Observation of modulational instability in Nd-laser beat-wave experiments.

The modulational instability of the high-phase-velocity plasma wave generated by a Nd-laser beat wave has been observed for the first time. Both ion and electron waves are detected when the electron density is close to the resonant density for which the plasma frequency is equal to the difference frequency between the two pump lasers. We also detect the presence of harmonics of the beat frequency which are a signature of high-amplitude density perturbations.