# 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
### Morphological Characteristics of Laser-Induced Damage in the Later Stage  
Based on the exploration of the transient behavior at the initiation of damage, the main damage morphology under different pulses can be shown in Figure 20.  
The laser energy deposited inside the crystal material can be transferred to the surrounding material area through thermal conduction under the irradiation of short laser. According to the analysis of the thermal conduction in Section 4.1., there will be stress near the erosion pit. Actually, the brittle crystal materials have strong fracture toughness against compressive stress, but much smaller against tensile stress. As a result, the cracks will occur around the molten pit if the tensile stress exceeds the fracture toughness of KDP crystals. However, KDP crystal materials are anisotropic, which will lead to the generation of non-directional cracks. Therefore, the damage process of the material is accompanied by melting, crack propagation et al., and the final morphology of the damage point is also more complicated, as shown in Figure 20a ([PERSON] and [PERSON], 2016). The experimental investigations of short pulse laser induced damage have verified the rationality of the damage mechanism that discussed in the above sections well. In contrast, the morphology of the damage pit under ultra-short pulse laser irradiation in  
Figure 20: Typical morphological comparison of the laser damage sites on KDP crystal surfaces induced by short and ultra-short laser pulses. (**a**) FWHM (Full Wave at Half Maximum) = 6.9 ns; (**b**) FWHM = 28 fs ([PERSON] and [PERSON], 2016). Figure 20b is obviously different from that of short pulse laser irradiation. The laser energy deposited inside the crystalline material is too late for thermal conduction to occur due to the ultra-short pulse period. In addition, the morphology of the damage pit that shown in Figure 20b is a good verification of the correctness of the inference that shockwave is generated by mechanical expansion for KDP under ultrashort pulse irradiation in Section 4.1. As a result, the laser damage only occurred in the local high-temperature plasma area near the focal point of the laser spot, and there was no obvious cracking around the damage point. In the study of nanosecond laser induced damage, [PERSON] et al. [103] found that the incident laser fluence has an effect on the morphology and size of the damage of KDP crystal. Based on the observation of damage morphology, these damages can be divided into three typical types: crack, shell, and crater, as shown in Figure 21.  
The crack-type damage always generates on the surface without any connection with the thermal explosion and fracturing inside the bulk under laser fluence smaller than 7.5 J/cm\({}^{2}\). [PERSON] et al. proposed that crack damage is caused by the residual stress during processing. Once the laser fluence more than 7.5 J/cm\({}^{2}\) is used to irradiate KDP crystal material, shell-type damage, and crater-type damage begin to appear. Among them, the mechanism of shell damage is the same as that of crack damage, which is caused by the residual stress during processing. Shell damage accounts for 80% of the total damage, while crater damage is mainly caused by a large amount of thermal absorption and conduction due to the residual defects in the previous processing. Based on the analysis of the morphology and composition of the damage surface, the morphology characteristics of LID has been further supplemented in Figure 22.  
Figure 21: Microscopic images of the typical damage morphology. (**a**) Crack, (**b**) shell, and (**c**) crater. \(\Lambda\) = 351 nm, pulse duration = 5 ns [102]. It can be seen in Figure 22 that the damage pit presents a cone-shaped crater with a melted core region and fractured periphery. The cracks stretch outward. It is clear that the laser heating and material structural deformation are coupled to each other and then contribute to the LID of KDP crystal. [PERSON] et al.