Picosecond Pulse Multiphoton Coherent Propagation in Vapors

We present a method to study multiphoton excitation by intense picosecond pulses in vapors. The technique, based on interferometric cross correlations of the multiphoton stimulated reradiation with the pulse itself, requires an interferometric delay line to generate accurately phased pulse sequences. Experimental data for two-photon coherent propagation in lithium vapor agree with our theoretical model.