Publication

In situ monitoring of femtosecond laser-induced modifications in dielectrics

Olivier Bernard
2023
Thèse EPFL
Résumé

Over the last decades, the progress made in the generation of laser pulses shorter than a picosecond (10^-12 s) has allowed us to reach extreme optical power intensities exceeding 10^15 W cm^-2. This tremendous power has triggered an abundance of original scientific and industrial applications. Chief amongst them is material processing, and in particular, in-volume processing of transparent materials, which motivates the present work. Femtosecond lasers induce a rich taxonomy of material modifications that can take diverse forms, including smooth densification, self-organised nanogratings, localised crystallisation, or amorphisation, that will vary in the processing parameter space, from one material to another. To date, effective methods for direct observation of laser-induced morphologies are missing. To address this need, this thesis work explores in situ methods for direct observation of femtosecond laser-modified zones. The first one consists in using a quantitative phase-contrast microscopy method: digital holographic microscopy. We propose a feedforward manufacturing method, which uses phase data acquired from the microscope to feed a semi-analytical model, a "digital twin". We demonstrate this resilience of this method to quill effects (directionality), and its increased inscription resolution.The second method consists in using full-field multiphoton microscopy. The interaction between the processing laser, with a decreased energy, and already-written structure, induces harmonics generation. Their signals and emission patterns change depending on the structures. Three different interaction regimes are identified in fused silica with third-harmonic generation, associated respectively with nanopores, nanogratings, and microexplosions. The former shows a correlation between the signal and wet etching rate. Full-field allows to identify the shape of the exposed modifications, and to study them by fast focal-plane tomography, highlighting their time-resolved formation.Finally, we present scientific demonstrations and potential applications for these methods. We show that we can inscribe large-scale refractive structures. We then show the validity of the incubation law, and highlight the stochastic nature of the interaction using the high contrast allowed by third-harmonic generation, with a survival analysis. We also show the ability of this method to detect otherwise optically undetectable laser-induced modifications, buried close to a surface. Finally, full-field third-harmonic generation microscopy allows to determine single-shot the nature of some modifications, particularly in the case of ultraviolet femtosecond laser processing.

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Concepts associés (35)
Laser
thumb|250px|Lasers rouges (660 & ), verts (532 & ) et bleus (445 & ). thumb|250px|Rayon laser à travers un dispositif optique. thumb|250px|Démonstration de laser hélium-néon au laboratoire Kastler-Brossel à l'Université Pierre-et-Marie-Curie. Un laser (acronyme issu de l'anglais light amplification by stimulated emission of radiation qui signifie « amplification de la lumière par émission stimulée de radiation ») est un système photonique.
Blocage de mode
Le blocage de mode ou verrouillage de mode désigne une technique de synchronisation de la phase des modes laser destinée à produire de courtes et intenses impulsions lumineuses. Le blocage de mode est réalisé à l'aide de différents éléments optiques : colorant à absorbant saturable, modulateur acousto-optique, cellule de Pockels... La principale application du blocage de mode est la réalisation de laser femtoseconde. Les premiers lasers à colorant délivrant de courtes impulsions sont apparus dans les années 1970, mais les impulsions qu'ils délivrent ne sont pas suffisamment stables .
Ablation laser
L'ablation laser est une technique utilisée pour la production de nanoparticules, certaines méthodes d'analyses de matériaux et/ou pour produire un dépôt en couche mince atomique. L' ablation laser complète ici la gamme des méthodes de dépôt physique de couches minces, telles l'évaporation, la pulvérisation cathodique ou le procédé sol-gel. Un faisceau laser pulsé est focalisé sur une cible constituée du matériau à déposer. L'interaction cible-faisceau entraîne l'arrachage de la matière constituant la cible, par pulvérisation, évaporation, voire fracturation mécanique.
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