## 5 Shadowgraph Technique for the Cavitation Bubble Investigation  
The laser-induced cavitation bubble dynamics can be studied by employing a high temporally resolved shadowgraph technique [9; 19; 20; 21]. In Figure 6, a typical sketch for the shadowgraph technique is shown. The bubble is generated by the fast energy exchanges between the plasma and the liquid and its evolution strictly depends on laser pulse characteristics as wavelength, output energy and focusing condition, pulse duration, external pressure [11; 22]. As an example, at atmospheric external pressure, the ns laser-induced bubble duration is usually in the order of hundreds of microseconds, that is orders of magnitude longer than the plasma duration itself [23]. The bubble dynamics consists of an early expansion stage, a maximum expansion, reached when the vapor pressure inside the bubble equals the surrounding water pressure, and a further shrinking stage where the bubble collapses. After the first collapse, a bubble rebound can occur with a higher order bubble formation, depending on the initial laser energy [24]. When higher order bubbles are induced, the expansion/compression stages resemble a damping oscillator, until all energy of the cavitation bubble is dissipated into the surrounding liquid. The evolution of the radius of a cavitation bubble can be experimentally determined from the shadowgraph images, as reported in Figure 6b where Ry is employed to determine the bubble radius, Rx is employed to determine the bubble front expansion, z is the shadowgraph detection direction and x is the laser direction. The determination of the radii  
Figure 5: Temperature maps of LIP induced on Ti target immersed in water (**left**) and the corresponding axial temperature profiles at radial distance = 0 \(\upmu\)m along the expansion axis of the plasma (**right**) at delay times from the laser pulse (**a**) 60 ns, (**b**) 240 ns and (**c**) 900 ns, respectively. (Laser pulse width = 6 ns, laser energy = 270 mJ, laser wavelength = 532 nm, laser frequency = 10 Hz, crater size = 1 \(\pm\) 0.2 mm, ICCD gate width = 20 \(\upmu\)s, 100 accumulations for each image). can be directly employed for the determination of the vapor pressure inside the bubble during the bubble dynamics. From the experimental point of view, Figure 7 shows the temporal evolution of a laser-induced bubble on Ag target immersed in water. Figure 8 shows the experimental bubble radius determined from the shadowgraph images reported in Figure 7.  
Figure 8: Percentages of ejected AgNPs as a function of time after laser pulse and the corresponding bubble radius evolution. (Laser pulse width = 6 ns, laser energy = 50 mJ, laser wavelength = 532 nm, laser frequency = 2 Hz, crater sizes = 1 \(\pm\) 0.2 mm, ICCD gate width = 20 μs, estimated initial AgNPs number = 2e10). Figure 6: (**a**) Sketch of the experimental set-up for temporally resolved shadowgraph images. (**b**) Sketch of cavitation bubble where Ry is employed to determine the bubble radius, Rx is employed to determine the bubble front expansion. (The z and x axes are the shadowgraph and laser direction, respectively). Figure 7: Time-resolved shadowgraph images of the laser-induced bubble on Ag target immersed in water. (Laser pulse width = 6 ns, laser energy = 50 mJ, laser wavelength = 532 nm, laser frequency = 2 Hz, crater sizes = 1 \(\pm\) 0.2 mm, ICCD gate width = 20 μs, 5 accumulations for each image). As a matter of fact, by determining the bubble radius at each observation time, the estimation of pressure and of the temperature inside the bubble with the hardcore Van der Waals model can be performed [8; 9]. In Ref. [25], the Rayleigh-Plesset equation was employed starting from the experimental parameters.