Abstract
The spatial evolution of plasma filaments in air induced by femtosecond laser pulses is investigated experimentally. Several major filaments and small scaled additional filaments are detected in the plasma channel. The complicated interaction process of filaments as splitting, fusion and spreading is observed. The major filaments propagate stably, and the small scaled additional filaments can be attracted to the major filaments and merged with them. The major filaments are formed due to the perturbation of initial beam profile and the small scaled filaments are mainly caused by the transverse modulational instability.
Highlights
Since the first observation of long propagation of femtosecond laser pulses in air by Braun et al, [1] there has been great interest in the formation of long plasma filaments in air induced by fs laser pulses [2,3,4,5,6,7,8,9,10,11,12,13,14,15]
We present our experimental investigations on the spatial evolution of filaments in air generated by prefocused fs laser pulse
We suggest that the local unbalance of the laser beam profile introduce the formation of primary filaments, the transverse modulational instability and strong interactions in the diffraction plane are mainly responsible for the complicated spatial evolution of filaments
Summary
Since the first observation of long propagation of femtosecond (fs) laser pulses in air by Braun et al, [1] there has been great interest in the formation of long plasma filaments in air induced by fs laser pulses [2,3,4,5,6,7,8,9,10,11,12,13,14,15]. The filaments result from the dynamic balance between the nonlinear Kerr self-focusing due to the nonlinear intensity-dependent refractive index and the plasma defocusing due to the high-order multiphoton ionization (MPI) and diffraction effects of the laser pulses in air. Of about 3.2 GW in air for the laser wavelength λ=800 nm, where n2 = 3.2×10−19 cm2 /W in air [2], the laser beam is self-focused before the geometrical focus. The increased laser intensity due to the self-focusing generates free electrons by the MPI process, and the electrons contribute negatively to the index of refraction of air: n plasma = −ω 2 p (r ) / (2)
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