JOURNAL OF RAMAN SPECTROSCOPY J. Raman Spectrosc. 2003; 34: 633–637 Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/jrs.1031 Brillouin scattering at high pressure: an overview Alain Polian ∗ Physique des Milieux Condens ´ es, CNRS–UMR 7602, Universit ´ e Pierre et Marie Curie, B77, 4 Place Jussieu, 75252 Paris Cedex 05, France Received 21 March 2003; Accepted 8 April 2003 Brillouin scattering allows the determination of acoustic velocities and adiabatic elastic moduli in matter. These data are crucial in many areas of science, such as fundamental physics, geosciences and technology, especially when measured as a function of the density (pressure). In this paper, we present a review of the work performed on Brillouin scattering under high pressure in diamond anvil cells, emphasizing the most recent results. Copyright  2003 John Wiley & Sons, Ltd. KEYWORDS: Brillouin scattering—High pressure—High temperature—Elastic properties INTRODUCTION Sound waves propagate in matter, independently of its struc- ture. Brillouin scattering consists in the inelastic scattering of light by the sound waves, or thermal excitation in a material, in particular acoustic phonons in a crystal. The energy of light is therefore modified, increased in the case of the anni- hilation of excitation, or decreased in the case of creation of excitation. The measurement of this energy difference gives information on the energy of the phonons and therefore on the interatomic potentials of the material. The theory of Brillouin scattering has been extensively developed in the literature, 1–4 so only the main ideas will be covered here. In the interaction process between a photon and thermal excitation, the energy and momentum are conserved: ¯ h Dš¯ hω i  ω s  1 E q Dš E k i  E k s  2 where ω and k are the wavenumber and the wavevector of the photon, the subscripts i and s refer to the incident and scattered beams and  and q are the wavenumber and the wavevector of the phonon, respectively, the plus sign corresponds to the creation of a phonon (Stokes) and the minus sign to annihilation (anti-Stokes). It should be noted that the equations of conservation hold for both Brillouin scattering and Raman scattering because the physical processes are identical. From the experimental point of view, the differences are large, coming from the difference in the energies involved in the processes: the wavenumbers of Ł Correspondence to: Alain Polian, Physique des Milieux Condens´ es, CNRS – UMR 7602, Universit´ e Pierre et Marie Curie, B77, 4 Place Jussieu, 75252 Paris Cedex 05, France. E-mail: alain.polian@pmc.jussieu.fr the optical phonons giving rise to Raman scattering are of the order of 10 – 1000 cm 1 (1.2 – 120 meV), whereas the Brillouin wavenumbers are of the order of 0.1–6 cm 1 . Because of this proximity with the elastically scattered Rayleigh line, apparatus of much higher resolution has to be used to discriminate the Brillouin scattered light compared with Raman scattering where a grating spectrometer is well suited. Such a high-resolution apparatus is the Fabry-P´ erot (FP) interferometer. Recent advances in Fabry-P´ erot spectroscopy have been reviewed 5 and will be described here only briefly. A Fabry-P´ erot system consists basically of two partially transmitting plane mirrors, separated by a distance d.A parallel beam of light is incident along the mirror axis. Owing to interferences between the two parallel mirrors, the light is transmitted only when nd D m  2 3 where n is the refractive index of the medium between the two mirrors,  is the wavelength of the light and m is an integer. Therefore, the wavelength of the transmitted light is selected by varying the optical path nd of the light, by changing either the distance between the mirrors or the refractive index of the inter-mirror medium. Two parameters describe the quality of an FP system: the first is the contrast (C), defined as the ratio of the maximum intensity transmission to the minimum intensity transmission, and this quantifies the ability of the set-up to measure signals that are small in comparison with the Rayleigh scattering. The second is the finesse (F), defined as the ratio of the distance between two successive transmission peaks to the full width at half-maximum of a transmission peak, and provides the capability of measuring peaks close to the Rayleigh peak. The most important parameter is the contrast. For a good quality FP system, one typically obtains Copyright  2003 John Wiley & Sons, Ltd.