The acoustic wave emitted from the plasma channel associated with a filament induced by a femtosecond laser pulse in air was detected with a microphone. This sonographic detection provides a new method to determine the length and the spatial profile of the free-electron density of a filament. The acoustic wave is emitted owing to the expansion of the gas in the filament, which is heated through collisions with high-energy photoelectrons generated by multiphoton ionization. Compared with other methods, the acoustic detection is simpler, more sensitive, and with higher spatial resolution, making it suitable for field measurements over kilometer-range distances or laboratory-scale studies on the fine structure of a filament.
© 2003 Optical Society of America
(010.1300) Atmospheric and oceanic optics : Atmospheric propagation
(190.4180) Nonlinear optics : Multiphoton processes
(190.5940) Nonlinear optics : Self-action effects
(190.7110) Nonlinear optics : Ultrafast nonlinear optics
(350.5400) Other areas of optics : Plasmas
Jin Yu, Didier Mondelain, Jérôme Kasparian, Estelle Salmon, Sylvain Geffroy, Catherine Favre, Véronique Boutou, and Jean-Pierre Wolf, "Sonographic Probing of Laser Filaments in Air," Appl. Opt. 42, 7117-7120 (2003)