The vibrational modes of glasses

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.