Filamentation Laser Femtoseconde dans l'Air et Application au Guidage de Decharges Electriques et à la Generation de Rayonnement Terahertz

摘要

Laser filamentation phenomena spontaneously occurs during the propagation of an infrared femtosecond laser beam in air, when the beam peek power exceeds a few Gigawatts. At such intensity level the beam collapses on itself due to the Kerr effect and a dynamic competition between diffraction, Kerr effect and plasma defocusing allows to maintain a very high intensity in the core of the beam over a considerable distance. It produces then a thin plasma column, whose length can reach hundreds of meters. Several applications relying on the remote generation of laser filaments have been proposed. In this case beam propagation and filament formation can be significantly affected by the atmospheric turbulence. To quantify the effects of turbulence on filamentation we performed experimental and numeric studies of the influence of air turbulence on the pointing stability of filaments and the filamentation distance. Moreover, we studied the properties of the plasma column in the filament and more especially its ability to initiate powerful electric arcs with minimal losses, which could be useful for contactless feeding of high speed trains. Finally, a complete study of the possibility to use femtosecond filaments as Terahertz radiation source has been made. Several mechanisms of THz generation have been observed. Their spectral properties and their energy allow to consider the development of applications such as remote THz spectroscopy or to reach the regime of nonlinear THz optics.