## 1 Introduction
### Opaque Films: Rippling of the Surface, Guided Modes and Relevant Thresholds  
The surf-BLS technique can be used also to determine the elastic constants of metallic films, where the penetration depth is only a few nanometers, so that the ripple mechanism is the dominant one while the elastooptic coupling is usually negligible. It is, therefore, recommendable to use p-polarized incoming light and relatively large angles of incidence to maximize the cross section [31]. The first investigations of opaque films were concerned with Au films of thickness from ten to hundreds of nanometers, supported by glass [48] where the Rayleigh and Sezawa guided modes were detected and the elastic constants estimated from a best fit procedure of their dispersion curves. In a successive study, the surf-BLS cross section of Au films on Si [49] was also analyzed, showing that the ripple mechanism gives the main contribution to the scattering process, even if the elastooptic contribution is not insignificant. Further studies of metallic films dealt with Al/NaCl [50] (where the authors were unable to fit our experimental results to obtain the elastic constants independently, since different combinations of the Cij lead to similar spectra) and with Mo [51] or Ni [52] films. More recently, other investigation on the limits and accuracy of the estimation of the constants have been proposed, with reference also the uncertainty connected with the film thickness [38; 53; 54]. Particularly relevant, during the late eighties and the early nineties, were the investigations of metallic superlattices, i.e., multilayers with many repetitions of elemental nanometric bilayers, where anomalous values of the elastic moduli were reported by conventional methods. Among the systems successfully investigated we recall here periodic superlattices: Mo/Ta [55], Cu/Nb [56] (quasi-periodic [57]), Ag/Pd [58], Fe/Pd [59], Co./Cu [60]. Co./Au [61], Ag/Ni [62], FeNi/Cu and FeNi/Nb [63] and Ta/Al [64] (quasi-periodic [65]). In all these systems, the overall film thickness was typically hundreds of nanometers, while the artificial periodicity ranged from a few nanometers to a few tens of nanometers. Therefore, the superlattice was modeled as an effective medium with hexagonal symmetry and five effective elastic constants, whose dependence on the artificial periodicity was investigated. Some of these constants, such as the shear constant \(C_{44}\), was found to be appreciably dependent of the artificial periodicity, because of strong interface effects. The analysis was generally based on the analysis of the dispersion curves of Rayleigh and Sezawa modes, as shown in Figure 9 for the case of Ag/Ni superlattices. Four of the five constants elastic constants (\(C_{11}\), \(C_{13}\), \(C_{33}\) and \(C_{44}\)) could be obtained by a best fit procedure of the measured frequencies on the calculated curves, while \(C_{66}\) was not accessible since it only affects Love modes, not detected in the case of metallic samples. In Figure 10 it is shown the effect of the different constants on the different modes, so one can see for instance that the shear constant \(C_{44}\) gives the main contribution to the velocity of the Rayleigh  
Figure 9: **(Left)** Brillouin spectrum taken on an Ag/Ni superlattice, 500 nm thick, having a periodicity of 2 nm, supported by a glass substrate. The Rayleigh and four Sezawa modes are seen. The incidence angle is 67\({}^{\circ}\), which corresponds to a value of h/\(\Lambda\) = 1.8; **(Right)** Experimental (circles) and calculated (solid line) velocity dispersion of the Rayleigh and Sezawa modes for the same sample. Adapted with permission from [62], AIP Publishing, 1992.  
mode at large film thickness, while its contribution to high-order modes is rather limited. One can also see that the contribution of \(C_{11}\) and \(C_{33}\) to the various modes is similar, so that it is not straightforward to disentangle these two constants from each other in the fitting procedure.