# Recent Advances in Laser-Induced Surface Damage of KH\({}_{2}\)Po\({}_{4}\) [PERSON]
## 1 Introduction  
Inertial confinement fusion (ICF) has gradually become one of the most promising clean energy sources that people are striving for [1; 2]. Lawrence Livermore National Laboratory (LLNL) built the largest large-scale laser fusion device: National Ignition Facility (NIF) [3; 4]. The Limeil Laboratory of France and the China Academy of Engineering Physics (CAEP) [5] also built laser fusion ignition devices successively. _Nature_ published two consecutive articles in 2014 [6] and 2019 [7] to report new advances in inertial confined fusion. In the driven laser system engineering for ICF devices, large quantity, caliber, and special performance of optical components are required [8]. To achieve favorable absorption of the laser by the target fuel, the ultraviolet (UV) light with shorter wavelength is preferred. The most effective way to obtain high power UV light is to convert the laser from fundamental frequency to multi-frequency and triple-frequency light through large diameter KH\({}_{2}\)PO\({}_{4}\) (KDP) crystals. As a result, the laser damage resistance of the optical crystal materials in the devices determines the transmission quality of the whole optical path, seriously restricting the quality of the output laser, which is an urgent technical problem in the engineering application [9; 10]. KDP crystals are used as electro-optical switches and frequency conversion elements at all levels of bridging optical paths in laser-driven fusion engineering due to the wide spectral transparency and high nonlinear conversion efficiency [11]. Laser-induced damage (LID) [12] will occur inside or on the surface of materials when the KDP crystals are exposed to the sufficiently high laser intensitythat exceeds the extreme value that the materials can withstand. [PERSON] et al. (2013) summarized two kinds of crystal damage depending on the location of damage: bulk damage and surface damage. The bulk damage is relatively evenly distributed inside the crystal. The porosity and the deposited impurities during the growth of crystal will absorb the sufficient laser energy and explode to form the central melted and fractured zone several to tens of micrometers in diameter ([PERSON] et al., 2013; [PERSON] et al., 2014; [PERSON] et al., 2015). In contrast, the surface damage is usually initiated by the near surface impurities, the defects, such as particles or scratches, remaining during the processing and surface contamination ([PERSON] et al., 2014; [PERSON] et al., 2015). In response to these varied damages with different laser intensity, the laser induced damage threshold (LIDT) is introduced to show the initiation of laser induced damage ([PERSON] et al., 2015; [PERSON] et al., 2015). At present, a series of studies on the damage mechanism and the damage threshold of fused silica and other optical elements in ICF devices under laser irradiation have been carried out. The bulk damage of KDP crystal materials has been mostly studied by [PERSON] et al. (2016, 2017). However, the impurity particles inside the crystal have been avoided gradually due to the improvement of the growth technology of KDP crystal materials. The intrinsic damage mechanism of the surface damage of crystal materials should be considered significantly with the advent of ultra-short and short pulse regime. In addition, the actual laser induced damage threshold (LIDT) of the KDP crystal used in the current project is about 12-15 J/cm\({}^{2}\) (1 ns) under the laser irradiation, which is still much smaller than the theoretical value of 147-200 J/cm\({}^{2}\) (1-3 ns). The reason is that the mechanism of laser-induced surface damage of materials has not been fully interpreted ([PERSON] et al., 2015). Because of the transparent properties of the KDP crystal material, part of the energy of the incident laser is reflected or scattered, the other part of the energy is absorbed by the material, and the rest continues to propagate forward through transmission.