Investigation Of The Air Plasma Induced By The Intense Femtosecond Laser

Femtosecond laser technique has been the hot research area since its emergence.The propagation of femtosecond laser in air has also attracted the great attention.During the propagation of femtosecond laser pulse in air, a thin filament will beformed due to the dynamic balance between the self-focusing resulting from Kerreffect and plasma defocusing resulting from ionization, which is the well-knownfilamentation process. After the filamentation of femtosecond laser in air, itspropagation is no longer limited by the Rayleigh length, making the long distancepropagation possible. Quite a few interesting physical phenomena are involved in thefilamentation, such as the Kerr self-focusing, self-phase modulation, plasmadefocusing, supercontinnum generation, third-order harmonic generation and conicalradiation etc. These phenomena can find extensive and promising applications, suchas the long distance detection, laser lightning, remote sensing, rain and snow making,laser induces fusion, laser weapon and so on.To explore the evolution of electron density during filamentation, we use thefocusing lens to focus to femtosecond pulse so as to generate filament, and measurethe fluorescence spectrum of plasmas. Using the spectral line at337nm tocharacterize the filamentation process, filamentation in case of different focal lengthsand initial energies is studied. Through the analysis of the experimental data, wearrive at the following conculsions: in the case of not changing the focal length, alongwith the decrease of the initial energy of laser pulse, the collapse distance deviatesmore and more with respect to the focal length of the lens, the intensity luminance andof filament get lower, and length of filament becomes shorter, as a result, we can notobserve filamentation any more eventually; In the case of not changing the initialenergy of the laser pulse, along with the decrease of the focal length of lens, thecollapse distance and filament length get shorter, while the intensity offilament increases gradually.