Mostafa HASSAN : "Ultrafast imaging of Bessel beam generated nano-plasmas within dielectrics"

Abstract : Ultra-intense femtosecond lasers are widely used for nano and micro-material processing. Explaining the laser-matter interaction is needed to control the processing. In this context, non-diffracting beams, such as Bessel beams, provided substantial advantages regarding the stability of the non-linear propagation within dielectrics. They allowed the generation of extremely high aspect ratio nanovoids. However, conventional models describing Bessel-dielectric interaction fail to explain experimental observations. Therefore, this thesis aims to develop a set of techniques to retrieve the parameters of laser-generated plasmas. The first part develops an approach to shape the polarization of the Bessel beam along its propagation based on spatially-varying waveplates. The second part of this work is dedicated to the imaging of the interaction between the plasma generated by the onset of the pulse and the trailing part inside sapphire. We confirmed a generation of nano-plasma with a density exceeding the critical density, and a diameter of typically 200 nm. Then, the third part of this work focuses on imaging the plasma using a pump-probe approach. This new concept is based on shaping the probe as a higher-order Bessel beam. The plasma parameters are extracted from a set of measurements of reflectivity in amplitude and phase for different orders and angles of the probe pulse.

Jury Composition:

M. François COURVOISIER, Directeur de recherche, Université Bourgogne Franche-Comté, Directeur de thèse

M. Yves BELLOUARD, Associate Professor, École Polytechnique Fédérale de Lausanne, Rapporteur

Mme Ottavia JEDRKIEWICZ, Chargée de recherche, CNR- Istituto di Fotonica e Nanotecnologie, Milano, Rapporteure

M. Sylvain LECLER, Professeur des universités, INSA Strasbourg, Examinateur

M. Maxime JACQUOT, Professeur des universités, Université Bourgogne Franche-Comté, Examinateur

Localization : Amphithéatre Jean-Jacques Gagnepain - Institut FEMTO-ST 15B, avenue des Montboucons 25000 BESANCON