PhD Defense of Bastien MULLER

Terahertz generation,Photo-induced plasma,Helium plasma jet,THz generation at metal surface,Polarization sensitive THz detection

Abstract :
This thesis project aims to study two innovative techniques for generating terahertz waves from a plasma. This is either a photo-induced filament or the surface of a metal. Even if a metal is not strictly an ionized gas, it is a medium composed of free carriers, whose response to electromagnetic excitation is quite close to that of a plasma. In both cases (breakdown in air and metals), we generated and detected a THz signal by excitation with an intense femtosecond pulsed laser, using an experimental bench of THz spectroscopy type in the time domain. We have achieved a complete characterization of the THz signal generated in the air as a function of the numerous experimental parameters. In particular, we are interested in the polarization of the observed phenomena. We also investigated the possibility of enhancing the generation efficiency by the addition of a helium plasma jet. The results obtained are perfectly described by a model we have developed. This takes into account many parameters of the photo-excited gases, such as their ionization energy, their chromatic dispersion and their second-order non-linear coefficient. In the case of metal, we performed for the first time a THz generation experiment by illuminating a metallic mirror by the laser and its second harmonic, highlighting a nonlinear phenomenon of the Kerr type.
 
Jury members :

• Jean-louis COUTAZ - Emeritus Professor, IMEP-LAHC, Savoie Mont Blanc University : Supervisor
• Emmanuel ABRAHAM - Professor, LOMA University of Bordeaux : Reviewer
• Francis HINDLE - Professor, LPCA, University of  Littoral Côte d'Opale : Reviewer
• Angela VELLA - Professor, GPM, University of Rouen Normandie : Examiner
• Ronan LE DANTEC -  Professor , SYMME, University of Savoie Mont Blanc :  Examiner