Strong field effects in rotational femtosecond degenerate four-wave mixing

We study theoretically strong field effects in rotational femtosecond degenerate four-wave-mixing (DFWM). First, we developed a perturbative approach and calculated the leading correction to the standard (weak field) formula for femtosecond DFWM signal. Second, we constructed a nonperturbative approach for computing femtosecond DFWM signals excited by (short) pulses of arbitrary intensity. Third, we worked out the theory to describe femtosecond DFWM with an extra aligning pulse. We show that the strong-pulse-induced nonadiabatic alignment does explain many experimentally detected features that develop in femtosecond DFWM signals with increasing laser intensity beyond the standard weak field regime. However, we also show that this nonadiabatic alignment cannot solely be responsible for the onset of the heterodyne detection and pronounced constant background in DFWM signals excited by high intensity laser pulses.