## 1 Introduction
In particular,  
* it does not require any external generation of acoustic waves (since it probes thermal acoustic phonons naturally present into the medium);
* it is a local probe, since the investigated portion of the sample is defined by the dimension of the focused laser spot and this permits to operate a scan of the elastic properties of inhomogeneous specimens;
* it is suited to analyze short wavelength and high-frequency phonons, up to several GHz, such as those of current and forthcoming ICT devices operating at microwave frequencies. The latter characteristic descends from the fact that, thanks to the wavevector conservation in the photon-phonon interaction, the wavelength of the revealed elastic waves is of the same order of magnitude of that of light. It is interesting to notice that most of the experimental investigations of layered structures, such as supported films and superlattices, have been carried out in the eighties and  
Figure 1: Schematic drawings of free-standing (**left**) and solidly mounted (**right**) bulk acoustic-wave resonators, named free-standing bulk acoustic wave resonators (FBAR) and solidly mounted resonators (SMR), respectively, that are currently used in filters and duplexers for mobile communication devices. early nineties, i.e., just after the availability of the high-contrast Tandem Fabry-Perot Interferometer. Later on, around the turning of the millennium, thanks to the growing importance of the rising fields of spintronics and magnonics, most of the research groups that had been active in the application of surf-BLS to acoustic phonons [such as the group of [PERSON] in Kaiserslautern (Germany), that of [PERSON] in Argonne (USA), and the groups of [PERSON] in Ferrara and mine in Perugia (Italy)] switched their main interest to the study of magnetic films and multilayers by detection of spin waves. In some sense, this caused a slowing down in the transfer of expertise and in the education of young researchers in the application of surf-BLS to the elastic characterization of layered samples. The result is that, according to my experience, there is currently a gap between the needs of the industrial manufacturers of advanced acoustic components (such as filters and duplexers) for mobile telecommunication devices and the availability of experts in the application of surf-BLS to their needs. The main aim of this paper is just to provide a contribution to fill this gap. The structure of the paper is as follows. In Section 2, I recall the physical mechanism that is at the origin of the experiments, focusing on the interaction between light and acoustic phonons and discussing in details the quantitative relations that hold in the backscattering geometry that is the most popular for the study of opaque media. Section 3 is devoted to the presentation of the situations that may occur when applying the surf-BLS technique to the elastic characterization of different types of films. In particular, the experimental methodology that apply to specific case studies are discussed together with the discussion of the amount of information that can be extracted, depending on the thickness of the film and on its transparency. A short presentation of the limitations of the technique and a generalization to the case of multilayered samples is also provided. In the last Section, the recent development of the micro-focused, scanning, version of the surf-BLS technique are shortly discussed, addressing the new opportunities that are at hands for the imaging of the spatial profile of the acoustic field in acoustic resonators and phononic crystals, with a deeply submicrometric resolution. Brillouin Scattering from Thin Films and Opaque Media: Historical Background and Interaction Mechanism  
As introduced in the previous paragraph, the surf-BLS technique is presently the most powerful tool to access the elastic characteristics of thin films and layered structures in the GHz range of is frequencies that are of growing importance in nowadays ICT devices.