Projet ANR-11-BS04-0008 – GlassPhon : Phonons sub-terahertz dans les verres
Programme : Blanc - SIMI 4 - Physique (Blanc SIMI 4) 2011
Année de financement : 2011
Coordinateur du projet : Madame Marie FORET (CENTRE NATIONAL DE LA RECHERCHE SCIENTIFIQUE - DELEGATION REGIONALE LANGUEDOC-ROUSSILLON)
Partenaires : Institut des Nanosciences de Paris; Laboratoire de Chimie de Coordination; Institut d'Électronique, de Microélectronique et de Nanotechnologie
Montant : 229360.0 euros
Date de début : 2011
Durée : 36 mois
Résumé : The main purpose of our project is to use the advent of reliable ultrafast optical techniques to explore acoustic modes in glasses in the difficult but crucial frequency region between around 0.1 and 1 THz. Brillouin scattering of light allows to measure velocity and absorption of sound up to frequencies around 100 GHz and coherent inelastic x-ray scattering (IXS) is limited to frequencies larger than  1 THz. A rather universal property of glasses is that a large excess of modes exists, with a maximum density of states near 1 THz. On the other hand, the frequency between 0.1 and 1 THz corresponds to acoustic wavelengths at the mesoscopic length scale where specific structural and vibrational inhomogeneities are expected in glasses. It is thus of crucial importance to investigate acoustic wave propagation in the above frequency range. It has been recently established, both by the advent of IXS experiments and of numerical simulations that acoustic modes loose their plane-wave character at frequencies larger than a limit \Omega_IR defined by the Ioffe-Regel criterion. This threshold must be the consequence of a strong damping mechanism, dominant in the 0.1 to  1 THz range. The onset of this effect, expected in this frequency range, has never been observed. We want to investigate this effect in several protypical glasses.
In a first task, we plan to extend the preliminary picosecond optical techniques (POT) measurements in vitreous silica up to 400 GHz, by using silica films on a diamond substrate and a probe wavelength in the UV range. We will also perform measurements at low temperature where the strong damping effect is expected to dominate over a large frequency range. In the second task, we will use GaAs/AlAs super-lattices as efficient generators and detectors of coherent acoustic waves. Such devices have been shown to be efficient at least up to 1 THz. Experiments will also be performed at low temperatures. The third task is devoted to the challenging study of transverse modes by the two above approaches, supplemented by high resolution Brillouin light scattering.
On the basis of these measurements, we first expect to establish the existence of a strong damping process in the sub-terahertz regime in vitreous silica as well as its temperature dependence. We want to check the universality of the strong damping process by investigating glasses of different chemical nature, as this effect is expected to be related to different structural organisation at the mesoscopic scale. We are confident that understanding the elastic properties of glasses at such mesoscopic length scale would be a decisive step towards a comprehensive picture of the intriguing vibrational properties of amorphous materials. Finally, we think that a better knowledge of the attenuation of high frequency acoustic waves has an interest in applied physics, as applications reach higher and higher frequencies.