# Recent Advances in Laser-Induced Surface Damage of KH\({}_{2}\)Po\({}_{4}\) [PERSON]
## 4 Simulation and Experimental Exploration of Laser-Induced Damage Behavior in the Later Stage
### Exploration of Transient Behavior at the Initiation of Damage  
As shown in Section 3 of this review, during the energy deposition, the plasma gradually formed and further promoted the energy deposition. The deposition energy in the damage area provides necessary conditions for subsequent plasma expansion. Then, the pressure will appear. Actually, the correspondingly increased of pressure is the fluid dynamic effect of the energy deposition process [83]. In addition, the process from the initial energy deposition stage to the final damage crater formation is quite varied under the subsequent irradiation of the ultrashort or short pulse laser [84]. In general, the plasma formed inside the material will eject with the deposited energy. [PERSON] et al. [85] obtained plasma emission images with different pulse energies and pulse durations by using the lateral imaging system. It can be found that the energy deposition inside the dielectric material will be more significant under the irradiation of high-intensity ultra-short pulse laser. In addition, the diffusion range of the energy deposited inside is small, which will result in a high concentration of regional temperature. The plasma with all the deposited energy will eject from a small area of material rapidly. It can be seen in Figure 16a that the emitted plasma carries all the energy, and the temperature on the body of the material drops suddenly without any thermal conduction, which  
Figure 15: Formation of crack-free surface in the processing by circular edge cutting tool [74]. also visually reflects the phenomenon that the plasma will carry energy and eject from the interior of the material. The investigation of the shock wave generated by the instantaneous expansion and the emission of plasma have been described in reference [86]. The plasma will expand rapidly after absorbing energy, i.e., energy deposition, as shown in Figure 16b. Due to the severe temperature difference, the outward released plasma will induce the stress, which also contributes to the generation of shock waves. Combined with the images detected by TRPP technology in Figure 16, it can be reasonable to infer that that the shockwave in KDP will be generated by mechanical expansion rather than thermal expansion under the irradiation of ultra-short pulse laser [86]. In contrast, the irradiation of the short pulse laser ensures the thermal conduction. Unlike the sudden emission of plasma under the irradiation of ultra-short pulse laser, the emission process of plasma under the irradiation of short pulse laser involves the parameters such as temperature and stress. At the end of the plasma formation process, the temperature and pressure in this region are significantly higher than that of the surroundings. When the heat transferred from the center of the laser beam to the surrounding area gradually, a non-uniform temperature field will form inside the crystal material, causing a temperature gradient inside the material [87]. The existence of temperature gradient inside the crystal will cause compressive stress in the material. The resulting compressive stress also makes the thermal expansion of the irradiation area on the upper side of the absorption layer larger than the expansion on the lower side. With the increase of the temperature in the irradiation area, the stress state in the material will also increase, which will result in the stress gradients. The resulting pressure gradients will lead to the formation of a blast wave that propagates into the background gas. It is worth noting that the pressure gradient in the vicinity of the precursor defect will be larger due to the large amount of energy deposition. Figure 16: The shockwave and plasma emission images of dielectric. (**a**) Plasma emission and the corresponding intensity cross sections along the z-axis. **a1**, **a2**, and **a3** obtained for pulses of energies of 9.6 \(\upmu\)J and pulse durations of 100 fs, 250 fs, and 550 fs, respectively.