Brillouin light scattering (BLS)

We constructed a unique High Resolution Brillouin Spectrometer with the highest precision on acoustic phonon frequencies and linewidth worldwide. It consists of a spherical interferometer in tandem with a planar one both controlled by laser light modulated at high frequency. This high resolution set-up enables one to detect very small sound velocity or elastic constant variations. Low sound wave attenuations can also be measured. The spectrometer is equipped with a confocal microscope that permits experiments on small size samples and µ-Brillouin cartography, with an improved spatial, lateral and depth resolution. For less demanding experiments we also have a Sandercock marketed Fabry-Perot tandem interferometer. We developed very recently a Brillouin spectrograph for fast data acquisition using imaging on a CCD.

Hyper-Raman spectroscopy (HRS)

Hyper-Raman scattering (HRS) is a non-linear process in which two incident photons scatter one photon after interaction with an excitation in the material. The main interest of this spectroscopy is that its selection rules are different from those of Raman scattering and infrared absorption. Although the theory was established 45 years ago, this spectroscopy develops very slowly, mainly owing to the poor efficiency of the non-linear process (about 106 weaker than the Raman one). The use of HRS really started 25 years ago as it provides a unique and very accurate way to measure soft polar vibrations in ferroelectric-type materials, in which HRS is very efficient. Nowadays, the advances of technology (High power pulse lasers, low noise CCD Camera, etc.) open the possibility to perform hyper-Raman in a much wider range of liquids and solids, including glasses. Our preliminary results obtained on a spectrometer in Sapporo (Japan) in 2000, motivated us to construct a HR setup at the LCVN. Our apparatus works since June 2004, with excellent performances. In oxide glasses, high quality spectra are obtained in less than 1 hour, which is very encouraging.

Sub-THz acoustic vibrations in Glasses

Involved researchers : M. Foret, B. Rufflé, B. Hehlen

Our project is to obtain very first direct evidences for the expected dramatic decrease of acoustic phonons mean free path of wavelength in the mesoscopic length scale domain or sub-THz frequencies region in some important model glasses. This strong decrease can lead rapidly to the end of acoustic plane waves at the Ioffe-Regel limit, which does provide an explanation for the universal plateau in the thermal conductivity of glasses around 10 K. The origin of this attenuation is currently debated. Several mechanisms have been invoked, such as scattering by elastic disorder [Schirmacher06] or resonance with quasi-local vibrations [Rufflé08]. In both cases, the explanation links to the boson peak, i.e. to an excess of low-frequency modes over the Debye expectation, another controversial feature of glasses. These unsettled questions clearly call for sound-absorption data in the crucial frequency region below 1 THz.
There are two ways to address the problem of phonon lifetimes : on the one hand spectroscopic experiments allow the determination of line widths ; on the other hand propagation of acoustic waves allows a direct determination of the mean free path. For this last approach in the sub terahertz range, a step was exceeded in the last two decades with the evidence that very short acoustic pulses could be produced and detected with femtosecond laser pulses and a pump-probe experiment giving birth to the field of “picoseconds acoustics”. Recently, it was demonstrated that superlattices could play the role of efficient generators and detectors of coherent acoustic waves at least up to 1 THz [Huynh06, Huynh08]. Our goal is to apply these very new concepts to the study of high frequency sound absorption in glasses. Analyzing time resolved Brillouin signals from a multilayer Al/SiO₂Si by means of picosecond optical pulses, allowed us recently to obtain for the first time accurate attenuation coefficients of SiO₂ at frequencies nearing 250 GHz [PRB2008]. This project is collaboration between LCVN/INSP/IEMN. A financial support of the ANR will be requested to the next round.