Sonographic probing of laser filaments in air

Abstract

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.

Acoustic Measurement of the Length of Air-plasma Filament Induced by an Intense Femtosecond Laser Pulse

“…Optical field enhances free electron temperature, Te, to the order of 10 4 -10 5 K, while the temperature of neighboring molecules or ions, Tm, is much lower [18]. Within the following 10 −9 -10 −8 s, Tm rapidly ascents through energy transfer due to elastic and inelastic collision between hot electrons and the surrounding air molecules (nitrogen and oxygen), until thermal equilibrium appears [19,20]. This sudden increase of localized gas temperature gives rise to a shock wave that persists for a short distance, then relaxing into acoustic wave [21,22].…”

“…Therefore, measure for plasma column is only indirectly implemented based on filament manifest phenomenon. Based on acoustic radiation from plasma induced by strong femtosecond laser pulses [15,16], acoustic measuring method is employed to examine the plasma filament, depending on our laboratory existing femtosecond laser system. Length of filament is examined through detecting acoustic pressure at different position of filament.…”

Optimal control of filamentation in air

“…The signal in three different spectral bands ͑360± 5, 400± 5, and 500± 5 nm͒ was the optimizing variable in the optical experiments. Alternatively, optimizations were performed on the electron density, recorded by sonometric measurements 26,27 after 28 m propagation of the laser beam. In both experiments, the signal was averaged over 50 shots.…”

“…3, the number of filaments increases steeply at the location of the microphone, which is consistent with our previous observation that the sound is maximal at the beginning of the filaments. 26 In other words, the genetic …”

Multijoule scaling of laser-induced condensation in air

“…24,25 The photon bath contribution to condensation is confirmed by the observation of ozone generation from filamentfree beams at an intensity as low as 150 GW/cm 2 . At this level, a sonometric setup 26 detects no ionization of the air. Ozone generation exhibits the same dual-component dependence on both fluence and intensity as observed for the generation of nanoparticles in the photon bath of the DRACO laser beam (Fig.…”