Benoit Rufflé

Brillouin spectroscopy is a non-invasive method technique for visualizing the mechanical properties of biological samples with sub-micrometer resolution. However, to obtain 2D or even 3D maps, a high-contrast spectrometer is required to effectively reject spurious signals and isolate the signal of interest. In this work, we present an optimized two-stage virtually imaged phased array-based Brillouin spectrometer capable of achieving a contrast of at least 80 dB thanks to a new single diffraction mask, and thus with an unprecedentedly high overall transmission. Numerical simulations of the electric field propagation through the spectrometer and dedicated experiments validate the increase in contrast. We have used the newly developed Brillouin spectrometer coupled to a confocal microscope to obtain 1 h high-resolution 2D images of about 104 spectra of transparent dental pulp stem cells near a MgF2 interface as well as of the much less transparent porcine dentin material, demonstrating its overall stability and reliability.

Many theories predict a quartic increase of the acoustic damping at sub-THz frequencies in glassy media, related to the excess vibrational modes known as the boson peak anomaly. Here by introducing phase-sensitive acoustic spectroscopy techniques with a THz bandwidth, we investigate the acoustic properties of vitreous silica at 15 and 300 K in the crucial but unexplored sub-THz gap region below the boson peak. Our results indicate a negative dispersion below the THz range and the onset of an athermal quartic frequency scaling acoustic attenuation term, which appears above all other thermal losses.

In situ high-pressure Brillouin light scattering experiments along loading-unloading paths are used to investigate the compressibility of vitreous silica. Below 9 GPA, the equation of state obtained from the sound velocities corrected for dispersion agrees with volume measurements. Conversely, huge anelastic effects are observed in the range 10–60 GPa, unveiling the reversible transformation from the fourfold-coordinated structure to the sixfold one. The associated density changes correlate with the average Si coordination number. Decompression curves from above 20 GPa reveal abrupt backward coordination changes around 10–15 GPa and significant hysteresis. Contrary to common wisdom, the residual densification of the recovered silica samples can be figured out from changes in elastic properties along pressure cycles, ruling out a plastic description of the latter process.

Under a high pressure of helium, the volume change of amorphous silica is much smaller than expected from its elastic properties. This is due to helium insertion in the free volume of the glass network. Here, we report spectroscopic experiments using either helium or neon as penetrating pressurizing media and molecular simulation that indicate a relationship between the amount of gas adsorbed and the strain of the network. A generalized poromechanical approach, describing the elastic properties of microporous materials upon adsorption, is shown to successfully describe the physics of deformation of such silica glasses in which the free volume exists only at the sub-nanometer scale.

Nowadays powerful X-ray sources like synchrotrons and free-electron lasers are considered as ultimate tools for probing microscopic properties in materials. However, the correct interpretation of such experiments requires a good understanding on how the beam affects the properties of the sample, knowledge that is currently lacking for intense X-rays. Here we use X-ray photon correlation spectroscopy to probe static and dynamic properties of oxide and metallic glasses. We find that although the structure does not depend on the flux, strong fluxes do induce a non-trivial microscopic motion in oxide glasses, whereas no such dependence is found for metallic glasses. These results show that high fluxes can alter dynamical properties in hard materials, an effect that needs to be considered in the analysis of X-ray data but which also gives novel possibilities to study materials properties since the beam can not only be used to probe the dynamics but also to pump it.