Cascaded free-induction decay four-wave mixing

We report the observation of cascaded optical free-induction decay four-wav e mixing (FID-FWM) signal. This process can take place when nonlinear optic al measurements are carried out with pulses that are orders of magnitude sh orter than the dephasing time of the sample. Experimental observations and theoretical calculations show that the coherent emission from the first las er pulse participates as a time-delayed local electric held to yield the ca scaded signal. We arrive at this conclusion based on pulse sequences of deg enerate noncollinear femtosecond pulses for which three-pulse FWM is forbid den. Further confirmation was obtained from experiments where the time dela y between two pulses were used to form ground or excited state populations, the signal reflected the corresponding ground or excited state dynamics. A lthough FID is long lived, the femtosecond resolution was found to be maint ained in our measurements on gas phase molecular iodine. This is because th e FID is modulated in the femtosecond time scale by the molecular dynamics of the system; its intensity and modulation were confirmed using femtosecon d time-gated upconversion measurements. (C) 2001 Elsevier Science B.V. All rights reserved.