Finite sample sizes increase the measured Brillouin scattering linewidths. This should not be mistaken for a genuine damping of sound. Using high-resolution interferometry, we show that in silica glass under ambient conditions and with visible light excitation, the additional width becomes observable for sample thicknesses below 0.2 mm. Similarly, light absorption reduces the scattering volume, producing a strong contribution to the width for excitation of bulk samples above their absorption edge. The resulting limitations are illustrated for UV Brillouin scattering from silica.

The dynamic structure factor of lithium-diborate glass has been measured at several values of the momentum transfer Q using high resolution inelastic X-ray scattering. Much attention has been devoted to the low-Q-range, below the observed Ioffe-Regel crossover q(IR) similar or equal to 2.1 nm(-1). We find that below q(IR), the linewidth of longitudinal acoustic waves increases with a high power of either Q, or of the frequency Omega, up to the crossover frequency Omega(IR) similar or equal to 9 meV that nearly coincides with the center of the boson peak. This new finding strongly supports the view that resonance and hybridization of acoustic waves with a distribution of rather local low frequency modes forming the boson peak is responsible for the end of acoustic branches in strong covalent glasses. Further, we present high resolution Brillouin light-scattering data obtained at much lower frequencies on the same sample. These clearly rule out a simple Omega(2)-dependence of the acoustic damping over the entire frequency range. (c) 2006 Elsevier B.V. All rights reserved.

High frequency sound is observed in lithium diborate glass, Li2O-2B(2)O(3), using Brillouin scattering of light and x rays. The sound attenuation exhibits a nontrivial dependence on the wave vector, with a remarkably rapid increase towards a Ioffe-Regel crossover as the frequency approaches the boson peak from below. An analysis of literature results reveals that the boson-peak frequency is closely related with a Ioffe-Regel limit for sound in many glasses. We conjecture that this relation, specific to glassy materials, might be rather common among them.

The phonon dynamics of wurtzite aluminum nitride contaminated by oxygen were investigated by employing the Raman back scattering, the Fourier Transform Infrared (FT-IR) reflectivity and absorption, and the Brillouin scattering techniques on unseeded polycrystalline samples of AIN built of single crystallites 1-5 mm in size. The six Raman active zone center optic modes were observed and identified. Throughout the oxygen contamination range (similar to 1, similar to 2, and similar to 6 at.%) of three samples investigated, the widths of the principal Raman modes were found to decrease with increasing the oxygen content in the single crystal. This behavior is interpreted as a change in the nature of the oxygen defect when the oxygen concentration exceeds 1s at.%. The FT-IR reflectivity spectrum exhibits two-mode behavior at low oxygen concentration, one-mode behavior tends to be dominant when the oxygen concentration increases, and only one-mode behavior can be observed at high oxygen concentration. These changes in the reststrahlen band with oxygen concentration support the hypothesis of a transition in the oxygen accommodation defect as the concentration of oxygen increases. The oxygen effects on the AIN optical parameters were investigated by calculating these optical parameters from the reflectivity data of single crystallites differing in their oxygen concentration. The FT-IR absorption measurements showed several absorption bands in the multiple-phonon region. A tentative interpretation is proposed in which these bands are considered to be due to oxygen impurity absorption and to a combination of several phonon branches at the Brillouin-zone boundaries. The absorption spectrum in the one-phonon region allowed us to obtain a reliable data on the phonon density of states function in bulk AIN. Lastly, three different configurations were used in Brillouin scattering. measurements to achieve a complete determination of the elastic stiffness constants of AIN. (c) 2006 Elsevier B.V. All rights reserved.