Génération et détection par couplage élasto-optique tridimensionnel de champs acoustiques picosecondes diffractés

L’absorption d’une impulsion laser crée un échauffement localisé suivi d’une brusque dilatation. Dès lors, un champ acoustique de plusieurs dizaines de gigahertz peut être généré. Cette méthode optique sans contact et non destructive possède des applications en micro-électronique pour la caractérisation de structures nanométriques, mais également dans des domaines plus fondamentaux. Jusqu’`à présent, la dimension latérale de la tache focale des impulsions laser était très grande devant l’épaisseur des films considérés. Dès lors, la génération était unidimensionnelle et seules des ondes acoustiques planes pouvaient être engendrées. Récemment, l’utilisation de sources laser focalisées a permis de générer par diffraction des champs acoustiques tridimensionnels (3D). Lorsque des impulsions d’une durée inférieure à la picoseconde sont employées dans les métaux, une approche macroscopique n’est plus suffisante. Il est alors nécessaire d’expliciter les évolutions microscopiques impliquées dans le processus de génération. Ainsi, une méthode semi-analytique basée sur un modèle à deux températures 3D est développée dans la première partie de ce mémoire afin de décrire les phénomènes électroniques. En se propageant, l’onde acoustique divergente module l’indice optique en temps et en espace par couplage élasto-optique. La propagation de la lumière est alors perturbée, et sa mesure permet de caractériser la propagation acoustique. Dans la seconde partie de ce mémoire, l’interaction 3D de l’impulsion laser gaussienne avec le champ acoustique diffracté est donc modélisée.The absorption of a laser pulse creates a localized heating, followed by a sudden dilatation. Thereby, an acoustic field of several tens of gigahertz can be launched. This optical method is applied to microelectronics to characterise nanometric films non-destructively and without contact, but also to more fondamental fields. Until now, the lateral size of the laser spot was very large compared to the films thicknesses. Thus, generation was one-dimensional, and only acoustic plane waves could be engendered. Recently, the use of focused laser sources has allowed the generation of three-dimensional (3D) diffracted acoustic fields. When pulses shorter than a picosecond are absorbed in metals, a macroscopic approach is not relevant anymore. It is then necessary to describe the microscopic evolutions implied in the generation process. Thus, a semi-analytical method based on a 3D two-temperature model is developed in the first part of this thesis to describe the electronic phenomena. While propagating, the divergent acoustic field modulates the optical indices in time and space through the elasto-optic interaction. The light propagation is thereby perturbed, and its measurement allows the characterization of the acoustic propagation. In the second part of this thesis, the 3D interaction of the Gaussian laser pulse with the diffracted acoustic field is therefore modeled

Nanoscale mechanical contacts probed with ultrashort acoustic and thermal waves

Using an ultrafast optical technique we measure coherent phonon-pulse reflection from - and heat flow across - a mechanical contact of nanoscale thickness between a thin metal film and a spherical dielectric indenter. Picosecond phonon wave packets at ∼50 GHz returning from this interface probe the pressure distribution, the contact area, and the indentation profile to subnanometer resolution, revealing the film deformation in situ. These measurements and simultaneous thermal-wave imaging at ≳1 MHz are consistent with significant enhancement of phonon transport across the near-contact nanogap

Oblique laser incidence to select laser-generated acoustic modes

The purpose of this paper is to study the effect of a non-normal optical penetration due to an obliquely incident laser source. It is shown that the loss of symmetry due to such a penetration influences specific bulk acoustic modes. For a given detection position, increase in shear wave amplitude is obtained by orienting the incident laser source. 1. Introduction Since the first quantitative approaches in the 60's [1], a large number of studies has been conducted to better understand the generation of laser-generated acoustic waves. Models for the acoustic generation in the thermoelastic regime, taking into account the optical penetration of the source [2] or the effect of source width [3], were developed. A more general approach based on Green's function formalism also included thermal diffusion in the medium [4]. These works have dealt with the modeling of a circular spot of laser illumination. However, modeling of the acoustic field generated by a line source is of interest, since the signal-to-noise ratio may be increased in this experimental situation [5]. All the works cited above assume a normal incidence of the laser beam with respect to the illuminated surface of the sample. The purpose of this paper is to analyse the effects of oblique incidence on acoustic waves generated by a line-focused laser source. We present a theoretical model accounting for the effects of optical penetration, finite width of the beam and pulse duration of the laser source. The absorbed energy density is first calculated by solving Maxwell's equations. Then the thermoelastic equations are solved to obtain the displacement field through a semi-analytical calculation based on a double Fourier transform in space and time. It is shown that oblique incidence increases shear wave amplitude. Changes of the longitudinal waveforms are also discussed

Effect of laser beam incidence angle on the thermoelastic generation in semi-transparent materials

When a laser beam is absorbed in a semi-transparent material, a volume acoustic source is created owing to penetration of the laser beam inside the material and to thermoelastic transduction. Many experimental and theoretical studies have been conducted to better understand this ultrasound generation process with normal laser light incidence on the material surface. The purpose of this paper is to analyze the effects of the asymmetry caused by oblique incidence of a laser line source on the generation of acoustic waves in semi-transparent isotropic materials. Experiments on a glass plate demonstrate that such an obliquely incident laser light strongly affects bulk acoustic waves generation. Compressional and shear waves are enhanced and the loss of symmetry of the acoustic source causes asymmetrical behavior of the acoustic waves. Surprisingly, compressional-wave amplitude decreases whereas shear-wave amplitude increases in the region where the electromagnetic energy is refracted. This feature is explained by semi-analytical calculations