# 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
### Outlook of the Prediction of Damage Threshold  
The three stages from the initiation to the finalization of LID (i.e., energy deposition, damage initiation, and damage forming) was described clearly in the above sections to understand the mechanism of LID and to predict the LIDT more accurate. Actually, the deposition of energy will cause the initiation of damage once it reaches a certain degree. Based on the analysis of the initial energy deposition under the irradiation of laser, it can be found that all the methods of embodying the peak energy the material can withstand are not intuitive. The electron density curve of KDP materials and the transmission or absorption of the incident laser can only reflect the trend of damage, which is not the great methods to predict the damage threshold. The technology of TRPP in Section 3.1.2. captures the momentary dynamic behavior of damage initiated on KDP materials. However, it is still not enough to improve the anti-laser damage ability of KDP crystal solely rely on the prediction of damage trends. In addition, some surface defects tend to be leaved on the surface of KDP after processing. Under these conditions, accurate and intuitive prediction of damage threshold remains a challenge. To overcome this difficulty, our research group has carried out some relevant research on the prediction of LIDT. In the early stage, [PERSON] et al. [76; 78] used the FTDT algorithm to explore the phenomenon of light intensity modulation inside the material under the analysis of the electric field. Based on the exploration of the damage trend, the multi-physics and electric field coupling technology was discovered and applied to the prediction of the LIDT of KDP crystals with different cracks defects in reference [88]. The multi-physics coupling technology reveals the mechanism of damage  
Figure 22: Typical morphology of the laser-induced damage. (**a**) Damage craters under the irradiation of nanosecond laser (\(\lambda=355\) nm) with **a1** before irradiation, **a2** fluences (\(17.3\) J/cm\({}^{2}\)), and **a3** fluences (\(19.5\) J/cm\({}^{2}\)) shows the central core [92]; (**b**) Damage under \(\lambda=355\) nm, and \(6.4\) ns pulse duration with **b1** reflection 3D scan image and **b2** transmission 3D scan image [29]. more comprehensively and gives the detailed damage thresholds and damage mechanism intuitively, as shown in Figure 23.  
In addition, in order to investigate the LID of KDP crystals more comprehensively, our research group subsequently carried out the corresponding verification of damage mechanism and damage threshold in engineering. The damage repair devices [105; 106] as well as repair strategies [107; 108], were explored with a view to achieving the significance of predicting the damage threshold. Figure 24 demonstrates the designed repair device and the repair process of damaged pits of our research group. Figure 24: Micro-milling set-up developed for the repairment of damage on KDP crystal. (**a**) Overall configuration of the 5-axis vertical spindle machine tool; (**b**) The repairing process [105]; (**c**) Schematic of the generation process of tool marks in the micro ball-end milling repair process of KDP optics [107]. Figure 23: Transient dynamic behavior induced by laser damage on the surface of KDP crystal with conical crack defect. (**a**) Multi-physics coupling simulation result with **a1** crater morphology, **a2** vapor velocity distribution, and **a3** temperature distribution of local laser damage area; (**b**) Experimental images captured by the system in the laser damage micro-region with **b1** material ejection and **b2** plasma expansion [89]. For the design of the repair strategies, [PERSON] [105, 109] and [PERSON] et al. [110] conducted various measurements on LIDT to ensure that the repair work is effective in raising the LIDT of KDP crystal.