Brillouin Scattering and its Application in Geosciences

Brillouin spectroscopy is an optical technique that allows one to determine the directional dependence of acoustic velocities in minerals and materials subject to a wide range of environmental conditions. It is based on the inelastic scattering of light by spontaneous collective motions of particles in a material in the frequency range between 10−2 to 10 GHz. Brillouin spectroscopy is used to determine acoustic velocities and elastic properties of a number of crystalline solids, glasses, and liquids. It is most commonly performed on transparent single crystals where the complete elastic tensor of the sample material can be derived. However, Brillouin spectra can be also measured from opaque materials, from which partial or complete information on the elastic tensor can be determined. It is a very flexible technique with many possible areas of application in research disciplines from condensed matter physics to biophysics to materials sciences to geophysics. Brillouin scattering can be performed on very small samples and it can be easily combined with the diamond anvil cell and carried out at high pressures and temperatures (see reviews by Grimsditch and Polian 1989 and Eremets 1996). This makes this technique the method of choice to study the elastic properties of deep Earth materials, relevant to construct a mineralogical model of the interior of our planet that is consistent with the constraints from seismology. Several of the candidate minerals of the Earth’s interior are not stable at ambient conditions, and only recently has there been substantial progress in their synthesis. Unfortunately, those deep earth minerals that can be metastably preserved at ambient pressure and temperature are only available as single crystals with sizes of the order of several tens of microns at most. However, crystals of this size are large enough for Brillouin scattering to be performed. In addition, more sophisticated methods …