## 1 Introduction
The physical principle that stands behind this technique was initially argued, almost a hundred years ago, by the pioneering and independent works of [PERSON] and [PERSON] [10; 11] concerning the scattering of light by acoustic waves in liquids and transparent media. Extension to thin films and metallic or semiconductor solids (i.e., opaque to visible light) were achieved in the seventies thanks to both the advent of lasers and the construction of high-contrast Fabry-Perot interferometers [12; 13; 14]. [PERSON] developed an original type of these machines in the so-called \"tandem\" configuration during the seventies, while working in the research division of the RCA company in Zurich, but a few years later he established his own company (formerly named \"JRS Scientific Instruments\" and now \"The Table Stable\") [15], which launched into the market the multi-pass tandem Fabry-Perot interferometer (TFP). After more than thirty years from its commercialization, this kind of interferometer is still without alternatives on the market for the detection of either phonons or magnons from thin films and opaque solids. Several upgrades of the setup have been performed during the last two decades to achieve better performance in terms of contrast, resolution and signal-to-noise ratio. Let us now recall some relevant characteristics of the physical mechanism lying below the surf-BLS from acoustic waves. As stated in the Introduction, it is based on the coupling between the electric field of a monochromatic incoming light beam and the periodic variation of the dielectric constant of the medium associated with acoustic waves. The modulation of the dielectric constant of the medium, induced by the acoustic wave, can be described according to the following equation:  
\[\Delta\varepsilon_{\alpha\beta}=k_{\alpha\beta\gamma\delta}\;\mathrm{S}_{ \gamma\delta} \tag{1}\]where \(S_{\gamma\delta}\) is the strain field associated with the wave and \(k_{\alpha\beta\gamma\delta}\) is the elastooptic tensor. Following the derivation in Ref. [16], the scattered electric field \(E\)s at large distance from the scattering volume \(V\) is proportional to the following integral:  
\[\iiint v\ \Delta\varepsilon_{\alpha\beta}\ \mathbf{E}_{i}e^{-i(\mathbf{K}_{i}-\mathbf{K}_{ s})\cdot\mathbf{r}}dV \tag{2}\]  
where \(\mathbf{E}_{i}\) is the electric field of the incident light, \(\mathbf{K}i\) and K\(s\) are the wavevectors of the incident and scattered light. The above integral leads directly to the conservation of momentum, because for a given acoustic excitation of wavevector \(\mathbf{Q}\), \(\Delta\varepsilon_{\alpha\beta}\) is proportional to \(e^{-i\mathbf{Q}\times r}\) so that, if the interaction volume \(V\) is large with respect to the acoustic wavelength, the integral becomes a delta function \(\delta(\mathbf{K}_{i}-\mathbf{K}_{s}\mp\mathbf{Q})\) and this means that the exchanged wavevector must be conserved. As for the intensity of the inelastic scattering from a bulk acoustic wave in an isotropic medium at temperature \(T\), the scattering cross section (scattered power divided by the solid angle), for an unitary interaction volume and incident power, can be expressed as [17]:  
\[\frac{dP}{d\Omega}=\frac{k_{B}T\pi^{2}}{4\varrho v^{2}\lambda^{4}}\ |\mathbf{g}|^{2} \tag{3}\]  
where \(\varrho\) is the mass density, \(\lambda\) is the light wavelength and \(v\) the acoustic velocity. Interestingly, \(\mathbf{g}\) is a vector that is determined by the values of the contracted elastooptic coefficients \(k_{\alpha\beta}\)[18] according to:  
\[g_{\alpha}=\frac{1}{QE_{s}}\ \big{[}(k_{11}-k_{12})(e_{\alpha}\mathbf{Q}\times\mathbf{E} _{i}+Q_{\alpha}\mathbf{e}\times\mathbf{E}_{i})+2k_{12}\delta_{\alpha\beta}E_{\beta} \mathbf{Q}\times e\big{]} \tag{4}\]  
where \(e\) is the unit polarization vector of the acoustic wave. It results that the scattered intensity depends on the values of the elastooptic coefficients and on the polarization direction of the incoming and scattered light beams, as well as of the acoustic wave.