Читать книгу Encyclopedia of Glass Science, Technology, History, and Culture - Группа авторов - Страница 198

Brillouin spectroscopy is an optical technique used to investigate the elastic properties and acoustic velocities in glasses and melts under a variety of temperature and pressure conditions (cf. [21]). The method relies on inelastic scattering of monochromatic incident photons by thermal acoustic phonon vibrations in the sample. Whereas Raman spectroscopy investigates inelastic scattering between ~5 and 3500 cm⁻¹ from an exciting laser, Brillouin spectroscopy measures the scattered light within 10⁻² to <10 cm⁻¹ (usually ±1–2 cm⁻¹) of the laser line with a resolution of 10⁻³ cm⁻¹. Measurements can be made with two different kinds of sample geometries. With the so‐called platelet geometry, the incident and scattered beams make the same angle θ with the normal to the in and out surfaces. One then derives the sound velocity from the relation

(5)

where v_s,p is the sound velocity; λ and c the wavelength and velocity of light, respectively; Δσ the observed Brillouin shift; and θ the angle between incident and the scattered light. Experiments made with the backscattering geometry are simpler to perform as they require only a polished surface, but the index of refraction then needs to be known independently to determine the elastic properties.

The spectra consist of peak doublets on either side of the laser line frequency. The position, intensity, full width at half maximum (FWHM), and polarization of the peaks give information on the acoustic velocities and viscoelastic properties, coupling coefficients (between fluctuations and the electromagnetic field), the lifetime of the interactions and the anisotropy of the interaction, respectively. Thus, one can use changes in acoustic velocities, for example, to infer the occurrence of polyamorphism/polymorphism at high pressures in glasses such as SiO₂ (e.g. [22]). Figure 10 shows a number of spectra for an anorthite composition (CaAl₂Si₂O₈) glass at various pressures. Note the structural change between 5.4 and 7.2 GPa (*indicates peaks due to the pressure transmitting medium) indicated by the shift in the peak at ~ ±40 GHz. The elastic line region is the exciting laser line and notch filter cutoff.

Figure 10 Evolution of Brillouin spectra with the pressures from which an anorthite glass (CaAl₂Si₂O₈) was quenched.