Direct measurement of the acoustic waves generated by femtosecond   filaments in air
  We present direct measurements of the gas acoustic dynamics following interaction of spatial single- and multi-mode 50 fs, 800 nm pulses in air at 10 Hz and 1 kHz repetition rates. Results are in excellent agreement with hydrodynamic simulations. Under no conditions for single filaments do we find on-axis enhancement of gas density; this occurs only with multi-filaments. We also investigate the propagation of probe beams in the gas density profile induced by a single extended filament. We find that light trapping in the expanding annular acoustic wave can create the impression of on-axis guiding in a limited temporal window. 
J. K. Wahlstrand, N. Jhajj, E. W. Rosenthal, S. Zahedpour, and H. M.   Milchberg
physics.optics