With the continuous advancement of inertial confinement fusion (ICF) research, the ultraviolet (UV) laser damage resistance of high-quality potassium dihydrogen phosphate (KDP)/deuterated potassium dihydrogen phosphate (DKDP) crystals has been steadily improving. However, the relationship between the laser damage performance of crystals and their growth parameters remains unclear. This is manifested in the lack of quantitative studies on the connections among various defects in crystals, growth parameters, and optical properties. Precursor defects inducing UV damage have not been experimentally observed or confirmed, resulting in numerous crystal damage phenomena that cannot be explained by physical models to date. In particular the nature of microscopic defects related to crystal damage under high fluence and the physical mechanisms by which they induce damage remain poorly understood. Consequently, research on improving crystal quality lacks fundamental microscopic understanding and technical approaches. The issue of UV laser damage caused by microscopic defects limits the application and service life of these crystals in high-power laser systems. Here we outline the laser damage mechanisms of KDP/DKDP crystals and summarize both experimental and theoretical researches on crystal damage-related defects, including recent progress in our own group.

In this paper, we first summarize the current domestic and international progress in damage researches on large-sized KDP/DKDP crystals and outline the primary physical mechanisms of laser damage. We then categorize the main types of defects present in KDP/DKDP crystals and their formation mechanism, summarizing the characterization methods for these defects reported in the literature as well as their impact on the optical properties of the crystal materials. Recently, we have analyzed the macro-distribution characteristics of hair inclusions using laser scattering techniques and have further conducted the statistical characterization of their micro-morphology. By combining the micro-Raman spectroscopy, we have identified the type of these hair inclusions. Based on their actual microscopic features, we have established a simulation model for defect-modulated light fields to study and analyze the field intensity modulation effects caused by inclusion defects within KDP crystals. Laser-induced damage testing was employed to investigate the influence of hair inclusions on the laser damage resistance of KDP-type crystals and analyze their damage mechanisms. Finally, we explored the origin of these hair inclusions. Regarding theoretical simulations, we employed first-principle calculations to study defect clusters composed of oxygen vacancies and surrounding intrinsic cation vacancies (hydrogen vacancies and potassium vacancies). We analyzed the impact of these defect clusters on KDP crystals from perspectives including their energetic stability, crystal structure, electronic structure, and linear absorption properties. Additionally, to theoretically study the damage generated on KDP crystal surfaces, we used first-principle methods to investigate the structural difference among KDP crystals and their dehydration products (K₂H₂P₂O₇ and KPO₃). We calculated the electronic structures and optical properties of these three crystals. The results provided a scientific basis for a deeper understanding of the intrinsic mechanisms behind laser-induced damage growth on KDP crystal surfaces.

Regarding UV laser damage under high-fluence, studies on the damage characteristics of crystal components reported to date clearly indicate that the damage initiation precursors in crystals under high-fluence laser irradiation are nanoscale defect clusters inside the crystal material. The efficient, precise, and multi-scale characterization method for defects in crystal components is fundamental for investigating the origin of damage initiation and supporting the preparation of high-quality crystal materials, which is represented as a current bottleneck. Achieving characterization of such nanoscale defect clusters provides crucial evidence for establishing the correlation among crystal growth parameters and laser damage performance.