Controlled interactions of femtosecond light filaments in air

We experimentally demonstrate coherent control of two light filaments in air generated by ultrafast laser pulses, as proposed by Xi et al. [Phys. Rev. Lett. 96, 025003 (2006)]. We show that depending on the relative phase and the incidence angle, the filaments can experience fusion, repulsion, energy redistribution, and spiral motion for propagation in air. By translating the initial beams on subwavelength scales, we achieve $~1$-mm transverse deflection of approximately $0.5$-mJ energy for parallel propagation and $7$${}^{^∘{}}$ angular deflection for spiral motion in $3$-m propagation. Our approach using the relative beam phases to reliably control propagation of femtosecond light filaments is potentially applicable to remote sensing and lightning guiding.