# 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
In contrast, the light intensity enhancement induced by the rear surface crack is mainly due to the total reflection or even double total reflection (DTR) effect of the incident laser at the crack wall. Although the light intensity distribution induced by the micro-cracks on the rear surface is relatively simple, a series of hot spots parallel to the rear surface were formed between the total reflected light wave and incident laser in the sub-surface of material, which will induce a strong phenomenon of light intensity enhancement, as indicated in Figure 12(a3). Thus, the energy deposition of laser on the rear surface is much more serious than the front surface. Based on the investigation of the energy absorption capacity of different defects to the incident laser, [PERSON] et al. [18] further summarized the trend of LIEF with the size parameters of surface crack defects, and then presented the degree of energy deposition. It can be seen in Figure 13 that the LIEF induced by the micro-crack defects on the surface of the KDP crystal is not only related to the location of the micro-cracks and geometric structure parameters, but also the wavelength of the incident laser. The LIEF of KDP rear surface with conical cracks in Figure 13c is more susceptible to have larger values in the extremes under the influence of crack depth than other types of defects. In contrast, the LIEF of radial cracks and lateral cracks in Figure 13a,b will be larger on the front surface than on the rear surface. In general, the calculated LIEF shows a gradually increasing trend with the increase of size parameters. Therefore, the number, geometric  
Figure 12: Exploration of the influence of defects on energy deposition. (**a**) Cracks; (**b**) Scratches; and (**c**) Plastic scratches with brittle cracks. **a1** and **b1** are the topography images of defects; **a2** and **b2** are the simplified defect models; and **a3** and **b3** are the intensity distribution with simplified defect model simulated by finite-difference time-domain (FDTD) [18, 78, 79]. parameters, and the positions of defects on the surface of crystal element can be controlled to ensure the laser damage resistance of KDP crystal elements. Correspondingly, [PERSON] et al. [18] used the R-on-1 laser irradiation mode to detect the surface LIDT of KDP crystals with different kinds of cracks, as shown in Figure 14. The cracks were established based on the exploration of references [18; 81; 82]. The results of experiments are consistent with the simulation data well. The existence of brittle cracks will greatly reduce the LIDT, which also verifies the rationality of FDTD algorithm simulation well. To avoid the occurrence of brittle defects and a large amount of initial energy deposition, [PERSON] et al. [74] conducted a lot of research on the brittle-plastic transition during the processing, which provided a basis for obtaining the surface without brittle damage. In the process of processing KDP materials, the first condition that needs to be satisfied is that the micro-cracks and craters formed in the brittle zone will not extend underneath the final surface,  
Figure 14: Experimentally tested laser induced damage thresholds (LIDTs) for crack-free and flawed KDP surfaces [18]. Figure 13: Influence of size parameter of surface cracks on light intensity under the irradiation of 1064 nm, 532 nm, and 355 nm lasers. (**a**) Radial cracks; (**b**) Lateral cracks; (**c**) Conical cracks [18]. another condition is that the feed rate used in the cutting process should be small enough to ensure the fracture zone formed in the previous cutting can be completely removed by the subsequent process, as shown in Figure 9:0 These conditions can be expressed as Equation (14). \[\left\{ \begin{array}{l}Y_{c}<h=\frac{2 fx}{\sqrt{R^{2}-(x+f/2)^{2}}+\sqrt{R^{2} -(x-f/2)^{2}}}\\ f\leq\sqrt{t_{c}gR}\end{array} \right.