École Nationale Supérieure de
Techniques Avancées

Abstract

The subject of this work is the study of filamentation of femtosecond laser pulses in active and passive transparent media, and the study of the plasma filaments as sources of secondary electromagnetic radiation in the Terahertz and Radiofrequency bands. After a brief overview of filamentation and state of the art in chapters I and II, a new technique (P-scan) is described in chapter III, which provide a useful method to observe and analyze non linear propagation regimes of femtosecond laser pulse in gases. In chapters IV and V the filamentation process in amplifying media in presence of population inversion is described. Evidence of amplification of energy and power in the filament core is obtained, while the multiple filamentation regime is always avoided. It was already known that a plasma filament can radially emit Terahertz radiation. The detailed study of the plasma filament as source of ! terahertz radiation is described in chapters VI, VII and VIII. This study has brought about the discovery of a different generation mechanism of radial terahertz radiation in presence of a static electric field applied along the filament axis, and the discovery of a new kind of Terahertz emission in the absence of applied field, conically emitted in the forward direction with respect to the filament axis. To explain this mechanism, a theoretical model has been developed in collaboration with Prof. Tikhonchuk of Bordeaux 1 University; very good agreement is found between the theoretical model and experimental results. Finally, evidence of dipole-like emission in the radiofrequency range from a laser induced plasma channel is presented in chapter IX.