| Potassium dihydrogen phosphate(KH2PO4,KDP)crystals and its isotopic compound deuterated potassium dihydrogen phosphate(K(DxH1-x)2PO4,DKDP)crystals are the only crystals that can be used for frequency doubling and Pockels cell in inertial confinement fusion(ICF)projects because of their high electro-optical coefficient,wide transmission band,good optical homogeneity,low half-wave voltage and easy to grow into large-aperture single crystals.In practical applications,the laser-induced damage threshold of KDP crystals is far below its theoretical value by an order of magnitude,and the damage is severe during UV laser irradiation,which reduces the energy of the output laser and the lifetime of the upstream and downstream components,and becomes a bottleneck for the further development of ICF engineering.Defects are the main factors contributing to the low laser-induced damage threshold of crystals in practical applications.Defects in crystals(e.g.point defects,line defects,surface defects and body defects)are complex and varied,and the intrinsic mechanisms that these defects affect laser-induced damage in crystals are distinct.The effect of intrinsic and impurity point defects on KDP crystals has been partially investigated,but the effect of defect clusters on the structure and properties of crystals is unclear and the mechanisms affecting laser-induced damage in crystals are also unknown.In addition,when damage occurs in crystals under laser irradiation,there is a large difference between the characteristics of surface and bulk damage.Surface dehydration products are thought to be responsible for the surface damage growth in crystals,but its mechanism needs to be further investigated.In this paper,based on the "point seed" rapid growth of KDP crystals,the Fe3+ defects centers in KDP crystals have been characterized with using the electron paramagnetic resonance technique,which provides experimental support for determining the cluster configuration.The effects of intrinsic and impurity cluster defects and surface dehydration products on the structure and properties of the crystals have been investigated by the first-principles calculations,and the mechanisms by which these defects reduce the laser-induced damage threshold of KDP crystals have been analyzed,providing theoretical references for improving the crystal quality and laser induced damage threshold.The main contents of this paper are as follows:1.The effects of oxygen vacancies cluster defects on the structure and properties of KDP have been investigated by using first-principles calculations.The stability of cluster defects in crystals composed of oxygen vacancies(VO2+)charge-compensated with hydrogen vacancies(VH-)or potassium vacancies(VK-)have been considered,and we have found that oxygen vacancies are more likely to complex with the nearest VH-or VK-as VP2++ 2VH-,VO2++2VK-and VO2++VK-+VH-cluster defects.The stabilities of the three cluster defects in the crystal are VO2++2VH->VO2++VK-+VH->VO2+2VK-,due to the greater lattice relaxation energy required for complexation with VK-.These three cluster defects lead to large distortions in the O-H bonds connecting the phosphorus-oxygen tetrahedral skeleton,which may lead a decrease in the stability of the crystal structure.Due to the electrons interactions between vacancies and nearby atoms,deep energy level defect states are introduced at the bottom of the conduction band(CBM)of the crystal,and the band gap of the crystal is shortened.For the VO2++2VH-and VO2++VK-+VH-cluster defects,the interaction of electronic states between VO2+ and the nearest VH-leads to additional linear absorption at 6.4 eV in the crystal.All of these result in lowing the laser-induced damage threshold of KDP crystals.2.The effect of Fe3+ impurities on the structure and properties of the KDP crystals has been investigated using the electron paramagnetic resonance technique and the first-principles calculations.The Fe3+ defects centre in KDP crystals before and after X-ray irradiation has been characterised by using the electron paramagnetic resonance technique.The Fe3+ defects centre are the substitution of Fe3+ ions for K+ ions(FeK2+)in complex with two VH-.The defects centre traps free electrons after X-ray irradiation and the intensity of the electron paramagnetic resonance centre is weakened.Combining the first-principles calculations,the most thermodynamically stable FeK2++2VH-cluster defect has been built,and the effects of the cluster defect on the crystal structure,electronic structure and optical properties of KDP crystals have been investigated.The cluster defects could cause large distortions in the O-H bonds connecting the phosphorus-oxygen tetrahedral skeleton in KDP crystals.Before and after the cluster defect capture free electrons,the electronic structure of KDP crystal has different variation,but both are introduced multiple defective energy levels by the cluster defect.The band gap of the crystal is shortened,which increases the non-linear absorption of the crystal.In addition to capturing a free electron,Fe3+ ions could lead some additional optical absorption peaks at 0-7.2 eV in KDP crystal and the linear absorption of the crystal increases.In conclusion,Fe3+ ions cause a decrease in the structural stability of KDP crystals,and could increase the linear and non-linear absorption of KDP crystals,as a result,adversely affects the laser-induced damage threshold of crystals.3.The effects of Zn2+and Al3+ ions on the structure and properties of KDP crystals have been investigated by using first-principles calculations.Theoretical calculations show that these two impurity ions mainly replace K+ions in KDP crystals and could complex with intrinsic point defects to form ZnK++VK-,ZnK++VH-and Alk2++2VH-cluster defects.The additional linear absorption peaks introduced by Zn2+ and Al3+ ions are very close to the absorption cut-off edge of the crystal(before 200 nm),so Zn2+ and Al3+ ions have little effect on the linear absorption of KDP crystals.Zn2+and Al3+ions cause a slight shortening of the band gap of KDP crystals.However,these ions could bring larger deformations of O-H bonds.As the concentration of Zn2+ or 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 Zn2+ or Al3+ ions could cause the instability of the crystal structure,which finally affects the laser-induced damage threshold of the KDP crystals.4.The differences in the crystal structures,electronic properties and optical absorpiton bewteen KDP crystal and it’s surface laser-induced damage dehydration products K2H2P2O7 and KPO3 crystals are investigated by using first-prinsples calculations.The results of theoretical calculations show that the non-linear optical effect structural motifs of KDP crystals are disrupted after irradiation dehydration to K2H2P2O7 and KPO3,which leads to the non-linear optical properties at the surface damage being broken.In terms of electronic structure,the dehydration products K2H2P2O7 and KPO3 have a shorter band gap compared with KDP crystal.In terms of optical properties,K2H2P2O7 and KPO3 have a wider optical absorption band than KDP crystals 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 growth of surface damage with subsequent irradiation,whereas the bulk damage generally does not continue to grow in subsequent laser irradiation.In summary,intrinsic and impurity defects in KDP crystals affect the crystal structure,electronic properties and optical absorption,which in turn lead to a lower laser-induced damage threshold.Appropriate control and reduction of the concentration of intrinsic and impurity defects during crystal growth and subsequent processing can help to improve the quality of the crystal and the laser-induced damage threshold. |
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