Attraction and repulsion of parallel femtosecond filaments in air

We present an approach to explore and control nonlinear interactions between two orthogonally polarized femtosecond filaments launched parallel in air. The self-phase and cross-phase modulations due to the Kerr effect and cross-(de)focusing induced by the plasma and molecular alignment were distinctly identified resulting in attraction and repulsion of parallel filaments with different spatiotemporal proximities. Their interaction ranges were analyzed by comparing the interaction-induced displacements of parallel filaments at different initial separations. As proved by field-free displacements of parallel filaments around the impulsive rotational Raman excitation of the diatomic molecules in air, the molecular alignment exhibited a relatively longer interaction range than that of the plasma defocusing and the Kerr effect.