## 4 New Perspectives from Micro-Focused Surf-BLS: Mapping the Spatial Profile of Acoustic Modes  
While the conventional surf-BLS technique, presented above, offers the possibility to measure the frequency of acoustic excitations with a well-defined wavenumber in a large frequency range (1-500 GHz), it has a poor lateral resolution, determined by the diameter of the laser spot, that is usually about 30-40 \(\upmu\)m. Moreover, the signal is proportional to the intensity of the acoustic field, but it has no phase sensitivity. This means that in the case of inhomogeneous samples, such as artificially defined phononic crystals (PC) [71] or acoustic resonators [1; 2; 3; 4], one cannot achieve the imaging of the spatial profile of the acoustic field. This limitation can be overcome using the recently developed micro-focused surf-BLS technique, whose schematic apparatus is drawn in Figure 14, that has been extensively used in the last decade to investigate the lateral localization of spin waves in ferromagnetic films and artificial magnonic crystals [72]. In fact, using a microscope objective with large numerical aperture, the laser light can be focused down to a diffraction-limited spot with a diameter below 300 nm and a submicrometric depth of focus. In addition, phase resolution can be achieved in the case of externally driven excitations, paving the way to the tridimensional mapping of magnetic or acoustic excitations by using surf-BLS as a scanning probe technique, in conjunction with an automated scanning stage for the sample. A coaxial viewing system based on a collimated LED light source (455 nm wavelength), a beam expander, and a CCD camera is used to obtain a direct visualization of the laser spot and of the sample region under investigation. Let's notice that, although micro-focusing permits to achieve spatial and phase resolution, the wavevector sensitivity is lost, because of both the small dimension of the focused spot and the very large collection angle. Therefore, micro-focused surf-BLS is well suited to analyze the acoustic field driven by an external signal (as in acoustic resonators), rather than to detect thermally activated phonons. The application to these aim is initiated and preliminary results have been just presented [73]. Further progress will be achieved in the near future, also because the operating frequencies of devices are expected to increase above a few GHz, so the availability of a technique operating in the frequency domain (rather than in the time domain as done up to now [74; 75]) can be advantageous.  
Figure 14: (**Left**) Scattering geometry of the conventional surf-BLS experiment. The wave vectors of the incident, reflected and scattered light are indicated, together with that of the acoustic surface phonon (in red). The illuminated area is a circle with a diameter of tens of microns (i.e., much larger than the wavelength of the detected phonons) so the wavevector conservation is guaranteed; (**Right**) Schematic drawing of the micro-focused apparatus. The light beams coming from either the laser or the auxiliary LED sources are represented by green and blue beams, respectively. The illuminated area is a spot of about 300 nm diameter and the collection angle is very large, so the conservation of momentum does not apply.