## 1 Introduction
### Opaque Films: Rippling of the Surface, Guided Modes and Relevant Thresholds
A relatively easier situation may occur if the total thickness of the superlattice film becomes much larger than the light wavelength, so that the number of Sezawa modes is very large and the spectrum becomes almost continuum, as it would happen at the surface of a semi-infinite medium. In this case, as illustrated in Figure 11, the projection of the vertical acoustic displacement at the free surface contains the discrete peak of the Rayleigh surface wave, followed by a continuum of modes. Interestingly, in this continuum spectrum one can identify a dip, labeled Longitudinal Threshold (LT), occurring in correspondence of the frequency of the longitudinal acoustic wave propagating parallel to the free surface (i.e., where one would have observed the LM peak in a transparent film). The experimental counterpart of this calculation is in Figure 12, where the spectrum relative to a 5 \(\upmu\)m thick AgNi superlattice film is shown [66]. It is clear that, in addition to the RW peak, one can see the dip corresponding to the LT and directly derive unambiguously the value of the longitudinal elastic constant \(C_{11}=\rho V_{LT}^{2}\). The same methodology, based on the detection of the discrete RW peak and the dip associated to the LT has been also used to selectively obtain the elastic constants \(C_{11}\) and \(C_{44}\) in polycrystalline Ta/Al superlattices with hexagonal elastic symmetry [67]. Finally, also for opaque semiconductors with a cubic elastic symmetry, such as epitaxial InGaAs films grown on GaAs analyzed in Ref. [68], the elastic constants have been obtained looking at both the RW and the LT. In addition, also the edge of the continuum shoulder (transverse threshold, TT) that is put in evidence in Figure 13, was measured. More recently, it has been shown that, taking advantage of the RW peak, the TT edge and the dip of the LT, a simple and robust fitting procedure has been presented for determining the three independent elastic constants of a cubic thick film from surf-BLS, using InAs0.91 Sb0.09 as the case study [69]. To summarize the discussion of this Section about the potentiality of the technique, we provide in Table 1 a schematic presentation of the experimental methodology that is suitable to gain information about the elastic constants of thin films, considering their thickness and their transparency. However,  
Figure 10: Relative changes of the phase velocity of the Rayleigh and first three Sezawa modes produced by a 1% increase of each elastic constant vs. the ratio between film thickness h and phonon wavelength. Calculations refer to an Ag/Ni superlattice film supported by a glass substrate. Reprinted with permission from [62], AIP Publishing, 1992.  
before concluding this section, we want also to put in evidence a few weak aspects of surf-BLS. The first one is the limited accuracy in the determination of the phase velocities and of the phonon lifetime (attenuation), which are derived from the frequency position and the line width of the Brillouin peaks in the spectra, respectively. The accuracy in the determination of these quantities is limited to be above about 0.5% in the best cases, due to the intrinsic limitations in the frequency and wavevector resolution. It follows that the derived elastic constants are affected by uncertainty larger that about 1%. Another relatively weak point is the rather long acquisition times of each spectrum, that may range from a few tens of minutes to several hours (in the worst case of shear horizontal (Love) modes). Finally, the unavoidable heating of the sample region by the focused laser light should be taken into account. In fact, typical incident powers are between 100 and 300 mW for conventional measurements and between 1 and 10 mW for micro-focused ones. This causes a temperature increase ranging from a few degrees to a few tens of degrees, depending on the specific incident power and on the dissipation characteristics of the specimen.