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Intentionally adding select ions such as Al3+ could be helpful in controlling the crystal habit of KDP crystal for high yield of optics. The study of how Al3+ ions affect crystal quality can provide a basis for selecting an appropriate doping level without negatively affecting the optical properties of crystals. Here, the influence of Al3+ ions on the crystal structure and properties of KDP crystals have been investigated by using first-principles calculations. Theoretical calculations show that Al3+ ions mainly replace K sites in KDP crystals and could complex with intrinsic VH− point defects to form AlK2+ + 2VH− cluster defects. The linear absorption spectra indicate that the presence of Al3+ ions has minimal impact on the linear absorption of KDP crystals, aligning well with the experimental findings. And Al3+ ions could cause a slight shortening of the band gap of KDP crystals. However, these ions could bring significant deformations of O-H bonds. As the concentration of Al3+ ions increase, more O-H bonds linking to PO4 groups are distorted in KDP crystals. As a result, the structural instability could be fast enhanced with increasing the defect concentration. Therefore, high concentrations of Al3+ ions could cause the instability of the crystal structure, which finally affects the laser-induced damage resistance of the KDP crystals. This manuscript contributes to a more comprehensive understanding of the physical mechanisms by which different impurity ions affect the optical properties of KDP crystals. [ABSTRACT FROM AUTHOR]

Laser induced damage sites on the surface of a KDP crystal component tend to grow with subsequent laser irradiation, which substantially decrease the lifetime of these optics. The structural investigation suggests that this may be related to the surface damage products of the crystals. In this work, the differences in the crystal structures, electronic properties and optical absorption between the KDP crystal and its surface laser-induced decomposition products K2H2P2O7 and KPO3 are investigated by using first-principles calculations. The theoretical results show that the nonlinear optical active units of KDP crystals are disrupted after irradiation dehydration to K2H2P2O7 and KPO3, which leads to the optical properties at the surface damage being deteriorated. In terms of the electronic structure, the dehydration products K2H2P2O7 and KPO3 have a shorter band gap compared with the KDP crystal. In addition, K2H2P2O7 and KPO3 have a wider optical absorption band than that of the KDP crystal and introduce more ultraviolet absorption in the optical elements. Compared to polycrystalline KDP at the bulk damage sites, the dehydration products at the surface damage increase the ultraviolet absorption of the crystals and cause the surface damage to continually grow under subsequent irradiation, whereas the bulk damage does not continue to increase in subsequent laser irradiation. [ABSTRACT FROM AUTHOR]

KDP and ADP crystals doped with different concentrations of Fe3+ ions were grown by the "point seed" technique and some KDP crystal samples were annealed. Positron annihilation lifetime spectroscopy and slow positron beam technique were applied to investigate the types and distributions of the defects with different states of charges for KDP and ADP crystals. It was found that the quality of the prismatic sector was better than that of the pyramidal sector, although Fe3+ was mainly enriched in the prismatic sector. The types and content of defects in the pyramidal sector were relatively larger. ADP crystals showed better structural stability, with fewer defects than KDP crystals. This might be one of the reasons for the higher damage threshold of ADP crystals. The results also indicated that thermal annealing could decompose large-size defects into small-size ones to improve KDP crystal quality. However, it could not reduce the influence of Fe3+, such as causing light absorption at 220–300 nm. In addition, the Raman spectroscopy, Fourier-transformed infrared spectroscopy, and ultraviolet absorption spectroscopy were implemented to illustrate the change in the structure and properties after doping in KDP crystals. Notably, this work focused on the types and distributions of microscopic defects, which would be significant for the crystal growth and quality improvement. [ABSTRACT FROM AUTHOR]