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Plants present a versatile engineering platform to design sustainable and multifunctional materials due, in part, to their natural abundance, self-growing capacity, and functional diversity. While plant-based materials technology has started to gain momentum in photonics, robotics, and soft electronics, it has not yet met phononic materials design. In this work, we investigate the phononic behavior of micro-structured plant cells scaffolds composed of dehydrated plant cell walls. Using laser-generated acoustic waves, we measure the dispersion curves of sub-GHz surface and guided waves in these biocomposites, and reveal the presence of phononic band gaps due to their interaction with compressional and flexural local resonances of the cell wall structure. Moreover, we show that these locally resonant phononic features can be controlled by altering the phenotype of the plant cells. Our results suggest a significant potential for acoustic manipulation of MHz-GHz frequencies using bio-derived surfaces, which we anticipate can lead to a wide range of green ultrasonic devices with tailorable properties.

The impact of sound and acoustic phenomena on daily life and its overall quality is not sufficiently perceived by the general public. In addition, in practically all fields of acoustics, there is a lack of professionals with the necessary knowledge and training to solve the problems that occur in everyday life. In order to raise general awareness of the importance of acoustics and to disseminate knowledge to both professionals and lay people, several universities and companies have joined forces in a consortium with the goal to facilitate the development and dissemination of contemporary teaching materials that go beyond the usual forms and approaches to knowledge transfer. As a result, the freely accessible platform Acoustic Courseware (ACOUCOU) was launched. Over the years, the consortium has developed different courseware on acoustics for specific user groups within the scope of several projects. The latest, ongoing project is the Acoustics Knowledge Alliance (ASKnow) project, funded by the ERASMUS+ programme. The aim is to develop modern, interactive content that will form a well-rounded package of teaching materials. These materials will be used in five specific areas of acoustics, i.e. in five corresponding courses: Acoustic Fundamentals, Psychoacoustics, Acoustic Simulations and Auralisation, Electroacoustics, and Room and Building Acoustics. This paper provides an overview of the ASKnow project, its tasks and objectives. It describes the nature and structure of the developed materials and discusses the development procedures used in their creation. The challenges encountered along the way are briefly outlined. The project has entered the phase of transforming the developed teaching materials into their final form, i.e. the interactive online content. This provides an opportunity to present selected extracts of the raw materials developed during the first half of the project, as well as examples of the finished prototypes of interactive online lessons.

International audience; Time-domain Brillouin scattering uses ultrashort laser pulses to generate coherent acoustic pulses of picoseconds duration in a solid sample and to follow their propagation in order to image material inhomogeneities with sub-optical depth resolution. The width of the acoustic pulse limits the spatial resolution of the technique along the direction of the pulse propagation to less than several tens of nanometres. Thus, the time-domain Brillouin scattering outperforms axial resolution of the classical frequency-domain Brillouin scattering microscopy, which uses continuous lasers and thermal phonons and which spatial resolution is controlled by light focusing. The technique benefits from the application of the coherent acoustic phonons, and its application has exciting perspectives for the nanoscale imaging in biomedical and material sciences. In this study, we report on the application of the time-domain Brillouin scattering to the 3D imaging of a polycrystal of water ice containing two high-pressure phases. The imaging, accomplished via a simultaneous detection of quasi-longitudinal and quasi-shear waves, provided the opportunity to identify the phase for individual grains and evaluate their crystallographic orientation. Monitoring the propagation of the acoustic waves in two neighbouring grains simultaneously provided an additional mean for the localisation of the grain boundaries.

International audience; The development of transducers to excite and detect coherent surface acoustic waves (SAWs) with picosecond periods and deeply-sub-optical, nanometer-level wavelengths and localization depths would be very useful for nanometrology, nanoimaging, sensing and manipulations in fundamental and applied research. The control of SAWs and Lamb waves up to the record frequencies of 90 and 197 GHz, respectively [1,2], was achieved by applying femtosecond pump-probe laser pulses to the metallic gratings deposited on the substrate. The spectrum of the pump-pulse intensity envelope contains the required frequencies, while the periodicity of the grating induces required wave numbers in the spatial distribution of the laser-induced mechanical stresses. The grating period should be shorter than 3 nm for 1 THz SAWs with velocity of 3 km/s. Yet the minimum period of the structures currently patterned in the laboratories is just approaching 10 nm. The key limitation in SAW generation at frequencies approaching 1 THz is therefore due to limitations of surface nanopatterning. Here, we suggest an engineering for SAWs/Lamb waves monitoring of unconventional transducers based on cleaved/sliced nanostructured bulk materials [3]. Currently, bulk superlattices (SLs) with a period of several atomic layers can be grown by epitaxy. They can be cleaved/sliced along the direction normal to their layers producing a periodically nanopatterned surface. We report first experimental monitoring of the coherent SAWs and surface skimming bulk acoustic waves up to 70 GHz on the cleaved cross sections of Ga1-x AlxAs/Ga1-yAlyAs SLs. We also discuss the optical detection mechanism of deeply-sub-optical wavelength acoustic excitations in the SLs. [1] M. Shubert, et al., Appl. Phys. Lett. 101, 013108 (2012). [2] M. Grossmann, et al., Appl. Phys. Lett. 106, 171904 (2015). [3] V. E. Gusev, In: D. Bićanić (eds) Photoacoustic and Photothermal Phenomena III. Springer Series in Optical Sciences, 69, 323 (Springer, Berlin, Heidelberg, 1992).

International audience; Photoacoustic methods of non-destructive testing and evaluation (NDTE) of materials with the use of lasers are of great interest due to their contactless nature. One of these methods is time-domain Brillouin scattering (TDBS) [1] where the monitoring of the acoustic pulse propagation in optically transparent media can provide a depth profile of the sample material inhomogeneity with nanometers spatial resolution [2, 3]. The information on the spatial profiles of the sound velocity, the optical refractive index [2, 4] and of the photo-elastic constant [3] can be revealed. In a case where the amplitude and the frequency of the oscillating part of the transient reflectivity signal are varying at the same characteristic times, their precise separation by measuring only the real part of the transient optical reflectivity signal is impossible, thereby, forbidding the independent depth profiling of different material parameters. For the independent depth profiling of several material parameters both real and imaginary parts of the optical reflectivity should be measured experimentally [5]. This is possible if the optical interferometry is applied. Basing on the advances in the application of the ultrafast optical interferometry [6], we have constructed an experimental setup that will lead to the first application of the ultrafast optical interferometry for the opto- acousto-optical depth profiling of materials. At first, the setup will be applied for the depth profiling of the materials with the known spatially inhomogeneous chemical composition. [1] H. T. Graham et al., IEEE J. Quantum Electron. 25, 2562 (1989). [2] C. Mechri et al., Appl. Phys. Lett. 95, 091907 (2009). [3] A. Steigerwald et al., Appl. Phys. Lett. 94, 111910 (2009). [4] A. M. Lomonosov et al., ACS Nano 6, 1410 (2012). [5] V. Gusev et al., J. Appl. Phys. 110, 124908 (2011). [6] O. Matsuda et al., J. Opt. Soc. Am. B, 30, 1911(2013).

International audience; Epoxies are the type of reactive polymers used as adhesive or paints in a wide range of industrial applications such as automotive and aerospace. The mechanical properties in the proximity of metal/epoxy interfaces play a crucial role in the bond strength and stability of adhesion. The metal/epoxy interfaces were generally studied using techniques like atomic force microscopy (AFM) and frequency-domain Brillouin scattering (FDBS) microscopy [1,2]. AFM provides information at nanoscale distances from the interface, but the destructive cut of the sample for the investigation could modify the epoxy, while the depth spatial resolution in the FDBS experiments was limited to 10 �m [2]. We report the implementation of the time-domain Brillouin scattering (TDBS) method with a fast data acquisition technique based on asynchronous optical sampling (ASOPS) to examine the metal/epoxy interface in its micrometers vicinity with nanometers depth resolution. We have performed experiments on 50 �m-thick epoxy resin film on Aluminum plate. For the epoxy, we used the combination of diglycidyl ether of bisphenol-A (DGEBA) and diethylenetriamine (DETA). We focused the laser pump pulse at 515 nm wavelength on the Aluminum plate from the epoxy side, which generates the acoustic waves. Their propagation through epoxy is detected from the same side using the probe pulse at 532 nm wavelength. We observed in the signal the Brillouin oscillations of up to ~0.5 ns duration, which implies gigahertz acoustic waves propagation to a depth of ~1.5 �m from the interface. The analysis of these oscillations as a function of time at various lateral locations of the epoxy could produce a three-dimensional image of the mechanical properties in nanometers depth resolution. Acknowledgments: The grant ANR-18-CE42-017 supports this research. [1] H. Kishi et al., J. Polym. Sci. Polym. Phys. 45, 1425 (2007). [2] J. K. Kr�ger et al., J. Adhes. 80, 585 (2004).

International audience; Biocompatible polymerized 2-(hydroxyethyl) methacrylate (pHEMA) is a very popular material for medical applications. There are still numerous questions to be answered concerning the polymerisation processes of HEMA upon compression [1]. For this reason, the application of non-destructive contactless time-domain Brillouin scattering (TDBS) pump-probe technique [2] to the evaluation of HEMA properties looks very promising. We report here the extension of the TDBS technique to the 3D imaging of evolution of HEMA compressed in a diamond anvil cell (DAC). First, experiments were performed using classical picosecond ultrasonic set-up with a mechanical delay line [2]. To polymerise HEMA, it was compressed to 6.5 GPa and then decompressed immediately to 0.5 GPa. After 10 minutes at this pressure it was decompressed to 0 GPa. This procedure led to the increase of the Brillouin frequency from 6.3 GHz in HEMA monomer to 11.3 GHz in pHEMA. The measurements by Raman spectrometry confirmed the polymerisation of HEMA. Then, to perform 3D imaging possible in a reasonable time, we have applied an ultrafast laser technique based on an asynchronous optical sampling (ASOPS). The maps of the Brillouin frequencies obtained using TBDS technique could significantly extend our knowledge about the earlier recognised two different states of HEMA above and below 6.5 GPa [1], which cannot be distinguished by any other technique. Moreover, it should potentially allow following in time the 3D spatial spreading of the polymerization reaction starting from the nucleation sites. In perspective, the 3D imaging of the transient processes at high pressures with nanometers depth resolution could be possible. Acknowledgments: This research is supported by the grant ANR-18-CE42-017. [1] E. Evlyukhin, et al Scientific Reports 5, 18244 (2015). [2] C. Thomsen, H.T. Graham, H.J. Maris, J. Tauc, Optics Communications 60, 55 (1986).

International audience; Biological composites are known to exhibit extraordinary mechanical properties, due to their complex, hierarchical internal structures composed of fibrous polymers, and other supra-molecular arrangements. While much is known about their quasi-static mechanical properties, the study of elastic wave propagation in biological structural materials is an almost pristine field. In this work, we investigate the phononic behavior of the anisotropic fibrillar network in micron-thick onion cell walls using laser-generated, sub-GHz guided acoustic waves. The measured dispersion curves reveal the presence of Rayleigh and Sezawa modes. By tracking their dispersion as a function of cell orientation, we find angular-dependent phase velocities due to mechanical anisotropy of the wall. Moreover, we show that the structural arrangement of the wall generates an anomalous low-frequency acoustic branch characterized by an ultra-slow group velocity. We demonstrate that this mode arises from the hybridization of modes guided within the wall and the resonance of subcell structures. Our findings pave the way to the design of new biologically-derived acoustic metamaterials.

International audience; Picosecond acoustic waves at ~365 GHz in a semiconductor saturable absorber mirror (SESAM) structure are optically controlled by femtosecond time-resolved spectroscopy. Picosecond acoustic waves are excited and detected in the In0.27Ga0.73As multiple quantum wells of the SESAM structure by femtosecond lasers operating at around ~1 �m wavelength. A second pump pulse with an adjustable time delay and intensity with respect to the first one, is used to exert control over the acoustic waves, which yields various acoustic patterns. The coherent acoustic spectral components can be manipulated in terms of enhancement, selective suppression, comb-spacing doubling, and tunable profiles. By the aid of a simplified model with identical generation and detection sensitivity functions in the quantum well stacks, the periodicity of modulation, the phase jump, the selection of suppressed modes, and the upper boundary in the time-delay or intensity-ratio dependent variations are well interpreted. The modelling agrees well with experimental results in a wide tunable range of the time delay and the intensity ratio. The results show promising applications in arbitrary acoustic wave synthesis and acoustic field filters. Additionally, a prism is inserted in the pump path to induce a subharmonic-like frequency component. By controlling the rotating angle of the prism, the amplitude of subharmonic components can be adjusted. The potential explanation of the induced acoustic change can be the excitation of subharmonic frequencies or the interference of acoustic waves induced by different pump spectral components with varied time delays.

International audience; Semiconductor saturable absorber mirrors (SESAMs) with N-ion irradiated In0.3Ga0.7As quantum wells are investigated by both degenerate and non-degenerate phonon spectroscopy. We found that the N-ion irradiation on quantum wells (QWs) has impact not only on the recovery time of the SESAM structures, but also on the laser-induced coherent acoustic phonons. The recovery time is shortened when more quantum wells are irradiated by N ions, down to 1.6 ps (for all-QW-irradiated sample) from 19.9 ps (for sample without N-ion irradiation). The reflectivity change induced by coherent acoustic phonons is enhanced in the sample with partially irradiated QWs, but further enhancement does not occur in the all-QW-irradiated sample. The modification of carrier concentration induced by N-ion implantation potentially accounts for the improvement of phonon-induced reflectivity change, while further enhancement is hindered by the suppression of the deformation potential effect due to the fact that the recovery time is shorter than the acoustic wave period. In the multiple quantum well region, overall consistent acoustic results are obtained in the degenerate and non-degenerate configuration, with slight differences caused by different pump photon energies or excitation functions. In the distributed Bragg reflector region, up to seven orders of the center-modes of folded acoustic phonons starting from ~31 GHz are observed in the non-degenerate configuration. The detected first-order component is evidently enhanced and detected higher- order modes are overall slightly suppressed in the all-QW-irradiated sample compared to those in the partial-QW-irradiated sample. Our results imply that N- ion irradiation distribution can be used to tailor coherent acoustic phonons, thus this can be directly applied to monitor the optimization of SESAM characteristics for mode-locking lasers.

International audience; We started in September 2019 a new master curriculum at Le Mans University, as part of a plan of actions in the frame of the Institut d?Acoustique - Graduate School. This Institute is related to a project funded by the French state, launched in 2018 in the frame of the national call ?Ecole Universitaire de Recherche? and which intends to offer ?Universities the possibility to reinforce the international impact and attractiveness of their research and training activities in one or several scientific domains, by integrating master and doctoral trainings with high level laboratories? and to introduce in France the model of Graduate Schools. One proposed action to implement all this, was to launch a new master curriculum focused on training by and through research, rethinking usual ways of organizing the training at master level in France, and inspired from several US and European universities. In this presentation, we will describe this new master curriculum on Wave Physics & Acoustics and in particular how we implemented a plan to favor education by and through research. We will highlight the necessary adjustments we had to make and the expected outputs for the students and the laboratory.

International audience; Time-domain Brillouin scattering uses ultrashort laser pulses to generate coherent acoustic pulses of picoseconds duration in a solid sample and to follow their propagation in order to image material inhomogeneities with suboptical depth resolution. The width of the acoustic pulse limits the spatial resolution of the technique along the direction of the pulse propagation to less than several tens of nanometres. Thus, the time-domain Brillouin scattering outperforms axial resolution of the classical frequencydomain Brillouin scattering microscopy, which uses continuous lasers and thermal phonons and which spatial resolution is controlled by light focusing. The technique benefits from the application of the coherent acoustic phonons, and its application has exciting perspectives for the nanoscale imaging in biomedical and material sciences. In this study, we report on the application of the time-domain Brillouin scattering to the 3D imaging of a polycrystal of water ice containing two high-pressure phases. The imaging, accomplished via a simultaneous detection of quasi-longitudinal and quasi-shear waves, provided the opportunity to identify the phase for individual grains and evaluate their crystallographic orientation. Monitoring the propagation of the acoustic waves in two neighbouring grains simultaneously provided an additional mean for the localisation of the grain boundaries

International audience; The non-destructive evaluation of adhesive bonding is a major issue in the aeronautical industry. Indeed, this assembly technique has many advantages compared to more conventional ones (riveting?): it allows lighter aircraft structures and therefore reduces fuel consumption and emissions. Furthermore, a better stress distribution can be achieved between assembled parts. Nevertheless, the significant deployment of this assembly technique is currently limited by the nonexistence of a method to certify the practical adhesion without damaging the structure. In the past 50 years, a large number of non-destructive methods have been investigated to measure bond strength quantitatively [1]. Concerning ultrasonic techniques, various approaches based on the reflection or the transmission of bulk waves at an imperfect interphase have been studied [2-3]. Other methods based on guided waves have been considered: shear horizontal modes [4], Lamb modes [5] (especially Zero-Group-Velocity (ZGV) modes [6]). In this research project, a laser ultrasonic method is proposed to non-destructively evaluate the adhesive bonding of plates. ZGV resonances are generated and detected experimentally on trilayer samples [alu. (1.5 mm-thick)/epoxy/alu. (3.0 mm-thick)]. The practical adhesion of these different assemblies is degraded by cohesive and adhesive defects. The temporal attenuations of these ZGV modes allow to discriminate the different samples. Moreover, quantitative parameters linked to the damping of ZGV resonances are identified. 1. M.D. Bode \textitet al., \textitLiterature review of weak adhesive bond fabrication and nondestructive inspection for strength measurement (U.S. Departement of Transportation, Washington, DC, 2015). 2. A. Baltazar \textitet al., J. Acoust. Soc. Am. \textbf114 (3), 1424 (2003). 3. E. Syriabe \textitet al., Ultrasonics \textbf79, 34 (2017). 4. B. Le Crom and M. Castaings, J. Acoust. Soc. Am. \textbf127 (4), 2220 (2010). 5. M.J.S. Lowe \textitet al., J. Acoust. Soc. Am. \textbf107 (3), 1333 (2000). 6. S. Mezil \textitet al., J. Acoust. Soc. Am. \textbf138 (5), 3202 (2015).

International audience; Picosecond laser ultrasonics methods provide non-contact, non-invasive imaging of &ₘu;m samples, serving in biology to map cell mechanics, likewise in material science to scan microstructures [1]. This work presents the 3D reconstruction of the cubic polycrystalline structure of ceria, with a pump-probe method: the time-domain Brillouin scattering (TDBS) [2]. In an opto-acoustic transducer (OAT), the pump fs laser pulse induces, via thermoelastic generation, an acoustic wave. When this wave is transmitted to the elastically anisotropic ceria, up to three mode-converted acoustic pulses could propagate. The delayed probe fs laser pulse is strongly reflected at the fixed OAT surface, and weakly reflected at the propagating acoustic wavefronts. The interferences between these reflections produce the so-called Brillouin oscillations which frequencies depend on the propagating acoustic pulses? velocities. Monitoring in time the Brillouin frequency changes of each acoustic mode, i.e. the velocity changes due to propagation in differently-oriented grains, give access to three different views of the 1D distribution of the grains along the z-direction (depth). The 3D reconstruction is obtained by gathering these 1D distribution views, combined with a 1 &ₘu;m step, 60 x 60 &ₘu;m2 scan (xy-directions) of the sample. Each gathered transient reflectivity signal is processed using a synchronous detection technique to estimate the instantaneous Brillouin frequency, eventually giving the distribution of the velocities as a function of depth and of xy-directions. To better differentiate and extract shapes of the grains, Otsu's and alphashape image processing methods are applied to each slice (xy velocity distribution at a given depth). Gathering the informations from the three acoustic modes improves grains differentiation. The expected changes in the TDBS signal when the acoustic pulse is incident on an inclined grain boundary are theoretically addressed. [1] M.Khafizov et al., Acta Materialia, 112, 209-215 (2016), [2] V.E.Gusev, P. Ruello, Appl. Phys. Rev., 5, 031101 (2018).

International audience; Time-domain Brillouin scattering (TDBS) is an experimental technique for generation and detection of nanometers-in-length acoustic pulses using ultrafast lasers, which was originally referred to as picosecond acoustic interferometry [1]. Experimental signals collected with the TDBS technique in transparent materials contain Brillouin oscillations, caused by interference of the probe laser pulses reflected from stationary interfaces and from the propagating ultrashort acoustic pulses. Frequencies of the Brillouin oscillations depend on the local refractive index and the local longitudinal sound velocity. Therefore, accurate estimates in a TDBS signal of the Brillouin frequency with respect to time give an access to the variations of the material parameters as a function of material depth. TDBS experiments could give access to acoustical, optical and acousto-optical parameters of the materials [2], by carrying experiments at several incidence angles for instance. Some applications of the TDBS technique for depth profiling of polycrystalline materials under very high-pressures [3,4] as well as its ability for real-time imaging of phase transition [5] will be presented. Limitations of this imaging technique, as well as the perspectives of its extension in multiple research areas [6] and industrial fields, will also be discussed. References[1]C. Thomsen, H. T. Graham, H. J. Maris, J. Tauc, Optics Communications 60, 55 (1986). [2]A. M. Lomonosov, A. Ayouch, P. Ruello, G. Vaudel, M. R. Baklanov, P. Verdonck, L. Zhao, V. E. Gusev, ACS Nano 6, 1410 (2012).[3]M. Kuriakose, S. Raetz, N. Chigarev, S. M. Nikitin, A. Bulou, D. Gasteau, V. Tournat, B. Castagnede, A. Zerr, and V. E. Gusev, Ultrasonics 69, 259-267 (2016).[4]M. Kuriakose, S. Raetz, Q. M. Hu, S. M. Nikitin, N. Chigarev, V. Tournat, A. Bulou, A. Lomonosov, P. Djemia, V. E. Gusev, and A. Zerr, Phys. Rev. B 96, 134122 (2017).[5]M. Kuriakose, N. Chigarev, S. Raetz, A. Bulou, V. Tournat, A. Zerr, and V. E. Gusev, New J. Phys., 19, 053026 (2017).[6]V. E. Gusev, P. Ruello, Appl. Phys. Rev. 5, 031101 (2018).

International audience; Time-domain Brillouin scattering (TDBS) is applied for the first time to perform the 3D imaging of polycrystalline water ice phases, VII and VI, coexisting at a pressure of 2.1 GPa in a diamond anvil cell (DAC) at room temperature.Materials and MethodsThe TDBS is a non-destructive opto-acousto-optic pump-probe technique [1] which allows the study of a variety of transparent materials [2]. In this method, an optical pump pulse from a femtosecond laser is absorbed by an optoacoustic transducer contacting the sample. As a result, a picosecond acoustic pulse is emitted into the sample. It scatters a time-delayed optical probe pulse from the same or another femtosecond laser. Thus, via detection of the changes of the transient optical reflectivity in time, the evolution of the acoustic pulse with its propagation distance can be probed when it traverses the sample. The TDBS signal contains information on the characteristics of the acoustic pulse and the parameters of the material in the current spatially localized position of the acoustic pulse. The length of this pulse is commonly at nanometers spatial scale. Therefore, the technique is suitable for imaging of materials along the pulse propagation path with a spatial resolution better than optical. Two-dimensional (lateral and depth) TDBS imaging has been earlier applied for revealing the texture of solid H2O [3] and Ar [4], the phase transitions [5] and the reliable pressure dependences of elastic moduli [6] in water ice. We report here the extension of the TDBS technique to the 3D imaging of samples compressed in a DAC.To accelerate the data acquisition and to make 3D imaging possible in reasonable time, we applied, for the first time, to a sample at high pressure in a DAC, an ultrafast laser technique called asynchronous optical sampling (ASOPS). In ASOPS technique, the time delay between the pump and the probe is controlled electronically by an offset of the repetition rate frequency of two lasers without the use of a slow mechanical delay line.The experiments were conducted on water ice in a DAC at 2.1 GPa. The opto-acoustic generator (iron plate of approximately 40 μm thickness [5]) inside the sample chamber has the diameter of ∼110 μm. The full width at half maximum of the laser beams at the surface of the generator is 1.4 μm. When irradiated by the pump optical pulses at 515 nm wavelength from our ASOPS-based picosecond acoustic microscope (JAX-M1, NETA, France), it emits picosecond acoustic pulses in 14.5 μm layer of water ice on the top of laser irradiated Fe surface [5]. The maximum analyzable time delay (1.9 ns) of the probe laser pulses at 532 nm wavelength provided opportunity for imaging the ice layers up to ∼10 μm distances from the generator. The image in the volume of 40x40x10 μm3 is obtained with the lateral step of 2 μm in 2 hours. ResultsMaps of the Brillouin frequencies in the moving time windows of the acoustic pulse propagation in the ice are presented in Fig.1. They were obtained using a time-frequency analysis based on the synchronous detection principle [6]. The delay times between the probe and the pump laser pulses inside the time windows are marked above each of the cross sectional maps (a)-(c) and, thus, correspond to different depth layers in the sample. The red line attracts the eyes to the variations of the Brillouin frequency with time in a selected lateral region. Note that, because acoustic waves propagate at different velocities in different lateral points, the same time window corresponds to different depth windows in different lateral points. The estimates demonstrate that the highest and the lowest detected Brillouin frequencies are the fingerprints of presence of the ice VII, while the intermediate frequencies in the lower half of the frequency spectrum could be due to the presence of ice VI. Fig.1. 2D maps of the Brillouin frequencies distribution in the 40x40 μm2 cross sections of the tested ice volume at 2.1 GPa for shifting time slices. The red line highlights one of the lateral regions with important variations of the Brillouin frequency as a function of the distance from the generator.ConclusionsWe demonstrated the ASOPS-based 3D TDBS imaging of water ice in a DAC. This 3D imaging allows us to visualize shapes of the crystallites formed in the sample volume and their transformation with increasing load. In perspective, the 3D imaging of the transient processes at high pressures with nanometers depth resolution could be possible.Acknowledgement This research is supported by the grant .References1.C. Thomsen, H.T. Graham, H.J. Maris, J. Tauc, Optics Communications 60, 55 (1986); doi:10.1016/0030-4018(86)90116-12. V. E. Gusev, and P. Ruello, Applied Physics Reviews 5, 031101 (2018) doi:10.1063/1.50172413. S. M. Nikitin, N. Chigarev, V. Tournat, A. Bulou, D. Gasteau, B. Castagnede, A. Zerr, V. E. Gusev, Scientific Reports 5, 9352 (2015); doi:10.1038/srep093524. M. Kuriakose, S. Raetz, N. Chigarev, S. M. Nikitin, A. Bulou, D. Gasteau, V. Tournat, B. Castagnede, A. Zerr, V. E. Gusev, Ultrasonics, 69, 201 (2016); doi:10.1016/j.ultras.2016.03.0075. M. Kuriakose, N. Chigarev, S. Raetz, A. Bulou, V. Tournat, A. Zerr, V. E. Gusev, New Journal of Physics 19, 053026 (2017); doi:10.1088/1367-2630/aa6b3d 6. M. Kuriakose, S. Raetz, Q. M. Hu, S. M. Nikitin, N. Chigarev, V. Tournat, A. Bulou, A. Lomonosov, P. Djemia, V. E. Gusev, A. Zerr, Physical Review B 96, 134122 (2017); doi:10.1103/PhysRevB.96.134122

International audience; In material science, the time-domain Brillouin scattering (TDBS) technique, initially known as the picosecond acoustic interferometry technique [1], allows imaging either of a continuous change of (or of inhomogeneities occurring due to a mismatch in) acoustical, optical, and/or photo-elastic parameters in transparent materials [2]. In this technique, acoustic waves are generated by an ultrashort optical (pump) pulse absorbed by the tested material or by an optoacoustic transducer in contact with it. In the most general case, up to three waves could be generated and propagate in the sample: one longitudinal (or quasi-longitudinal) and two shear (or quasi-shear) waves. Propagation of these waves is monitored by a second time-delayed ultrashort optical (probe) pulse with the wavelength in the transparency region of the tested material. In that case, a part of the probe beam is reflected at by stationary surfaces/interfaces and another part is reflected at by the propagating acoustic pulses. These reflected parts interfere on the photodetector and give rise to an oscillatory signal having the same frequencies as the Brillouin oscillations. In the most general case, several frequencies could be present in the signal, the values of which being defined by the combinations of the acoustic sound velocity, refractive index? and probe light wavelength. In a polycrystalline material, tracking of the changes of those frequencies with lateral position and with delay time should allow a precise 3D? imaging of the crystallites (or of crystallites groups having the same preferential orientation) and of the boundaries between them. Yet, contrast in the TDBS imaging, such as in the evaluation of texture [3], of position of grain boundaries [4], and of elastic moduli [5] in polycrystalline materials, comes usually from the value of the Brillouin frequency associated to the longitudinal acoustic pulses only. However, a parallel monitoring of both longitudinal and transverse acoustic pulses has recently demonstrated an enhanced ability in determining grains orientation near the surface of a polycrystalline ceria (CeO2) sample [6].We propose here to extend the technique to three dimensions by making use of the three acoustic pulses, if available, and/or by taking benefit from the fact that Brillouin frequencies depend on the local refractive index and the local sound velocities. Indeed, accurate and simultaneous estimates of the variations of the three Brillouin frequencies with respect to time in collected TDBS signals gives access to the variations of the material parameters as a function of the sample depth. Combining the latter with a 2D lateral scanning over the sample leads to a three dimensional imaging. Some applications of the TDBS technique for 3D imaging of polycrystalline materials, both at atmospheric and at high pressures, will be presented, together with the signal processing procedure/software developed for this purpose. The experimental results have been collected with an ASOPS-based picosecond acoustic microscope (JAX-M1, NETA, France), which allows a fast image acquisition. The fFigure 1 presents examples of the images for ??? obtained with the Brillouin frequencies associated with (a) longitudinal, (b) fast and (c) slow quasi-shear acoustic pulses, demonstrating their complementarity in determining the grain boundary positions in the vicinity of the sample surface. The nanoscale imaging ability of TDBS is improved by the use of shear acoustic waves, which is expected to be even more evident by the gain in contrast obtained on the imaging of buried grain boundaries. Fig. 1. Examples of the images obtained with the Brillouin frequencies associated with (a) quasi-longitudinal, (b) fast and (c) slow quasi-shear acoustic pulses.AcknowledgementsThis research is supported by the project , the Acoustic Hub® program and the LMAc project NANOSHEAR.References 1.C. Thomsen, H.T. Graham, H.J. Maris, J. Tauc, Optics Communications 60, 55 (1986); doi:10.1016/0030-4018(86)90116-12.V. E. Gusev, and P. Ruello, Applied Physics Reviews 5, 031101 (2018); doi:10.1063/1.50172413.S. M. Nikitin, N. Chigarev, V. Tournat, A. Bulou, D. Gasteau, B. Castagnede, A. Zerr, V. E. Gusev, Scientific Reports 5, 9352 (2015); doi:10.1038/srep093524.M. Khafizov, J. Pakarinen, L. He, H. Henderson, M. Manuel, A. Nelson, B. Jaques, D. Butt, D. Hurley, Acta Materialia 112, 209–215 (2016); doi: 10.1016/j.actamat.2016.04.0035.M. Kuriakose, S. Raetz, Q. M. Hu, S. M. Nikitin, N. Chigarev, V. Tournat, A. Bulou, A. Lomonosov, P. Djemia, V. E. Gusev, A. Zerr, Physical Review B 96, 134122 (2017); doi:10.1103/PhysRevB.96.1341226.Y. Wang, D. H. Hurley, Z. Hua, G. Sha, S. Raetz, V. E. Gusev, M. Khafizov, Scripta Materialia 166, 34– 38 (2019); doi: 10.1016/j.scriptamat.2019.02.037

International audience

International audience; Water ice is a molecular solid whose behavior under compression reveals the interplay of covalent bonding in molecules and forces acting between them. This interplay determines high-pressure phase transitions, the elastic and plastic behavior of H 2 O ice, which are the properties needed for modeling the convection and internal structure of the giant planets and moons of the solar system as well as H 2 O-rich exoplanets. We investigated experimentally and theoretically elastic properties and phase transitions of cubic H 2 O ice at room temperature and high pressures between 10 and 82 GPa. The time-domain Brillouin scattering (TDBS) technique was used to measure longitudinal sound velocities (V L) in polycrystalline ice samples compressed in a diamond anvil cell. The high spatial resolution of the TDBS technique revealed variations of V L caused by elastic anisotropy, allowing us to reliably determine the fastest and the slowest sound velocity in a single crystal of cubic H 2 O ice and thus to evaluate existing equations of state. Pressure dependencies of the single-crystal elastic moduli C ij (P) of cubic H 2 O ice to 82 GPa have been obtained which indicate its hardness and brittleness. These results were compared with ab initio calculations. It is suggested that the transition from molecular ice VII to ionic ice X occurs at much higher pressures than proposed earlier, probably above 80 GPa.

International audience

International audience