Pierre-Jean CHARPIN : "Modeling laser-plasma interaction in femtosecond Bessel beams"

Abstract : Femtosecond pulses shaped as Bessel beams can create nano-plasmas in dielectrics, leading to the formation of nano-voids with very high aspect ratio for applications in microelectronics and laser cutting of transparent materials. Modelling the laser-plasma interaction is of prime importance for understanding the spatio-temporal evolution of plasma formation and energy deposition by the femtosecond laser pulse. However, previous attempts of modelling using conventional codes based on pulse propagation equation or hydrodynamic codes could not hitherto reproduce experimental diagnostics. The central objective of this thesis was to modify the Particle-In-Cell numerical code EPOCH in order to simulate the laser-matter interaction in dielectric materials at femtosecond timescales. This modification involved implementing a relative background permittivity to be able to simulate the laser pulse propagation within solid dielectrics and replacing the native ionization rates, which are not adapted to solid-state physics, with the ionization rates from Keldysh theory. A study of the ionization dynamics of sapphire using femtosecond Bessel beams was carried out and led to results in good agreement with experimental ones. This study elucidates the dynamics of nano-plasma formation at femtosecond timescales. We observed a bifurcation between two regimes of the dynamics of plasma formation, yielding either a lateral plasma expansion or an increase in density. In the last case, extremely dense plasmas can be generated together with high energy density deposition, due to the fast ignition of strong impact ionization leading to resonant absorption.

Jury Composition :

François COURVOISIER, Directeur de recherche, Université Bourgogne Franche-Comté, PhD Director

Rachel NUTER, Directrice de recherche, Commissariat à l'énergie atomique et aux énergies alternatives, Reporter

Aurélien HOUARD, Chargé de recherche, Ecole polytechnique, Reporter

John DUDLEY, Professeur, Université Bourgogne Franche-Comté, Reviewer

Localization : Institut FEMTO-ST 15B Avenue des Montboucons 25030 Besançon Cédex - Amphithéâtre Jean-Jacques Gagnepain