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A detailed discussion of the shortcomings in the sample preparation, characterisation, data analysis, and measured equation of state in two publications of Niwa et al.

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; 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; 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; 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

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.

Electronic band structures of two novel semiconducting nitrides of the group-IV elements of Th3P4-type crystal structure, c-M3N4, where M[DOUBLE BOND]Zr or Hf, is investigated using an element specific soft X-ray spectroscopy for the first time. From the pairs of N 1s X-ray absorption and N 2p [RIGHTWARDS ARROW] 1s resonant X-ray emission spectra partial densities of states (PDOS) of nitrogen, predicted to be strongly hybridized with those of the metals, are obtained for both compounds. From these data the electronic band gaps of oxygen bearing c-Zr3N4 and of c-Hf3N4, predicted before to be direct or nearly direct, are derived to be Eg = 1.6 eV and Eg = 1.8 eV, respectively. While the experimentally determined Eg for c-Hf3N4 agrees with the theoretical one obtained using the local density approximation (LDA) method, the Eg measured for oxygen bearing c-Zr3N4 is significantly higher than those calculated using both the LDA and the generalized gradient approximation method. The examined compounds, having high hardness, elastic moduli and oxidation resistance, appear to be multifunctional materials suitable also for applications as (opto)electronic materials. Band gap evolution in c-M3N4 nitrides as a function of the cation type, Zr or Hf.

Time-domain Brillouin scattering technique, also known as picosecond ultrasonic interferometry, which provides opportunity to monitor propagation of nanometers to sub-micrometers length coherent acoustic pulses in the samples of sub-micrometers to tens of micrometers dimensions, was applied to depth-profiling of polycrystalline aggregate of ice compressed in a diamond anvil cell to Megabar pressures. The technique allowed examination of characteristic dimensions of elastic inhomogeneities and texturing of polycrystalline ice in the direction normal to the diamond anvil surfaces with sub-micrometer spatial resolution via time-resolved measurements of variations in the propagation velocity of the acoustic pulse traveling in the compressed sample. The achieved two-dimensional imaging of the polycrystalline ice aggregate in-depth and in one of the lateral directions indicates the feasibility of three-dimensional imaging and quantitative characterization of acoustical, optical and acousto-optical properties of transparent polycrystalline aggregates in diamond anvil cell with tens of nanometers in-depth resolution and lateral spatial resolution controlled by pump laser pulses focusing., 32 pages, 5 figures