## 6 NP Visualization during the PLAL Processes  
In the last years, part of the literature on the PLAL was dedicated to the NP formation mechanism during the different PLAL steps occurring before the NPs releasing in solution. The study of the PLAL processes coupled with the investigation of NP location and the mechanisms of release in solution is a very important goal for the control and the systematic production of NPs. The NPs before the release in solution are trapped inside the cavitation bubble following, of course, the bubble dynamics as discussed in the previous section. The visualization of the NPs inside the cavitation bubble during its evolution by the use of optical diagnostics is extremely difficult just because the bubble surrounds the NPs and light scattering is not so efficient when scatters are smaller than the incident wavelength. One of the most powerful techniques for obtaining information on the size and mass of particles inside the bubble is the small-angle X-ray scattering (SAXS), usually obtained by synchrotron-radiation [27]. A simpler way [10] to check qualitatively the location of the NPs in the bubble is to shoot a second laser pulse focused inside the bubble produced by the first laser pulse in order to produce a secondary plasma from the material ablated by the first pulse. This technique, that is double pulse laser ablation in liquid (DP-LAL), can be efficiently used as a diagnostic tool for understanding the distribution of NPs in the bubble volume. Finally, also the light-scattering-based technique [9; 28] has been used for the inspection of the materials inside the cavitation bubble during its evolution. In the following, a brief description of the techniques mentioned above is given. In Ref. [29], small-angle X-ray scattering was used during the cavitation bubble evolution to identify the species inside the bubble generated by laser ablation of a silver target in water with temporal and spatial resolution. Since the X-rays can penetrate the bubble interior, X-ray scattering can analyze the nanometer-sized particle distributions. Figure 9a shows a sketch of the employed experimental set-up, and Figure 9b, the main resultsobtained with this technique. The authors of Ref. [29] show in detail how, in the bubble volume, during the bubble evolution, two different groups of particle size distributions can be identified. One contains particles around 8-10 nm size and the other one contains bigger agglomerates around 40-60 nm size. Further research has been done by the same group, improving the temporal resolution of the acquisition [30] and the quantification as well as a survey of the crystallinity of NPs during the release in solution [31]. In Figure 10a, a sketch of the experimental set-up for the DP-LAL technique is shown [32; 33]. Using the double pulse technique as a diagnostic of the materials inside the cavitation bubble, the first laser pulse is employed for the PLAL, while the second laser pulse is fired collinearly to the first one and focused on the target at different interpulse delays between the two laser shots. By simultaneously acquiring the shadowgraph of the bubble and the images of the plasma generated by the second pulse, it is possible to visualize the second plasma at a different lifetime of the bubble. Depending on the position of the NPs inside the bubble, the second plasma is generated. In part on the target, in part on the materials that occupy the bubble. Figure 10b clearly shows this phenomenon, described in detail in Ref. [10], where it is possible to discriminate between both the NPs inside the bubble, located immediately below the expansion bubble front, and the materials ejected in solution after the bubble collapse. Figure 11a shows the sketch of the experimental set-up for the laser scattering technique. The continuum laser employed for scattering is perpendicular to the bubble axis propagation, and the images of the scattering are acquired during the cavitation bubble evolution.