Abstract
Extending the longitudinal range of plasma channels created by ultrashort laser pulses in atmosphere is important in practical applications of laser-induced plasma such as remote spectroscopy and lightning control. Weakly focused femtosecond Gaussian beams that are commonly used for generating plasma channels offer only a limited control of filamentation. Increasing the pulse energy in this case typically results in creation of multiple filaments and does not appreciably extend the longitudinal range of filamentation. Bessel beams with their extended linear foci intuitively appear to be better suited for generation of long plasma channels. We report experimental results on creating extended filaments in air using femtosecond Bessel beams. By probing the linear plasma density along the filament, we show that apertured Bessel beams produce stable single plasma channels that span the entire extent of the linear focus of the beam. We further show that by temporally chirping the pulse, the plasma channel can be longitudinally shifted beyond the linear-focus zone, an important effect that may potentially offer additional means of controlling filament formation.
Highlights
Since the original report on generation of extended plasma channels by intense femtosecond laser pulses in air [1] this phenomenon has been the subject of active research motivated by various potential applications such as remote spectroscopy [2], generation of few-cycle optical pulses [3], lightning control [4], and generation of THz radiation [5]
Of particular practical interest in remote spectroscopy and lightning control is the creation of extended filaments
The multiple filaments are randomly distributed within the laser beam and their locations fluctuate on the pulse-to-pulse basis
Summary
Since the original report on generation of extended plasma channels by intense femtosecond laser pulses in air [1] this phenomenon has been the subject of active research motivated by various potential applications such as remote spectroscopy [2], generation of few-cycle optical pulses [3], lightning control [4], and generation of THz radiation [5]. The fundamental mechanisms responsible for the stable self-guided propagation of the ultrafast high-intensity laser pulses in Kerr media are well understood, particular details can still be puzzling due to the richness and complexity of the highly nonlinear physics involved [6] It has been found by numerical simulations and later confirmed experimentally that only a small fraction of the intensity of the ultrafast laser beam is confined in the plasma channel, while the remaining portion of the beam is propagating in the close to linear regime and is subjected to ordinary diffraction. The single stable filament is pinned to the geometrical axis of the beam and its location experiences negligible pulse-to-pulse fluctuations This behavior is compared with the case of filamentation of weakly focused Gaussian beams, in which increased pulse energy is shown to create multiple filaments. The experimentally observed longitudinal extension of the plasma channel by pulse chirping may offer additional means of control over filament formation
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