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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; An electronic-driven liquid-liquid phase transition in rubidium is revealed by picosecond acoustic measurements combined with ab initio calculations. Picosecond acoustics were used to measure the melting line up to 10 GPa, finding the maximum in the melting curve at 7 GPa and 555 K. We observe the onset of a continuous liquid-liquid phase transition beginning around the melting maxima through until 16 GPa. Sound velocity shows a softening similar to that reported for liquid caesium, caused by a change in the bulk modulus during a crossover from the low-density to the high-density liquid. Guided by the ab initio calculations, we relate the changes in the thermo-elastic properties to the progressive localization of the valence electrons in the pressure range of 6-16 GPa. At high pressure rubidium forms an electride liquid quantified by the appearance of interstitial quasi atoms (ISQs) localised in the valence electron density.

Using a combination of picosecond acoustics and synchrotron x-ray diffraction, the sound velocities and equations of state of Fe-Si alloys have been determined over a wide range of compositions (10--29 at. % Si) and pressures (up to 65 GPa) under quasihydrostatic conditions. We observe marked variation in the elastic properties of the alloys depending on synthesis method and degree of Si ordering. In particular, it is observed that there is a sharp change in the density dependence of the sound velocities which coincides with the observation of long-range ordering by diffraction methods. This change in elasticity is probably due to a change in bond character of the Fe-Si alloy. Furthermore, the bcc-hcp transition in these alloys has been mapped to high pressures and Si contents. We observed that the onset of the transition changes by more than 6 GPa per at. % for alloys containing more than $\ensuremath{\sim}15$ at. % Si.

International audience; Liquid cesium (l-Cs) sound velocity at high densities was investigated along a 500 K isotherm using high-pressure picosecond acoustics measurements. At 2.0 GPa, the liquid sound velocity goes through a maximum versus pressure without any change on the reflectivity and interferometry acoustic signals. Upon further compression, a softening of the l-Cs visco-elastic properties is observed from 2.0 up to 4.0 GPa, pressure at which the reflectometric signal is abruptly reversed whereas the interferometric signal remains qualitatively the same. This anomalous behaviour could be related to an electronic transformation within the l-Cs state, which here again could reflect what happens at lower temperature within the solid state. If so, such liquid-liquid transition may be driven by the progressive collapse of the 6s electronic orbital onto the 5d ones. Above 4.0 GPa, the l-Cs sound velocity starts again to increase as commonly expected upon compression.

International audience; Depuis 4 ans, de plus en plus d'enseignants-chercheurs physiciens de l'UPMC se sont lancés dans l'enseignement interactif. Ils ont été initiés au départ par le Pr A. Rudolph en visite à l'université. Nous montrerons comment cette pratique pédagogique s'est implantée dans la Faculté, comment elle est utilisée, comment les étudiants réagissent. Nous discuterons aussi des effets en montrant quelques statistiques.

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

Based on the original combination of picosecond acoustics and diamond anvils cell, recent improvements to accurately measure hypersonic sound velocities of liquids and solids under extreme conditions are described. To illustrate the capability of this technique, results are given on the pressure and temperature dependence of acoustic properties for three prototypical cases: polycrystal (iron), single-crystal (silicon) and liquid (mercury) samples. It is shown that such technique also enables the determination of the density as a function of pressure for liquids, of the complete set of elastic constants for single crystals, and of the melting curve for any kind of material. High pressure ultrafast acoustic spectroscopy technique clearly opens opportunities to measure thermodynamical properties under previously unattainable extreme conditions. Beyond physics, this state-of-the-art experiment would thus be useful in many other fields such as nonlinear acoustics, oceanography, petrology, in of view. A brief description of new developments and future directions of works conclude the article. (C) 2014 Elsevier B.V. All rights reserved.

Ultrafast acoustics measurements on liquid mercury have been performed at high pressure and temperature in diamond anvils cell using picosecond acoustic interferometry. We extract the density of mercury from adiabatic sound velocities using a numerical iterative procedure. The pressure and temperature dependence of the thermal expansion, the isothermal compressibilty, the isothermal bulk modulus and its pressure derivative are derived up to 7 GPa and 520 K. In the high pressure regime, the sound velocity values, at a given density, are shown to be only slightly dependent on the specific temperature and pressure conditions. The density dependence of sound velocity at low density is consistent with that observed with our data at high density in the metallic liquid state.

International audience; High-pressure method combining diamond anvil cell with picosecond ultrasonics technique is demonstrated to be a very suitable tool to measure the acoustic properties of iron up to 152 GPa. Such innovative approach allows to measure directly the longitudinal sound velocity under pressure of hundreds of GPa in laboratory, overcoming most of the drawbacks of traditional techniques. The very high accuracy, comparable to piezoacoustics technique, allows to observe the kink in elastic properties at the body-centered cubic-hexagonal close packed transition and to show with a good confidence that the Birch's law still stands up to 1.5 Mbar and ambient temperature. The linear extrapolation of the measured sound velocities versus densities of hcp iron is out of the preliminary reference Earth model, arguing for alloying effects or anharmonic high-temperature effects. A comparison between our measurements and shock wave experiments allowed us to quantify temperature corrections at constant pressure in ∼−0.35 and ∼−0.30 m s−1/K at 100 and 150 GPa, respectively. More in general, the here-presented technique allows detailed elastic and viscoelastic studies under extreme thermodynamic conditions on a wide variety of systems as liquids, crystalline, or polycrystalline solids, metallic or not, with very broad applications in Earth and planetary science.

International audience

International audience; We report picosecond acoustic measurements in silica-glass films grown by wet thermal oxidation on a (111) silicon substrate. The longitudinal acoustic phonons are observed over the range from 150 to 300 GHz using an infrared pump and a second harmonic blue probe. The transducer is an aluminum thin film deposited on top. Multiple interference effects are analyzed and fully taken into account. They lead to a signal presenting rapid oscillations as a function of the sample thicknesses. The latter are determined by separate interferometry. Our remarkably precise acoustic attenuation results are found to follow rather well a model combining thermally activated relaxations and anharmonicity. New values for the optical absorption of silicon in the 400-to-500-nm region of the spectrum are obtained as a by-product.

International audience; We show that the propagation of coherent acoustic phonons generated by femtosecond optical excitation can be clearly resolved using a probe laser in the middle UV (MUV) range. The MUV probe is easily produced from a highrepetition- rate femtosecond laser and a homemade frequency tripler. We present various experimental results that demonstrate efficient and high frequency detection of acoustic phonons. Thus, we show that the MUV range offers a unique way to reach higher frequencies and probe smaller objects in ultrafast acoustics

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 attenuation of longitudinal acoustic phonons up to frequencies nearing 250 GHz is measured in vitreous silica with a picosecond optical technique. Taking advantage of interferences on the probe beam, difficulties encountered in early pioneering experiments are alleviated. Sound damping at 250 GHz and room temperature is consistent with relaxation dominated by anharmonic interactions with the thermal bath, extending optical Brillouin scattering data. Our result is at variance with claims of a recent deep-UV experiment which reported a rapid damping increase beyond 100 GHz. A comprehensive picture of the frequency dependence of sound attenuation in $v$-SiO$_2$ can be proposed., Comment: 4 pages, 3 figures

Valorization of AFBC fly ash in cement and concrete industry; En France, les chaudières à lit fluidisé circulant (LFC) ont une vingtaine d'années d'existence. Malgré leur âge, l'intérêt de leur système de désulfuration des fumées est amoindri du fait de la difficulté à valoriser les cendres qu'elles produisent. A l'heure actuelle, les voies de valorisation envisageables pour ces co-produits sont faiblement consommatrices de cendre et la filière du génie civil, c'est-à-dire la confection de ciment et de béton, leur reste inaccessible. Ces travaux de thèse visent à démontrer que sous réserves de plusieurs précautions, il est possible et même intéressant de confectionner un ciment contenant la cendre volante LFC. Du point de vue minéralogique, les cendres volantes LFC se différencient clairement des cendres volantes silico-alumineuses issues des chaudières à charbon pulvérisé (CP) normalisées par la présence de minéraux argileux, proches de ceux contenus dans les combustibles. A l'aide de la diffraction des rayons X et d'un logiciel de traitement de données par méthode de Rietveld, plusieurs couples de combustible et de cendre ont été caractérisés afin d'étudier leur filiation minéralogique. Du point de vue chimique, le système de désulfuration des chaudières LFC conduit à une grande diversité de composition chimique, de par l'ajout en quantité variable d'une fraction sulfo-calcique aux résidus silico-alumineux de la combustion des charbons. Cette fraction, constituée d'anhydrite et de chaux vive, est le principal frein à la valorisation de la cendre dans la filière du génie civil. Que la cendre soit hydratée seule ou en mélange avec du ciment, son anhydrite conduit à la formation d'ettringite, hydrate responsable de désordres dans la pâte de ciment. Les travaux de thèse eurent comme objectif d'étudier ce phénomène, en tâchant de le relier aux variations dimensionnelles de mortier et de béton. Au final, nos résultats dressent une liste de spécifications, suffisantes pour rendre réalisable la confection d'un ciment composé destiné à une utilisation industrielle. De plus, des essais mécaniques montrent que la nature physique, chimique et minéralogique de la cendre constitue un avantage en terme de résistance mécanique pour confectionner des ciments composés. En détaillant les principaux facteurs qui influencent le gonflement ettringitique et en apportant un nouvel éclairage sur la nature minéralogique de la cendre, cette étude pourrait aider, à terme, à la prise en compte de cette addition minérale dans les normes relatives à la confection de ciment et de béton.