Electron Paramagnetic Resonance Spectroscopy Study Of Point Defects In KDP/DKDP Crystals

KDP/DKDP crystal is the only nonlinear optical material that can be applied to large-scale laser devices in inertial confinement fusion(ICF).With the continuous development of ICF devices,the output laser energy continues to increase,which puts forward higher and higher requirements for the production efficiency,cost control,and laser irradiation resistance of crystals.How to grow crystals resistant to high-power laser damage quickly and efficiently has always been the focus of research in this field.In this paper,the rapid and efficient growth method of KDP/DKDP crystal and the characterization of point defects closely related to the damage of KDP/DKDP crystal are studied.The research results obtained are summarized as follows.The traditional method is the main growth method of high-quality THG(Type Ⅱ)wafers.In this paper,the traditional method is optimized by changing the tangential direction and shape of the seed crystal.The change of the tangential direction and shape of the seed crystal leads to the change of the final crystal shape,which improves the utilization rate of the THG wafer and saves raw materials.Through three-dimensional modeling,it is found that the crystal grown by the traditional method of 0° tangential seed crystal can produce more than 10 THG wafers,and the utilization rate of the crystal is increased to 26.34%.The small size KDP and DKDP crystals with different tangential seed crystals grown by the traditional method were characterized by spectrophotometer.It was found that the transmittance of different tangential seed crystals to KDP and DKDP crystals was close to 90%.The refractive index inhomogeneity of KDP and DKDP crystals was characterized by the orthogonal polarization interference technique.The RMS values of refractive index deviation of crystals were close to the order of 106,and the RMS values of DKDP crystals with 30° and 45° tangential directions were larger.The defects and their relative concentrations in KDP crystals were characterized by positron annihilation lifetime spectroscopy.The results show that the average positron lifetime τaν in the crystal remains at 0.27 ns with the change of the seed crystal tangential direction.The positron capture rate of the crystal with 30° tangential direction is higher,and it is easier to capture positrons.The relative concentration of vacancy defects in the crystal is the highest,which has a very adverse effect on the application of the crystal.The 0° tangential rectangular seed crystal can be selected as the seed crystal of DKDP crystal for producing THG wafer.Point defects in KDP and DKDP crystals include impurity ions,non-stoichiometric defects,electronic defects,etc.Electron paramagnetic resonance(EPR)is a detection method that can qualitatively and quantitatively detect the unpaired electrons contained in atoms or molecules of a substance and explore the structural characteristics of its surrounding environment.It has a wide range of applications in detecting impurity ions and intrinsic point defects in crystals.EPR experiments show that KDP and DKDP crystals produce a large number of hole and electron defects under irradiation conditions.The formation of these defects is closely related to the[H2PO4]-([D2PO4]-)group,which can directly form selfbinding holes and interstitial H0、(D0),and can also be affected by H(D)vacancies and oxygen vacancies to form defect centers such as[HPO4]-、([DPO4]-)and PO32-.The results of spin number density show that a total of about 100 ppm of various defect centers can be generated in DKDP crystals during irradiation.The spin number density of defect centers in KDP crystals is lower than that in DKDP crystals due to the enhancement of thermal stability of various defect centers caused by deuteration.The thermal stability experiments at the low-temperature show that the thermal stability of[DPO4]-and[D2PO4]0 centers are much higher than that of[HPO4]-and[H2PO4]0,and the same is true for interstitial H0、D0 and PO32-.The enhancement of thermal stability makes the number of these defect centers continue to increase during laser irradiation,resulting in linear absorption of the crystal and adversely affecting the application of the crystal.The DKDP sample was irradiated by continuous wave X-ray,and a set of signals with a maximum splitting of 588.3G in the crystal axis direction was obtained.The angular distribution signals of the signal in the c-a,a-b,and b-c planes were obtained at 35K.These resonance signals are divided into two groups according to the peak intensity.They come from two crystal domains of the ferroelectric phase of DKDP crystal.These two crystal domains satisfy a certain symmetry relationship.The spin Hamiltonian parameters are extracted according to the magnetic unequal positions in the two crystal domains by the least square method.It is found that there is a set of spin Hamiltonian parameters that satisfy the resonance positions of the two peaks at the same time.The three g factors in the principal axis direction are 2.0263,1.9796 and 2.0038,respectively,and the signal belongs to a hole.Through analysis,it is believed that the hole is captured by the O atom on the PO4 group,distributed on four oxygen atoms,and is affected by the adjacent 31P nucleus to produce hyperfine splitting.This defect generated at the irradiation condition line may have adverse effects on crystal applications.The characteristics of potassium dihydrogen phosphate(KDP)crystals with the Cr(NO3)3 doping at～10 ppm level were studied by the ultraviolet-visible(UV-Vis)absorption spectrum,electron paramagnetic resonance spectroscopy(EPR),and the first-principles calculations.According to the UV-Vis absorption spectrum,there are three additional absorption peaks in the 200-800 nm regions on KDP:Cr(NO3)3 crystals compared with undoped KDP crystals.The angular-dependence EPR spectra of a Cr3+paramagnetic center of KDP:Cr(NO3)3 crystal were obtained at room temperature.The spin Hamiltonial parameters(g and D tensors)of the Cr3+ center were extracted using a least-squares fitting code.The first-principles calculations show that the formation energy is the lowest when the Cr atom substitutes the K atom.In addition,the d-d orbital electron transition Cr3+ ion may contribute to the two absorption peaks(451 nm and 650 nm)in the UV-Vis absorption spectrum.Based on the experiments and theoretical calculations,an inference is that in the KDP:Cr(NO3)3 crystal,the Cr atoms occupy an average position of the K atoms,and the charge compensation is reached through the two adjacent H vacancies to meet the electrical neutrality requirement.In summary,KDP and DKDP crystals have high transmittance and high refractive index inhomogeneity in the traditional growth method.The efficiency of the crystal in the production process can be further improved by optimizing the seed crystal.Electron paramagnetic resonance results show that the crystal has a variety of point defects.These point defects may be from impurity ions(such as Cr3+ ions),non-stoichiometric defects(H vacancies),or their composite defects entering during the growth process;it is also possible that the crystal is produced during the irradiation process(such as selfbinding hole center),and their existence makes the crystal produce a large amount of linear absorption during laser irradiation.Therefore,the regulation of defects in crystals is of great significance.