Marie Foret

The vibrational modes of glasses

Abstract Our recent experimental observations on the harmonic vibrational modes of glasses are presented. Emphasis is placed on normal and densified silica. These results are discussed within the broader current knowledge, including thermal properties and other spectroscopic data that are critically assessed. We find that propagating acoustic modes enter a regime of strong scattering as their wavelength is reduced, and that this leads to an Ioffe–Regel crossover at frequencies of the order of the terahertz, corresponding to wavelengths of several nanometers. At similar frequencies, an excess in the density of states of optical modes, generally called the Boson peak, is observed. Hyper-Raman spectroscopy on these modes clearly shows that in silica they are due to the rocking of small groups of tetrahedra. These findings provide unique and unexpected information on the structure of glasses at the extended length scale, about which so little is known otherwise. The strong elastic inhomogeneity found at this scale might be decisive in determining glass properties, and even stability, and this will justify further studies.

Anharmonic versus relaxational sound damping in glasses. II. Vitreous silica

The temperature dependence of the frequency dispersion in the sound velocity and damping of vitreous silica is reanalyzed. Thermally activated relaxation accounts for the sound attenuation observed above 10 K at sonic and ultrasonic frequencies. Its extrapolation to the hypersonic regime reveals that the anharmonic coupling to the thermal bath becomes important in Brillouin-scattering measurements. At 35 GHz and room temperature, the damping due to this anharmonicity is found to be nearly twice that produced by thermally activated relaxation. The analysis also reveals a sizeable velocity increase with temperature which is not related with sound dispersion. A possible explanation is that silica experiences a gradual structural change that already starts well below room temperature.

Hypersound damping in vitreous silica measured by picosecond acoustics

The attenuation of longitudinal acoustic phonons up to frequencies nearing

Scaling the temperature-dependent boson peak of vitreous silica with the high-frequency bulk modulus derived from Brillouin scattering data.

250\phantom{\rule{0.3em}{0ex}}\mathrm{GHz}

Erbium emission properties in nanostructured fibers

is measured in vitreous silica with a picosecond optical technique. By taking advantage of interferences on the probe beam, difficulties encountered in early pioneering experiments are alleviated. Sound damping at

Observation of the onset of strong scattering on high frequency acoustic phonons in densified silica glass.

250\phantom{\rule{0.3em}{0ex}}\mathrm{GHz}

Small-angle neutron scattering of aerogels: simulations and experiments

and room temperature is consistent with relaxation dominated by anharmonic interactions with the thermal bath, extending optical Brillouin scattering data. Our result is at variance with claims of a recent deep-UV experiment which reported a rapid damping increase beyond

Vitreous silica distends in helium gas: acoustic versus static compressibilities.

100\phantom{\rule{0.3em}{0ex}}\mathrm{GHz}

Anharmonic versus relaxational sound damping in glasses. I. Brillouin scattering from densified silica

. A comprehensive picture of the frequency dependence of sound attenuation in

Colloidal vs. polymeric aerogels: structure and vibrational modes

v\text{\ensuremath{-}}\mathrm{Si}{\mathrm{O}}_{2}