We report on high-resolution Brillouin scattering experiments performed under hydrostatic pressures ranging from 0 to 10 GPa in vitreous silica. The hypersound-attenuation coefficient exhibits a sharp maximum located at 2 GPa, which appears to coincide with the well-known minimum in sound velocity. Two main processes contribute to sound damping: the thermal relaxation of local defects and the anharmonic interactions with thermal vibrations. By analyzing the temperature dependence of the velocity and subtracting the changes produced by the above mechanisms, the unrelaxed velocity is found. It is constant at low temperatures and increases anomalously above a temperature onset. We show that the onset strongly depends on pressure, which is in agreement with a dynamical origin of the structural changes that produce the hardening.[1] [1]S. Ayrinhac, B. Rufflé, M. Foret, H. Tran, S. Clément, R. Vialla, R. Vacher, J. C. Chervin, P. Munsch, and A. Polian, “Dynamical origin of anomalous temperature hardening of elastic modulus in vitreous silica”, Phys. Rev. B 84, 024201 (2011)

Recently we proposed an alternative way of measuring hypersound attenuation in silica thin films using ultrafast acoustics [1-2]. This way, the attenuation of longitudinal acoustic phonons up to 300 GHz has been measured in vitreous silica [3]. Our method is based on the emission of a short acoustic pulse from the absorption of an ultrashort optical pulse in a thin metallic layer deposited on top of the sample. The high frequency content of the pulse is detected in the substrate after propagation in silica using another laser pulse. By comparing the signal obtained for various film thicknesses, we can extract sound attenuation in silica in the sub-terahertz range. Multiple interference effects in the multilayer are analyzed and fully taken into account[3]. Our accurate acoustic attenuation results are found to follow rather well a model combining thermally activated relaxations (TAR) and anharmonicity. In the present work, we first show how to reach higher and higher frequencies. For that it is needed to improve both acoustic emission and detection. Concerning emission, it is governed by the thin metallic layer deposited on top which converts the ultrashort optical pulse in an acoustic pulse. Usually Al is preferred due to its efficiency in light-sound conversion. We will also report the results obtained using the same experimental scheme but at low temperatures. On cooling, the TAR contribution is expected to first increase, and this should be rather easily observable. As shown, an important difficulty to overcome is the thermal heating induced with the laser technique. We will present different experimental evidences which attest that the sample is at the expected low tempetature and then some hypersound attenuation measurements.

We have performed new Brillouin scattering measurements of the velocity and attenuation of hypersonic waves as a function of pressure P in vitreous silica [1]. The hypersoundattenuation coefficient exhibits a sharp maximum located at 2 GPa which appears to coincide with the well-known minimum in sound velocity. The results are analyzed together with previous literature data. Internal friction measurements are described satisfactorily using a model including dissipative effects of anharmonicity and thermally activated relaxation of structural defects. From this model, the associated velocity changes were calculated, and subtracted from the data to obtain the bare velocities (the so-called unrelaxed velocities) as a function of T at different P. v∞ (T) is almost constant at low T, then increases linearly above a temperature onset T0 . We find evidence for an increase of T0 with pressure P [1]. This is in agreement with numerical simulations in which the observed hardening is related to changes of the ring conformations with T [2,3]. [1] S. Ayrinhac, B. Rufflé, M. Foret, H. Tran, S. Clément, R. Vialla, R. Vacher, J.-C. Chervin, P. Munsch, A. Polian, Phys. Rev. B84, 024201 (2011). [2] L. Huang and J. Kieffer, Phys. Rev. B69, 224203 (2004). [3] Y. Liang, C.~R. Miranda, and S. Scandolo, Phys. Rev. B75, 024205 (2007).

Sound attenuation in glasses exhibits a rich variety of phenomena which depend on the frequency and temperature. Typically above 10 K, the thermally activated relaxations of defects which can be modelled by two-well potentials are the major source of attenuation. At higher temperatures, the anharmonic interaction of sound waves with the thermal phonon bath becomes important. This initially varies with the square of the frequency, making it the dominant source of attenuation at high frequencies, typically above 10 GHz in the case of vitreous silica. How the attenuation evolves at still higher frequencies remains currently debated. Picosecond optical techniques now offer a possible approach to access sub-THz sound properties. We report recent accurate measurements of attenuation of longitudinal acoustic phonons up to frequencies near 300 GHz in vitreous silica. At very high frequencies, in the THz range, sound changes in nature, loosing its wavelike character. This Ioffe-Regel crossover nearly coincides with the excess of vibrational modes manifested by the boson peak.