Competition of multiple filaments during the propagation of intense femtosecond laser pulses

We observed a universal phenomenon of the competition among multiple filaments generated during the propagation of intense femtosecond laser pulses in air. We show that the fluorescence signal from the excitation of nitrogen molecules inside the plasma channel contains important information pertaining to the formation and interaction of multiple filaments. The detected backscattered nitrogen fluorescence from inside the filaments yielded irregular changes from shot to shot which cannot be explained by fluctuation arising from the initial laser pulse itself. Numerical simulations reveal a complex dynamics of multiple filament propagation and interaction dynamics that depends strongly on the initial perturbations of the laser beam. The irregular changes of the fluorescence signal are attributed to the interference between adjacent hot spots that evolve into filaments which give rise to new hot spots (filaments) in between, and thus give the appearance of the fusion or branching of filaments.