# Recent Advances in Laser-Induced Surface Damage of KH\({}_{2}\)Po\({}_{4}\) [PERSON]
## 5 Summary and Outlook  
In summary, the emergence of chirped pulse amplification technology has greatly promoted the development and application of rapid ignition in laser-driven inertial confinement fusion. However, it is necessary to consider the limitations caused by the low LIDT of core optical components such as KDP crystals when constructing a laser system with high output power density. The impurities in the process of the formation of crystal materials can be controlled greatly due to the advance of technology. However, the occurrence of defects or scratches on surface is inevitable during the application of crystalline optical materials. As a result, it is very necessary to understand the intrinsic damage mechanism of crystal materials under the laser irradiation. Only understanding the mechanism of laser induced damage of the crystal material, can the comprehensive repair of the damaged points be proceeded. The accurate mitigation can ensure that there is no residual mechanical or thermal stress to fail the repair.  
To address this problem, the principles of LID has been reviewed clearly. The mechanism is mainly summarized as impurity thermal damage and ionization damage. Then, the process from the initiation to the finalization of LID has been divided into three stages (i.e., energy deposition, damage initiation, and damage forming) to ensure the understanding of LID. It is worth noting that the three stages  
Figure 25: Morphologies of the various KDP surfaces before (**a1**, **b1**, and **c1**) and after (**a2**, **b2**, and **c2**) 355 nm-laser pulse irradiation. (**a**) Initial crack damaged surfaces; (**b**) Repaired pit without tool marks; (**c**) Repaired pit with tool marks. **a3**, **b3**, and **c3** are the light intensity enhancement factor (LIEF) distribution [18, 105].  
are complementary to each other. The energy deposition inside the KDP crystal material is serious under high-intensity laser fluence, especially when there are various processing defects remaining on the surface of KDP. The influence of various initial defects on the forming of damage also has great difference. In addition, different degrees of initial energy deposition under the subsequent irradiation with different wavelengths and laser fluence will also have a great impact on damage forming.  
The damage has various morphology. It is mainly composed of cracks, bursts, and melting pit under short pulse laser irradiation, which also means the thresholds for damage initiation under various types of initial defects are different. Thus, the transient dynamic behavior of multi-physics coupling simulation and some experiments are still needed to further explore to determine the threshold in different situations.  
Conceptualization, M.C. and W.D.; methodology, M.C.; investigation, H.Y. and Q.L.; validation, J.C.; writing--original draft preparation, W.D.; writing--review and editing, M.C., W.D., and J.C.; Funding acquisition, M.C. and J.C. All authors have read and agreed to the published version of the manuscript.  
We acknowledge the support of Science Challenge Project [No. TZ2016006-0503-01]; the National Natural Science Foundation of China [Nos. 51775147, 51705105]; the Young Elite Scientists Sponsorship Program by CAST [No. 2018 QNKe001]; the China Postdoctoral Science Foundation [Nos. 2017M621260, 2018T110288]; the Heliognjiang Postdoctoral Fund [No. LBH-Z17090]; the Fundamental Research Funds for the Central Universities [No. HIT.NSRFI2019053]; and the Self-Planned Task Foundation of State Key Laboratory of Robotics and System (HIT) of China [No. SKLRS201718A, SKLRS201803B].  
The authors declare that they have no conflict of interest.