The influence of intense control laser pulses on homodyne-detected rotational wave packet dynamics in O ₂ by degenerate four-wave mixing

We illustrate how the preparation and probing of rotational Raman wave packets in O ₂ detected by time-dependent degenerate four-wave mixing (TD-DFWM) can be manipulated by an additional time-delayed control pulse. By controlling the time delay of this field, we are able to induce varying amounts of additional Rabi cycling among multiple rotational states within the system. The additional Rabi cycling is manifested as a change in the signal detection from homodyne detected to heterodyne detected, depending on the degree of rotational alignment induced. At the highest laser intensities, Rabi cycling among multiple rotational states cannot account for the almost complete transformation to a heterodyne-detected signal, suggesting a second mechanism involving ionization. The analysis we present for these effects, involving the formation of static alignment by Rabi cycling at moderate laser intensities and possibly ion gratings at the highest intensities, appears to be consistent with the experimental findings and may offer viable explanations for the switching from homodyne to heterodyne detection observed in similar DFWM experiments at high laser field intensities (>10 ¹³ W/cm ²).