# 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
Some researchers summarized the various defects that introduced by SPDF technique on the surface of KDP crystal, and further explored the effects of different defects on energy deposition using simulation and experimental methods. Figure 10: Optical microscopy images of the five kinds of KDP surface defects. (**a**) Fracture pits; (**b**) Convex scratches; (**c**) Surface protuberances; (**d**) Cracks; and (**e**) Plastic scratches [20]. [PERSON] et al. [18; 78] conducted the detailed investigations on surface cracks (Figure 12a) and scratches (Figure 12b) by establishing the simplified defect models. The surface scratches produced by flying cutting were simplified into circular and triangular to explore the light intensification distribution, as shown in Figure 12(a2,b2). At present, the light intensity enhancement factor (LIEF) was introduced internationally to characterize the intensity of distortion caused by defect inside the materials. \[LIEF=I_{\text{max}}/I_{0} \tag{14}\]  
where \(I_{max}\) is the largest light intensity inside the KDP crystal with various defects, and \(I_{0}\) is the light intensity inside the KDP crystal with ideal surface. A series of ionization effects such as photoionization and avalanche ionization of the material will be more likely to occur when the LIEF reaches a certain critical value. The electron density will also increase significantly, resulting in more energy deposition inside the material. The finite-difference time-domain (FDTD) was used by [PERSON] to describe the light intensification distribution near the defects to reflect the difference in the initial energy deposited of different types of defects. FDTD algorithm can maturely deal with non-uniform and complex shapes of target radiation and electromagnetic scattering. In addition, it is very convenient to give a demonstration process of the advancement of each electromagnetic field with time. The light intensification can be solved and visualized. Visualization can clearly simulate the distortion caused by light intensification with the influence of defects, and FDTD will not be limited by the size of the simulated object, which brings convenience to the analysis [30; 62]. Figure 11: The temporal and spatial (T and S) evolution of the predicted lattice temperature, the electronic densities, and the absorbed power density of the precursor. (**a**) Lattice temperature with **a1** temporal scale, **a2** spatial scale, and **a3** the physical quantities depend both on space and time; (**b**) Electronic densities with **b1** temporal scale and **b2** spatial scale; and (**c**) Absorbed power density with **c1** temporal scale and **c2** spatial scale. The position of precursor of **a1**, **b1**, and **c1** is in the center, and the actuation duration of **a2**, **b2**, and **c2** is 500 ps [77]. It can be seen from Figure 12a,b that all the defects of KDP crystal will produce diffraction enhanced ripples with the irradiation of laser. Although the light intensity distribution caused by the triangular scratch is more complex and intense than that of the circular scratch due to the existence of manufacturing-induced front surface scratches, the LIEF of scratches is still smaller than that of cracks, which is similar to reference [20]. As shown in Figure 12c, [PERSON] et al. [79] investigated the influence of surface scratches on the incident light intensity of KDP and found that once the brittle cracks appeared on the side of scratches, the LIEF would increase significantly, which also validated the hazards of brittle damage [19]. [PERSON] and [PERSON] et al. [80] also noticed that cracks randomly existed on the front or rear surfaces of the KDP crystal, which would lead to different degrees of absorption of light intensity. The enhancement of light intensity induced by microcracks on the front surface is mainly caused by diffraction effect, standing wave effect and interference ripples effect.