## 7 Conclusions  
Several optical techniques for the diagnostics of the PLAL process have been presented and discussed. A summary of the advantages and shortcomings is reported in Table 1.  
The use of OES allows the determination of the plasma parameters that may be extremely useful for investigating the source of matter of the all PLAL processes. Moreover, OES can be coupled with imaging in order to obtain a temporally and spatially resolved map of the plasma parameters. On the other hand, shadowgraph imaging permits the investigation of the cavitation bubble dynamics. The cavitation stage has focused the attention of many researchers because it is related with the production of the final NP form and with the releasing of the nanoparticle in solution. Shadowgraph imaging enables the determination of the bubble shape and radius. These are crucial pieces of information for investigating the cavitation \begin{table}
\begin{tabular}{c c c} \hline \hline _Technique_ & _Advantages_ & _Shortcomings_ \\ \hline \multirow{2}{*}{OES} & Determination of plasma parameters (T, & No spatial resolution, no information on \\  & Ne) with relatively high accuracy & plasma composition \\ \hline \multirow{2}{*}{Imaging} & Investigation of plasma expansion & Not possible to determine \\  & features and shockwave velocity & plasma parameters (T, Ne) \\ \hline \multirow{2}{*}{Spectrally resolved imaging} & Maps of plasma parameters & Low accuracy from the quantitative point of view \\ \hline \multicolumn{3}{c}{**CAVITATION BUBBLE**} \\ \hline _Technique_ & _Advantages_ & _Disadvantages_ \\ \hline \multirow{2}{*}{Shadowgraphy} & Determination of bubble radius and in & No linear relation with the density of the crossed \\  & turn of cavitation temporal evolution & medium \\ \hline \multicolumn{3}{c}{**NANOPARTICLE VISUALIZATION**} \\ \hline _Technique_ & _Advantages_ & _Disadvantages_ \\ \hline \multirow{2}{*}{Laser scattering} & Easy and cheap to set-up & Low sensitivity and selectivity \\ \hline \multirow{2}{*}{ SAXS} & Excellent selectivity and sensitivity & Need for synchrotron radiation. Limited spatial \\  & & and temporal resolution \\ \hline \multicolumn{3}{c}{Double pulse} & Good spatial resolution & Low selectivity \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Summary of the advantages and shortcomings of the various techniques applied in the different stages of the PLAL process. Figure 12: (**a**) Sketch of the experimental set-up for laser-scattering technique. (**b**) Laser-scattering images superposed on the shadowgraph images at different delays after the pump pulse. The probe laser is (upper) horizontally and (lower) vertically to-paper polarized, as indicated by the arrow and cross mark. (Target = Ag film, pump laser pulse width = 5 ns, pump laser energy = 3.7 mJ, pump laser wavelength = 532 nm, probe laser pulse width = 0.8 ns, probe laser energy = 0.1 mJ, probe laser wavelength = 1064 nm,) (Reproduced from Ref. [35] with permission from the Royal Society of Chemistry). thermodynamics and the rate of the expansion and compression, as the radius and in turn the volume can be related with the thermodynamic parameters with different real gas-state equations. Finally, the visualization of the produced nanostructure during the PLAL process can be investigated with scattering techniques or double pulse techniques. Among the scattering techniques, SAXS provides the best information, as light scattering has poor efficacy with scatters smaller than the employed wavelength. The double pulse technique can be a good option for a qualitative investigation of the position of the material in the bubble as well as for visualizing the released NPs. [PERSON] and [PERSON] have equally contributed to conceptualization, methodology, formal analysis, writing--original draft preparation and writing--review and editing of the paper. All authors have read and agreed to the published version of the manuscript. This research received no external funding. Not applicable.