# Recent Advances in Laser-Induced Surface Damage of KH\({}_{2}\)Po\({}_{4}\) [PERSON]
## 3 Exploration of the Initial Energy Deposition
### Effects of Surface Defects on Initial Energy Deposition of KDP Crystal  
Due to the characteristic of KDP crystal materials, micro-nano-scale defects will be introduced into the surface when the single-point diamond ultra-precision fly-cutting (SPDF) is carried out on the surface of material. Various types of defects will exist on the surface of KDP crystal during the flying cutting. As a typical soft and brittle material, KDP crystal is more prone to produce brittle or plastic defects during processing [73; 74]. Figure 9 shows the topography of KDP chip generated in the finishing stage. Figure 8: Schematic diagram of time Resolved Pump and Probe (TRPP) technology [69]. Figure 9: Topography of the machined zone and chip in groove cutting test [74]. Based on the characteristics of KDP crystal materials, [PERSON] et al. [20] further summarized all the defects that appeared on the surface of eight KDP components during processing and classified them into five categories, as shown in Figure 10.  
Figure 10a shows the fracture pits defects usually with the widths of 20-200 \(\upmu\)m and the depths of 2-6 \(\upmu\)m. Figure 10b,c present different typical forms of protrusion damage. The cracks that are shown in Figure 10d with the lengths of 20-120 \(\upmu\)m are also a kind of defect that occurs more frequently. Due to the characteristics of KDP, the typical plastic scratch defects are also often present during processing, as shown in Figure 10e. [PERSON] et al. used the fluorescent emission detection technology to characterize the impact of KDP crystals on energy absorption under these five defects [20; 75] and found that the fluorescence intensity of the defect-free area is significantly smaller than that of the defect area. Among the above defects, the fluorescence intensity of plastic scratches is greatly weaker than the other four defects. The surface defects in Figure 10a-d have a stronger absorption capacity than the defect-free KDP surface, and there will be more energy deposition near the defects. Based on the investigation of the influence of surface defects on the LID of KDP crystal materials, [PERSON] et al. [76] showed that the light intensity is seriously affected by surface defects, and the characteristics of the light field modulation are strongly dependent on the geometry of defects. The modulation of the light field inside the material will cause an uneven distribution of light intensity, which in turn affects the difference in initial energy deposition inside the material. Therefore, it is very important to consider the effect of defects in the energy deposition stage of induced damage. [PERSON] et al. [77] adopted a new mechanism based on thermally driven secondary point defect formation to simulate the process of energy absorption in precursor defects and surrounding materials. The existence of precursor defects affects the absorption of energy by electrons, which in turn leads to a rapid increase in lattice temperature. As shown in Figure 11a, the temperature exhibits nonlinear behavior as a function of temporal and spatial (T and S) scales. The energy deposition takes place inside the material before the irradiation time of 0.5 ns. At the same time, the plasma formed inside the material expands at an almost constant rate, which makes the volume of energy deposition gradually increase, resulting in a significant increase in temperature. After 0.5 ns, the thermal explosion behavior is initiated, and the subsequent damage is caused. The effect of precursor defects on the initial energy deposition is well verified. Therefore, it is necessary to further explore the influence of defect morphology and size parameters on LIDT, and to find out the most threatening surface defects. Then, the methods for suppressing or repairing surface defects during the subsequent processing can be proposed to improve the LID resistance of KDP crystals.