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International audience; Picosecond acoustics is an optical pump-probe technique allowing to access thermoelastic properties and sound velocities of a large variety of materials under extreme conditions. Coupled with diamond anvil cells and laser heating, picosecond acoustics measurements offer the possibility to probe materials over a pressure and temperature range directly pertinent for the deep planetary interiors. In this paper we highlight the capabilities and versatility of this technique by presenting some recent applications on materials of geophysical interest. All the independent components of the elastic tensor of MgO are simultaneously determined by measurements on a single crystal at ambient conditions. Compressional sound velocity is measured at high pressure on an iron-carbon alloy and on polycrystalline argon. First laser heating test measurements performed on molybdenum at high pressure are also presented. These examples demonstrate that picosecond acoustics is a valuable alternative to already existing techniques for determining the physical properties of samples under extreme pressure and temperature conditions.

International audience; Nowadays, microelectronics is making progress in miniaturization and diversification of chips by 3D integration: dies are stacked together to gain space and performances. The stacking used here is called hybrid bonding [1], as stacked surfaces are heterogeneous: part Cu and part oxide. After the bonding, it exists some defects at the interface which size can be milimetric to nanometric. Acoustic waves are very useful for detecting defects. In the field, the most used technique is the Scanning Acoustic Microscopy (SAM) which makes an image of the interface using ultrasonics (10-1000 MHz) with a XY resolution of 20 �m [2]. To detect smaller defects, Transmission Electron Microscopy (TEM) is used. Its resolution is about 0,1 nm, making this technic very local compared to SAM. Like the SAM, Colored Picosecond Acoustics (APiC) is based on acoustic waves but at higher frequencies (10-500 GHz) using of a femtosecond laser in a pump-probe scheme [3,4]. Such a technique is closer to TEM resolution. In this work, both acoustic techniques (SAM and APiC) are applied to the characterization of the same samples made of bonded wafers. APiC is first used to characterize the stack (thickness, velocity). Then, the interface contributions is focused on: Cu/Cu, Cu/oxide, oxide/oxide. As the technic makes a local measurement (typically 1-2 �m), the process is repeated at different points of the surface to get an interface image comparable to SAM result. In this work will be presented the APiC results obtained on wafers also studied by SAM to compare both approaches, especially about the resolution enhancement ApiC brings. The minimal size of the defects that APiC is able to detect will be explored.

We describe here a time resolved pump-probe laser technique&mdash, picosecond interferometry&mdash, which has been combined with diamond anvil cells (DAC). This method enables the measurement of the longitudinal sound velocity up to Mbar pressure for any kind of material (solids, liquids, metals, insulators). We also provide a description of picosecond acoustics data analysis in order to determine the complete set of elastic constants for single crystals. To illustrate such capabilities, results are given on the pressure dependence of the acoustic properties for prototypical cases: polycrystal (hcp-Fe-5 wt% Si up to 115 GPa) and single-crystal (Si up to 10 GPa).

International audience; Solitons are self-preserving traveling waves of great interest in nonlinear physics but hard to observe experimentally. In this report an experimental setup is designed to observe and characterize acoustic solitons in a GaAs(001) substrate. It is based on careful temperature control of the sample and an interferometric detection scheme. Ultrashort acoustic solitons, such as the one predicted by the Korteweg–de Vries equation, are observed and fully characterized. Their particlelike nature is clearly evidenced and their unique properties are thoroughly checked. The spatial averaging of the soliton wave front is shown to account for the differences between the theoretical and experimental soliton profile. It appears that ultrafast acoustic experiments provide a precise measurement of the soliton velocity. It allows for absolute calibration of the setup as well as the response function analysis of the detection layer. Moreover, the temporal distribution of the solitons is also analyzed with the help of the inverse scattering method. It shows how the initial acoustic pulse profile which gives birth to solitons after nonlinear propagation can be retrieved. Such investigations provide a new tool to probe transient properties of highly excited matter through the study of the emitted acoustic pulse after laser excitation.

We study propagation and attenuation of sound waves in silica with two complementary experimental techniques, Brillouin light scattering and picosecond acoustics. Analysis of new accurate Brillouin data obtained in a large temperature range (4-1300 K), supplemented with literature data on a broad frequency range, shows that internal friction results from two main mechanisms: the interaction of sound with relaxing defects and with modes of the thermal bath (anharmonicity). Brillouin spectroscopy at high pressure (0-6 GPa) reveals the existence of a structural modification around 2~GPa in silica at room temperature. High frequency sound is observed for the first time around 250~GHz in silica with a new picosecond acoustic technique. These data demonstrates that the sound attenuation is still dominated by the anharmonicity at these mesoscopic wavelengths. The relation with a strong scattering regime of the attenuation at THz frequencies is discussed.; Nous étudions la propagation et l'atténuation du son dans la silice avec deux techniques expérimentales complémentaires, la diffusion Brillouin de la lumière et l'acoustique picoseconde. L'analyse de nouvelles données Brillouin précises obtenues dans une large gamme de températures (4-1300 K), complétées avec les données de la littérature sur un large intervalle de fréquences, montre que le frottement interne résulte de deux principaux mécanismes : l'interaction du son avec les défauts relaxants et avec les modes du bain thermique (anharmonicité). La spectroscopie Brillouin à haute pression (0-6 GPa) révèle l'existence d'une modification structurale à environ 2 GPa à température ambiante. L'atténuation du son est observée pour la première fois autour de 250 GHz avec une nouvelle technique acoustique picoseconde. Ce résultat démontre que l'atténuation du son est encore dominée par l'anharmonicité à l'échelle des longueurs d'onde mésoscopiques. La relation avec un régime de forte atténuation au THz est discutée.

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.; 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.; 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.