Measurement of the Elastic Properties of Solids by Brillouin Spectroscopy

Brillouin Light Scattering (BLS) is a non-contact measurement technique that exploits light scattering to probe the properties of ultrasonic waves, either bulk waves propagating in transparent solids or liquids, or surface acoustic waves (SAWs) propagating at the surface of homogeneous solids or of thin layers, either supported or free standing. In BLS the scattering geometry selects a specific wavevector and probes the ‘thermal noise’ at that wavevector, therefore performing a sampling of the dispersion relation of waves. This is done by illuminating the surface with a laser beam and examining the spectrum of the scattered light.. From the spectrum of the ‘inelastically’ scattered light and the scattering geometry, one can derive the dispersion relation for the ultrasonic waves, and then infer the elastic properties of the material. How this is done is the subject matter of this chapter. The light scattering nature of BLS measurements has three main consequences. First, mechanical contact with the sample is not needed: only optical access is required. Second, scattering occurs locally, in a volume of the order of tens of micrometers. Third, in BLS it is the acoustic wavelength that is determined by the experimental conditions. With visible light the explored acoustic wavelengths are sub-micrometric, meaning that with typical surface and bulk acoustic modes the probed frequencies range from a few GHz up to several tens of GHz. Such small wavelengths give a peculiar sensitivity to thin and ultra-thin. Brillouin scattering techniques also have drawbacks. First, thermally excited fluctuations have small amplitude, requiring time consuming measurements; second, the probed wavelengths are small and span a limited range. Therefore, Brillouin scattering techniques are the preferred technique for materials such as thin films and whenever the contactless nature of measurements makes BLS the only, or almost the only, available choice, as it happens in extreme conditions like thye diamond anvil cell. This chapter is a review of the application of BLS for the evaluation of the elastic properties of materials in the above two main areas—thin coatings and solids under extreme conditions. It should be mentioned that BLS is also actively exploited for the characterization of magnetic materials through the detection of magnons or spin waves. However, this is a different field that deserves a review by itself and will not be considered here