## 1 Introduction  
The problem of a detailed knowledge of the elastic constants of thin film materials has attracted much attention in recent years, because of the growing importance of single- and multi-layered structures in advanced applications such as in devices for information and communication technology (ICT). For instance, current filters and duplexers in mobile communication devices exploit either bulk or surface acoustic wave (BAW or SAW) resonators operating at several GHz [1; 2; 3; 4], whose design and optimization require a detailed knowledge of the elastic properties of the constituent layers. In fact, as illustrated in Figure 1, free-standing bulk acoustic wave resonators (FBAR) and solidly mounted resonators (SMR) consist of a stack of several materials, including the active resonator (usually a film of AlN, ScAlN or ZnO), metallic layers for electrical electrodes (Ti, Au, etc.) and other materials for temperature compensation (SiO\({}_{2}\)) or for Bragg mirrors (SiO\({}_{2}\) and W). The thicknesses of the above layers range from a few tens of nanometers up to a few microns. It follows that it is crucial to dispose of advanced techniques and methods to provide information about the elastic constants of the constituent layered materials and to image the spatial profile of the acoustic field, both inside and outside the resonator. In this paper, I will review the capability of the surface Brillouin light scattering technique (surf-BLS) to the above aims and in particular to successfully achieve a selective determination of the elastic constants of various types of films and multilayers. This technique is based on the inelastic scattering of photons by thermal phonons, naturally present in any material at room temperature, through the modulation of the optical constants of the medium, which is induced in transparent media via the elasto-optic effect [5; 6]. However, in the case of opaque media, either metals or semiconductors,the rippling of the free surface, caused by the presence of thermal phonons, gives a substantial contribution to the light scattering [7]. In fact, the periodic modulation of the optical constant (either in the interior or at the surface of a medium) associated to a propagating acoustic wave results in a moving grating so that Brillouin scattering can be understood in very simple terms as a Doppler shift of the diffracted light. From the quanto-mechanical point of view, however, one considers a photon-phonon collision in which the frequency and the momentum are conserved through phonon creation or annihilation, corresponding to either Stokes or anti-Stokes processes, respectively. It follows that in a typical Brillouin light scattering experiment, the probed phonons have wavelength comparable to that of light sand this implies that their frequencies are in the range between about 10 and 150 GHz. The cross section of the scattering process is relatively low, so that in order to extract the tiny inelastic component of light from the elastically scattered contribution, a high-resolution spectrometer is required. To this aim, the best combination of high resolution and good throughput is achieved using a multipass Fabry-Perot interferometer. Measurement of the whole set of constants is usually out of the reach of conventional static techniques, such as micromechanical test or nanoindentation, which can only give information about specific elastic moduli [8]. On the other hand, use of acoustic techniques based on surface acoustic waves (SAW) presents technical difficulties connected with the fabrication of the acoustical delay line. In addition, one has to consider that in the usual frequency range of hundreds of MHz, the large values of the acoustic wavelength, compared to the film thickness, require a careful consideration of the effect of the substrate on the propagation of the SAW [9]. These limitations can be overcome using surf-BLS, because it has specific advantages over conventional techniques.