Abstract
Our experiments show that external focusing and initial laser energy strongly influences filament generated by the femtosecond Ti–sapphire laser in air. The experimental measurements show the filament length can be extended both by increasing the laser energy and focal length of focusing lens. On the other hand, the plasma fluorescence emission can be enhanced by increasing the laser energy with fixed focal length or decreasing the focal length. In addition, the collapse distance measured experimentally are larger than the calculated ones owing to the group-velocity-dispersion effect. In addition, we find that the line widths of the spectral lines from $\text{N}_{2}$ is independent of filament positions, laser energies and external focusing.
Highlights
A stable plasma channel will be formed when the Kerr self-focusing and plasma defocusing resulted from the multiphoton ionization reach a dynamical balance, which is called filament
The filament-induced fluorescence is collected by lenses and the spectra are guided to the spectrometer (Spectra Pro 500i, PI Acton, and the grating is 2400 and 150 grooves/mm) through an optical fibre fixed on the moving stage
It is found that the filament length can be effectively extended by increasing the laser energy and focal length of focusing lens
Summary
A stable plasma channel will be formed when the Kerr self-focusing and plasma defocusing resulted from the multiphoton ionization reach a dynamical balance, which is called filament. The plasma generation balances the Kerr self-focusing effect and leads to a limited beam diameter as well as limited peak intensity around 5×1013 W/cm The intensity of filament is high enough to stimulate and ionize air molecule emitting the characteristic spectral line of atoms and molecules, which has found applications in sensing atmospheric trace species[9,10,11]. The plasma fluorescence generated by femtosecond laser pulses is measured along the propagation path to investigate the femtosecond filamentation process. The influence of pulse energy and focal length of the external lens on plasma fluorescence emission as well as filamentation process in air is studied
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