Induced frequency shifts by counterpropagating subpicosecond optical pulses

We report time-resolved measurements of induced phase modulation that lead to frequency shifts in counterpropagating pump–probe experiments in a glass target with laser pulses of 0.7 ps FWHM and peak intensities of as high as 1 × 1013 W/cm2. Experimental results can only be well explained by including, in addition to the shift induced during copropagation, a frequency shift proportional to the pump-pulse intensity profile during counterpropagation. This additional shift is shown to offer a new approach for mapping of temporal pulse profiles. The effects of pulse asymmetry are also examined.