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International audience; The main goal of this lecture is to give an outlook of the numerous and various applications that the picosecond acoustic technique has met in the industrial world. The story started a few years after its invention by Prof. H. Maris. Indeed, Rudolph Technologies (now ONTO) rapidly realized that such a technique was the missing one in the industrial world for controlling the thickness of thin metal films. That was the beginning of the Metapulse® story, a full automatized tool dedicated to microelectronics applications and that can be found in any production company worthy of the name. More recently, picosecond acoustics and especially its variant based on laser-tuning, the so-called Colored Picosecond Acoustics or APiC, have been shown to be very useful for many other industrial applications in topics as different as space mirror, radio-frequency filtering, flat and smart glass, solar cells,…

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; A large number of methods have been developed to characterize the adhesion of a thin film to a substrate. Among them, tape test is still one of the most popular methods. Acoustic waves and especially ultra-high frequency acoustic waves are also sensitive to adhesion defects as they affect the way acoustic waves are reflected at such an interface. In this work, we first prepare a series of identical thin-film samples with a variable adhesion at the interface between the film and its substrate. To modulate adhesion of some samples, an ultrathin gold layer is deposited on the substrate before thin-film sputtering. Si+ ion implantation is also used to reinforce locally gold adhesion. The samples are then characterized using two much different techniques: picosecond acoustics (PA) for measuring thickness and acoustic reflection coefficient (R) at the interface and tape test to have an independent evaluation of the adhesion through the peeled area (P). On the samples series, R is found to vary from −0.46 the expected value for a perfectly bonded Ni/Si interface to almost −0.83 not far from −1 which corresponds to delamination. An excellent correlation is found between R and P: high |R| value samples are easily peeled off and P is large. That work demonstrates the capability of PA to perform adhesion test in a totally non-destructive manner and furthermore locally.

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; This paper presents some quantitative measurements of the adhesion energy of thin WTi films deposited on Si substrate. Two different techniques are applied to the same sample series. One is a mechanical test based on the analysis of spontaneously formed defects. The second is based on acoustic waves whose reflection at the interface between the thin-film and the substrate is sensitive to the adhesion. An excellent correlation is obtained between both approaches: the adhesion energy measured by buckles analysis and acoustic reflection coefficient measured by picosecond acoustics. The acoustic approach offers several advantages among which a non-destructive character, a compatibility with complex stacks and a sensitivity to detect adhesion anomaly even if no defect is formed.

Acoustic oscillation provides useful information regarding the interfacial coupling between metal transducer layers and substrate materials. The interfacial coupling can be significantly reduced by a mechanically soft layer between the transducer and substrate. However, preserving a thin, soft layer at the interface during fabrication is often challenging. In this study, we demonstrate that an amorphous CoB alloy on top of a sapphire substrate can substantially amplify acoustic oscillations. By analyzing the attenuation of acoustic oscillations, we show that a thin, soft layer with a thickness of &gt, 2 ± 1 Å exists at the interface. The intermediate layer at the interface is further verified by investigating heat transport. By analyzing the slow decrease of the temperature of the transducer layer, we determine a thermal conductance of 35 ± 5 MW m−2 K−1 at the transducer/substrate interface. This low value supports the existence of a thin, soft layer at the interface. Our results demonstrate that an amorphous metal with B alloying effectively preserves the soft nature at the interface and detects the acoustic propagation and heat transport across it.

Here, we experimentally characterize acoustic and optical modes of GaAs/AlAs Bragg mirror planar and micropillar cavities. The goal is to study the optomecanical coupling of such sys- tem with optomecanically induced transparency experiments. To characterize the high frequency acoustic modes (20 GHz) of the cavity, we use picosecond acoustic experiments, along with a subharmonic excitation technique, at low temperature. It allows to overcome the frequency resolution limitation of the experiment. Thanks to this technique, we measure acoustic quality factors as high as 3 · 10 ^4 , which gives a quality factor frequency product Q m f m of 5 × 10 ^14 Hz at 20 K. The optical mode characterization is performed with a reflectometric setup, which is also used for the induced transparency measurements. We did not experimentally observed any induced transparency signal. From calculation realized to simulate the result we expected, we were able to estimate an upper bound of the optomecanical coupling of the studied GaAs/AlAs Bragg mirror planar cavity at low temperature, g 0

For nearly 40 years, there has been a robust seismological model (PREM - Preliminary Reference Earth Model) which reports the elastic properties (compressional velocity - Vp, shear velocity - Vs, and density - ρ) of the Earth’s core, and yet there is still no clear consensus on the chemical composition which can match such properties at relevant pressures and temperatures. While iron has elastic properties which are close to those of the solid inner core, iron alone is too heavy to match the density of the inner core. In light of this, and based on a series of cosmochemical and geochemical arguments, Si has been proposed for many years in varying quantities as a material which could explain the difference in material properties between Fe and PREM. Using a combination of Synchrotron X-ray Diffraction and Picosecond Acoustics, ρ and Vp of Fe-Si and Fe-Ni-Si alloys have been measured to pressures and temperatures exceeding 1 Mbar and 2000 K. Obtained data allows an accurate determination of Velocity-Density relations, both P-V and P-V-T equations of state, and axial ratios of a variety of geophysically important Fe-Si and Fe-Ni-Si alloys. These results are used to discuss the elastic properties of Fe-Si alloys at high pressure - high temperature conditions to place tight constraints on Si abundance in the Earth’s inner core.; Depuis environ 40 ans, il existe un modèle séismologique fiable (appelé PREM pour Preliminary Reference Earth Model) qui rend compte des propriétés élastiques (vitesse de compression - Vp, vitese transverse - Vs, et masse volumique - ρ) du noyau terrestre, mais il n’existe pas de consensus clair sur la composition chimique qui peut correspondre à de telles propriété s aux températures et pressions souhaitées. Tandis que le fer a des propriétés élastiques qui sont proches de celles du noyau interne, le fer est trop lourd pour expliquer, à lui seul, la masse volumique du noyau terrestre. À la lumière de ces connaissances, et grâce à une série d’arguments cosmochimiques et géochimiques, le silicium comme impureté a été proposé pour expliquer la différence entre les propriétés du Fe seul et du modèle PREM. En utilisant une combinaison de diffraction des rayons-X en synchrotron et la technique acoustique picoseconde, ρ et Vp des alliages Fe-Si et Fe-Ni-Si ont été mesurés à des pressions dépassant 1 Mbar et 2000 K. Les données obtenues nous permettent une détermination précise des relations vitesse-masse volumique, des équations d’état P-V et P-V-T, ainsi que des rapports axiaux pour ces alliages. Ces résultats sont utilisés pour discuter des propriétés élastiques des alliages Fe-Si à haute pression – haute température pour fixer des contraintes sévères sur l’abondance de Si dans le noyau terrestre interne.

For nearly 40 years, there has been a robust seismological model (PREM - Preliminary Reference Earth Model) which reports the elastic properties (compressional velocity - Vp, shear velocity - Vs, and density - ρ) of the Earth’s core, and yet there is still no clear consensus on the chemical composition which can match such properties at relevant pressures and temperatures. While iron has elastic properties which are close to those of the solid inner core, iron alone is too heavy to match the density of the inner core. In light of this, and based on a series of cosmochemical and geochemical arguments, Si has been proposed for many years in varying quantities as a material which could explain the difference in material properties between Fe and PREM. Using a combination of Synchrotron X-ray Diffraction and Picosecond Acoustics, ρ and Vp of Fe-Si and Fe-Ni-Si alloys have been measured to pressures and temperatures exceeding 1 Mbar and 2000 K. Obtained data allows an accurate determination of Velocity-Density relations, both P-V and P-V-T equations of state, and axial ratios of a variety of geophysically important Fe-Si and Fe-Ni-Si alloys. These results are used to discuss the elastic properties of Fe-Si alloys at high pressure - high temperature conditions to place tight constraints on Si abundance in the Earth’s inner core.; Depuis environ 40 ans, il existe un modèle séismologique fiable (appelé PREM pour Preliminary Reference Earth Model) qui rend compte des propriétés élastiques (vitesse de compression - Vp, vitese transverse - Vs, et masse volumique - ρ) du noyau terrestre, mais il n’existe pas de consensus clair sur la composition chimique qui peut correspondre à de telles propriété s aux températures et pressions souhaitées. Tandis que le fer a des propriétés élastiques qui sont proches de celles du noyau interne, le fer est trop lourd pour expliquer, à lui seul, la masse volumique du noyau terrestre. À la lumière de ces connaissances, et grâce à une série d’arguments cosmochimiques et géochimiques, le silicium comme impureté a été proposé pour expliquer la différence entre les propriétés du Fe seul et du modèle PREM. En utilisant une combinaison de diffraction des rayons-X en synchrotron et la technique acoustique picoseconde, ρ et Vp des alliages Fe-Si et Fe-Ni-Si ont été mesurés à des pressions dépassant 1 Mbar et 2000 K. Les données obtenues nous permettent une détermination précise des relations vitesse-masse volumique, des équations d’état P-V et P-V-T, ainsi que des rapports axiaux pour ces alliages. Ces résultats sont utilisés pour discuter des propriétés élastiques des alliages Fe-Si à haute pression – haute température pour fixer des contraintes sévères sur l’abondance de Si dans le noyau terrestre interne.

International audience; We present a detailed experimental and theoretical study of the acoustic phonon modes in rolled-up multilayers with thickness of the layers in the nanometre and diameters in the micrometre range. We compare our results to planar, unrolled multilayers grown by molecular beam epitaxy. For the planar multilayers the experimentally obtained acoustic modes exhibit properties of a superlattice and match well to calculations obtained by the Rytov model. The rolled-up superlattice tubes show intriguing differences compared to the planar structures which can be attributed to the imperfect adhesion between individual tube windings. A transfer matrix method including a massless spring accounting for the imperfect adhesion between the layers yields good agreement between experiment and calculations for up to five windings. Areas with sufficient mechanical coupling between all windings can be distinguished by their acoustic mode spectrum from areas where individual windings are only partially in contact. This allows the spatially resolved characterization of individual tubes with micrometre spatial resolution where areas with varying interface adhesion can be identified.

Picosecond optoacoustics is a technique that can probe properties of matter on a submicronscale. The aim of this work is to synthesize shear waves at GHz frequencies in a thin isotropiclayer in order to analyse its shear properties. Transverse waves are synthesized using diffraction.In the first part we develop theoretical tools in order to analyse the spatial repartition of theacoustic field generated by the laser-matter interaction. Directivity patterns predict the particulardirections for which amplitude of the shear waves is higher. Thus, we synthesized shearwaves in these particular directions using a dedicated post processing method. To investigateshear properties of a film lying on the sample, we study the reection of shear waves betweenthe transducer and the lying film. Starting with either simulated or experimental recordedwaveforms, we demonstrate the method in the specific case of the titanium/glycerol interface., L'optoacoustique picoseconde est une technique qui permet de sonder la matière à l'échelle submicronique. L'objectif de ce travail est la synthèse d'ondes de cisaillement de fréquences GHz dans des couches minces isotropes pour l'analyse des propriétés transverses de la matière. C'est grâce à la diffraction du champ acoustique à l'intérieur du transducteur que ces ondes de cisaillement sont synthétisées. Nous exposons dans une première partie le développement des outils théoriques nécessaires à l'analyse de la répartition spatiale du champ acoustique généré lors de l'interaction laser-matière. Les diagrammes de directivité ainsi établis permettent de prévoir les directions de rayonnement privilégiées. Par une méthode de post traitement adaptée,nous synthétisons ensuite des ondes transverses dans ces directions particulières. Lorsque le transducteur est chargé par un milieu dont on cherche à connaître les propriétés transverses, les ondes de cisaillement synthétisées sont réfléchies à l'interface entre le transducteur et ce milieu. L'analyse du coefficient de réflexion permet alors de déterminer les propriétés mécaniques du milieu à sonder. Nous illustrons cette méthode numériquement et expérimentalement dans le cas d'une interface titane/glycérol.

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.

By means of an ultrafast opto-acoustic technique we study the nanoindentation of thin chromium films on sapphire substrates using a ceramic ball bearing. Acoustic pulses at approximately 40 GHz returning from the film-indenter interface allow the film indentation profiles to be probed to sub-nanometer resolution over contact areas approximately 25 microm in radius. The deformation of the films during loading is thereby revealed. Furthermore, thermal wave imaging of the contact at megahertz frequencies is simultaneously achieved.

The purpose of this work is to develop a fully passive in-line common-path femtosecond interferometer for the detection of high frequencies acoustic waves (GHz-THz). This work falls within the picosecond acoustic domain. The generation and the detection of ultrasounds are performed through the combined use of pulsed lasers and optical pump-probe time-resolved setups. The originality of the interferometric detection we propose lies in the use of a single birefringent crystal frst to generate a pair of phase-locked pulses and second to recombine them to interfere. We present in the first chapter a state-of-the-art of the interferometric setups currently used in picosecond acoustics. In the second chapter, we describe the principle of operation of the interferometer we have developed. Then we model the sensitivity of the interferometer inthe framework of the Jones formalism. We show that it is possible to measure independently the pump-induced modification of either the real or imaginary parts of the complex reflection coefficient of the sample. Finally, in the third chapter, the experimental characterization of the setup is detailed and we illustrate the performance of the interferometer through picosecond opto-acoustic measurements. Experiments are performed on two different samples : an opticallyabsorbing thin film and a transparent substrate coated with a metallic transducer., L'objectif de ce travail est le développement d'un interféromètre à chemin commun entièrement passif pour détecter des ondes acoustiques hautes fréquences (GHz-THz). Ce travail s'inscrit dans le domaine de l'acoustique picoseconde. La génération et la détection des ultrasons sont réalisées par l'utilisation combinée de lasers impulsionnels et de dispositifs pompe-sonde résolus en temps. L'originalité de la détection interférométrique proposée réside dans l'utilisation d'un unique cristal biréfringent pour créer une paire d'impulsions séparée de quelques picosecondes puis les recombiner et générer les interférences. Nous dressons dans le premier chapitre un état de l'art des différentes méthodes interférométriques actuellement utilisées en acoustique picoseconde. Dans le second chapitre nous détaillons le principe de l'interféromètre développé et nous modélisons son fonctionnement à l'aide du formalisme de Jones. Nous montrons que, suite à l'excitation induite par l'impulsion pompe, il est possible de mesurer indépendamment la dérivée temporelle de l'amplitude ou de la phase du changement relatif de réflectivité de l'échantillon. Enfin, le troisième chapitre est consacré à la caractérisation expérimentale de l'interféromètre puis à l'illustration des potentialités de cet interféromètre en acoustique picoseconde. Les expériences ont été réalisées sur deux échantillons différents : un film mince optiquement absorbant puis un film transparent déposé sur un transducteur métallique.

Ultrafast optical pump-probe technique, by exploiting ultrashort laser pulses (femtosecond), allows to initiate and monitor ultrafast processes in matter. Picosecond acoustics is a research field that focuses on the generation and detection mechanisms of high frequency coherent acoustic phonons in different media, as well as on their application in testing of nanomaterials and nanostructures. This PhDs research project was devoted to study of electron-acoustic phonon coupling in ferroelectric BiFeO3 (bismuth ferrite, BFO) by ultrafast acoustics. We have evidenced that depending on the BFO crystal orientation it was possible to tune the coherent phonons spectrum with in particular variable amplitude of longitudinal (LA) and transverse (TA) acoustic modes. In some grains with particular crystallographic orientations much stronger TA than LA signal was observed. Spectacularly, we have revealed an efficient coupling between electron and transverse acousticphonon. Such high ratio never reported before in any metal, semiconductor or nanostructure before, can be principally attributed to the photoinduced inverse piezoelectric effect (Lejman et al Nature Communications 2014). In a second part, we have shown that BFO as well as another birefringent ferroelectric LiNbO3 (LNO) can be used for ultrafast acousto-optic modeconversion (manipulation of light polarization at the picosecond time scale with coherent acoustic phonons). This effect, never reported at GHz up to now, can be potentially applied in photonics for ultrafast manipulation of light polarization bycoherent acoustic phonons in next generation photonic/phononic devices., La technique d’optique ultra-rapide pompe-sonde, qui repose sur l’emploi de lasers à impulsion ultracourte(femtoseconde), permet de déclencher et étudier des processus ultrarapides dans la matière. L’acoustique picoseconde concerne pour sa part l’étude des processus de génération et détection de phonons acoustiques haute fréquence ainsi quel’analyse des nanomatériaux avec ces phonons (nanoéchographie). Les travaux de recherche de cette thèse avaient pourbut l’étude des couplages électronphonon acoustique dans le matériau ferroélectrique BiFeO3 par acoustique ultrarapide. Nous avons pu mettre en évidence que selon l’orientation du cristal photoexcité, l’émission des phonons acoustiques cohérents longitudinaux (LA) et transverses (TA) pouvait être modulée. De manière spectaculaire, nous avons purévéler un couplage électron-phonon acoustique transverse très efficace comme cela n’avait jamais été observé jusqu’alors dans les métaux, semiconducteurs ou nanostructures artificielles. Une étude détaillée indique que le mécanismepiézoélectrique inverse semble être le moteur de ce couplage électron-phonon (Lejman et al, Nature Communications, 2014). Dans une seconde partie, nous avons montré que BFO, ainsi qu’un autre ferroélectrique biréfringent LiNbO3 (LNO), peuvent être utilisés pour la conversion de mode ultra-rapide par processus acousto-optique (manipulation de la polarisation de la lumière à l’échelle de la picoseconde avec des phonons acoustiques). Cet effet, jamais mis enévidence jusqu’alors dans le domaine GHz, pourrait potentiellement être exploité dans de nouveaux dispositifs photoniques/phononiques pour des modulations acousto-optiques à haute cadence.

International audience; Due to discrepancies in the literature data the thermodynamic properties of liquid gallium arestill in debate. Accurate measurements of adiabatic sound velocities as a function of pressureand temperature have been obtained by the combination of laser picosecond acoustics andsurface imaging on sample loaded in diamond anvil cell. From these results thethermodynamic parameters of gallium have been extracted by a numerical procedure up to10 GPa and 570 K. It is demonstrated that a Murnaghan equation of state accounts well for thewhole data set since the isothermal bulk modulus BT has been shown to vary linearly withpressure in the whole temperature range. No evidence for a previously reported liquid–liquidtransition has been found in the whole pressure and temperature range explored

This study relates to the physical, chemical and mechanical characterization of silicon nitride thin film. These films are used to increase the performances of the transistors by inducing a strain into the channel. With this intention the films are strongly stressed by the depositions conditions. To improve even more the performances of the transistors, the film stress was increased by using multi-layer films with plasma treatment. This treatment generates a hydrogen desorption allowing the recombination of new Si-N links, leading to a stress increase. At the time of this first phase of densification, one notes in addition that the Young's modulus increases as well as in plan as out-of plan. Then, if the treatment is prolonged, desorption stops and one notes a second phase during which the porosity of film strongly decreases. This reduction seems correlated with the higher increase in the Young's modulus in the plan compared to that out-of plan, leading to film anisotropy. It is thus supposed that the pores are column-like form in the direction out-of plan, Cette étude porte sur la caractérisation physique, chimique et mécanique de films minces de nitrure de silicium. Ces films sont utilisés pour accroître les performances des transistors en induisant une déformation dans le canal. Pour ce faire les films sont fortement contraints par les conditions de dépôts. Pour améliorer encore plus les performances des transistors, la contrainte du film a été augmentée en utilisant des films multicouches avec traitement plasma. Ce traitement engendre une désorption d'hydrogène permettant la recombinaison de nouvelles liaisons Si-N, conduisant à l'augmentation de la contrainte. Lors de cette première phase de densification, on constate par ailleurs que le module de Young augmente aussi bien dans plan que hors-plan. Ensuite, si le traitement se prolonge, la désorption s'arrête et on constate une deuxième phase au cours de laquelle la porosité du film diminue fortement. Cette diminution semble corrélée à l'accroissement supérieur du module de Young dans le plan par rapport à celui hors-plan, conduisant à une anisotropie du film. On suppose donc que les pores sont de forme colonnaire dans la direction hors-plan.

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.

We report time-resolved acoustic measurements in the picosecond range performed with a 100 nm lateral resolution. A near field optical microscope operating in reflection geometry and coupled with a standard picosecond acoustic setup is described. Results obtained on thin film are discussed and prove that the sensitivity of our system allows the observation of thermal features and acoustic signals such as echoes and surface waves. Patterned structures made of submicrometric metallic dots have also been studied: it is demonstrated that, owing to the high lateral resolution which we obtained, vibrational modes of a single dot are detected.

A 100 fs laser pulse passes through a single transparent cell and is absorbed at the surface of a metallic substrate. Picosecond acoustic waves are generated and propagate through the cell in contact with the metal. Interaction of the high frequency acoustic pulse with a probe laser light gives rise to Brillouin oscillations. The measurements are thus made with lasers for both the opto-acoustic generation and the acousto-optic detection, and acoustic frequencies as high as 11 GHz can be detected, as reported in this paper. The technique offers perspectives for single cell imaging. The in-plane resolution is limited by the pump and probe spot sizes, i.e. approximately 1 microm, and the in-depth resolution is provided by the acoustic frequencies, typically in the GHz range. The effect of the technique on cell safety is discussed. Experiments achieved in vegetal cells illustrate the reproducibility and sensitivity of the measurements. The acoustic responses of cell organelles are significantly different. The results support the potentialities of the hypersonic non-invasive technique in the fields of bio-engineering and medicine.

Here, we experimentally characterize acoustic and optical modes of GaAs/AlAs Bragg mirror planar and micropillar cavities. The goal is to study the optomecanical coupling of such sys- tem with optomecanically induced transparency experiments. To characterize the high frequency acoustic modes (20 GHz) of the cavity, we use picosecond acoustic experiments, along with a subharmonic excitation technique, at low temperature. It allows to overcome the frequency resolution limitation of the experiment. Thanks to this technique, we measure acoustic quality factors as high as 3 · 10 ^4 , which gives a quality factor frequency product Q m f m of 5 × 10 ^14 Hz at 20 K. The optical mode characterization is performed with a reflectometric setup, which is also used for the induced transparency measurements. We did not experimentally observed any induced transparency signal. From calculation realized to simulate the result we expected, we were able to estimate an upper bound of the optomecanical coupling of the studied GaAs/AlAs Bragg mirror planar cavity at low temperature, g 0