Two-beam coupling between filament-forming beams in air.

We experimentally demonstrate two-beam coupling between nearly identical filament-forming beams intersecting in air. A 7% amplification of one beam occurs at the energy expense of the other in a single interaction, controllable by adjusting their relative delay by tens of femtoseconds. The data are consistent with the impulsive Raman nonlinear response of the air molecules as the coupling mechanism. The filament conical emission is controllably enhanced or suppressed by the interaction, indicating that two-beam coupling may be an effective means for filament regeneration and control.