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Many theories predict a quartic acoustic attenuation increase at sub-THz frequencies in glassy media for 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 300K in the crucial but unexplored sub-THz gap region below the boson peak. Our results indicate a strong negative dispersion starting at \SI{500}{GHz} and the onset of an athermal quartic-frequency-scaling acoustic attenuation term, which emerges above all other thermal losses., Comment: 4 figures, 11 pages

Ultrahigh-frequency acoustic-phonon resonators usually require atomically flat interfaces to avoid phonon scattering and dephasing, leading to expensive fabrication processes, such as molecular beam epitaxy. In contrast, mesoporous thin films are based on inexpensive wet chemical fabrication techniques. Here, we report mesoporous titanium dioxide-based acoustic resonators with resonances up to 90 GHz, and quality factors from 3 to 7. Numerical simulations show a good agreement with the picosecond ultrasonics experiments. We also numerically study the effect of changes in the speed of sound on the performance of the resonator. This change could be induced by liquid infiltration into the mesopores. Our findings constitute the first step towards the engineering of building blocks based on mesoporous thin films for reconfigurable optoacoustic sensors., Comment: 13 pages, 4 figures

The coherent manipulation of acoustic waves on the nanoscale usually requires multilayers with thicknesses and interface roughness defined down to the atomic monolayer. This results in expensive devices with predetermined functionality. Nanoscale mesoporous materials present high surface-to-volume ratio and tailorable mesopores, which allow the incorporation of chemical functionalization to nanoacoustics. However, the presence of pores with sizes comparable to the acoustic wavelength is intuitively perceived as a major roadblock in nanoacoustics. Here we present multilayered nanoacoustic resonators based on mesoporous SiO$_2$ thin-films showing acoustic resonances in the 5-100 GHz range. We characterize the acoustic response of the system using coherent phonon generation experiments. Despite resonance wavelengths comparable to the pore size, we observe for the first time unexpectedly well-defined acoustic resonances with Q-factors around 10. Our results open the path to a promising platform for nanoacoustic sensing and reconfigurable acoustic nanodevices based on soft, inexpensive fabrication methods.

Inspired by concepts developed for fermionic systems in the framework of condensed matter physics, topology and topological states are recently being explored also in bosonic systems. The possibility of engineering systems with unidirectional wave propagation and protected against disorder is at the heart of this growing interest. Topogical acoustic effects have been observed in a variety of systems, most of them based on kHz-MHz sound waves, with typical wavelength of the order of the centimeter. Recently, some of these concepts have been successfully transferred to acoustic phonons in nanoscaled multilayered systems. The reported demonstration of confined topological phononic modes was based on Raman scattering spectroscopy, yet the resolution did not suffice to determine lifetimes and to identify other acoustic modes in the system. Here, we use time-resolved pump-probe measurements using an asynchronous optical sampling (ASOPS) technique to overcome these resolution limitations. By means of one-dimensional GaAs/AlAs distributed Bragg reflectors (DBRs) as building blocks, we engineer high frequency ($\sim$ 200 GHz) topological acoustic interface states. We are able to clearly distinguish confined topological states from stationary band edge modes. The detection scheme reflects the symmetry of the modes directly through the selection rules, evidencing the topological nature of the measured confined state. These experiments enable a new tool in the study of the more complex topology-driven phonon dynamics such as phonon nonlinearities and optomechanical systems with simultaneous confinement of light and sound., Comment: 6 pages, 3 figures

The effect of picosecond acoustic strain pulses (ps-ASP) on a thin layer of (Ga,Mn)As co-doped with phosphorus was probed using magneto-optical Kerr effect (MOKE). A transient MOKE signal followed by low amplitude oscillations was evidenced, with a strong dependence on applied magnetic field, temperature and ps-ASP amplitude. Careful interferometric measurement of the layer's thickness variation induced by the ps-ASP allowed us to model very accurately the resulting signal, and interpret it as the strain modulated reflectivity (differing for $\sigma_{\pm}$ probe polarizations), independently from dynamic magnetization effects., Comment: 6 pages, 5 figures

Experimental phonon imaging in diamond anvils cell is demonstrated to be an adequate tool to extract the complete set of elastic constants of single-crystalline silicon up to the ZB$\rightarrow \beta-$Sn transition (10 GPa). Contrary to what was commonly admitted, we demonstrate that the development of the strain-energy density in terms of strains cannot be stopped, for silicon, after the terms containing the third-order elastic constants. Nonlinear elasticity, degree of anisotropy and pressure-induced mechanical stability of the cubic silicon structure are thus revisited and investigated in more detail.

Three regimes of granular avalanches in fluids are put in light depending on the Stokes number St which prescribes the relative importance of grain inertia and fluid viscous effects, and on the grain/fluid density ratio r. In gas (r >> 1 and St > 1, e.g., the dry case), the amplitude and time duration of avalanches do not depend on any fluid effect. In liquids (r ~ 1), for decreasing St, the amplitude decreases and the time duration increases, exploring an inertial regime and a viscous regime. These regimes are described by the analysis of the elementary motion of one grain.

We investigate the effects of lateral walls on the angle of movement and on the angle of repose of a granular pile. Our experimental results for beads immersed in water are similar to previous results obtained in air and to recent numerical simulations. All of these results, showing an increase of pile angles with a decreasing gap width, are explained by a model based on the redirection of stresses through the granular media. Two regimes are observed depending on the bead diameter. For large beads, the range of wall effects corresponds to a constant number of beads whereas it corresponds to a constant characteristic length for small beads as they aggregate via van der Waals forces.