Propagation of twin laser pulses in air and concatenation of plasma strings produced by femtosecond infrared filaments

Two consecutive ultrashort laser pulses separated by 100 fs and polarized in orthogonal directions are shown to form a long femtosecond filament that connects both filaments created independently by each pulse from the interplay between photoionization and self-focusing. Measurements of the electron density generated by photoionization of air show that this connection is obtained for a very specific time delay between the pulses. A strong shortening of one of the pulses is associated to this connection of plasma channels. This phenomenon is modeled via numerical simulations of the propagation and interaction of the pulses.