Study On Ultraviolet Laser-Induced Damage Of KDP Crystal

The laser induced damage of large aperture potassium dihydrogen phosphate(KDP)crystal limits the load capacity of Inertial Confinement Fusion(ICF)which becomes the bottleneck of laser device.Laser induced damage is a hot issue for domestic and overseas scholars.It is of great significance to study the factors of influence on laser induced damage threshold and understand the mechanism of laser induced damage.It is important to grow and process the optical elements.In this thesis,the morphology of damage,the distribution of elements and the damage process are studied by scanning electron microscope,synchrotron radiation technology and photofluorescence emission spectrum.Firstly,the near-edge absorption structure spectra of synchrotron radiation proved that the damage structures are different between high fluences laser irradiation and low fluence multi pulse irradiation(retired component).Secondly,the understanding of damage process and mechanism of KDP crystals irradiated by high and low-fluences laser irradiation is strengthened.Finally,the research on defects of KDP crystal irradiated by different fluences is deepened.In this paper,the basic theory of inertial confinement fusion and the damage of KDP crystal are introduced.The structures and growth process of KDP crystal are investigated systematically.At the base of the current research status of laser induced damage of KDP crystal based,the content and innovation of the paper are summarized.The principles and methods of experiments using in this paper are briefly described.In the experimental section,the scanning electron microscopy and Raman technology were used to obtain the morphology and Raman spectra of KDP crystals irradiated with different fluences.And then,using X-ray absorption near-edge structure,the surface electronic structure of retired components and KDP crystals irradiated with high fluences were researched.The P L2,3-edge XANES spectra are analyzed with the Gaussian fitting.We found the samples irradiated by 11.50 J/cm2 only have medium chains on their surfaces,whereas the retired samples have short,medium and long chains on their surfaces.The chain length of KDP crystals increased,and several oxygen bridges are formed,which in turn increased the lengths of the P-O bonds.The long tetrahedral bonds prevent the overlap between phosphorus 3s orbitals and oxygen 2p orbitals.Therefore,the probability of probing states decreases with energy band of p characters.And for KDP crystals irradiated with high fluences,the probability of transition from 2p to 3p(energy band of p characters)decreases with the fluence increase.For retired samples,the damage is inhomogeneous on the surface,and the electron transition is complicated.We acquire the distribution of elements around the damage craters of KDP crystals by using the time of flight secondary ion mass spectroscopy.Then combining the synchrotron radiation micro-X-ray fluorescence with micro-X ray diffraction,the distribution of elements and changes of structures of KDP crystals irradiated with different fluences are investigated.KDP crystals are irradiated by high fluences and retired component.The changes of distribution between phosphorus and potassium were the same in KDP crystals irradiated by high fluences but were different in retired components.The KDP crystals irradiated by high fluences produce K2H2P2O7 and KPO3 crystals during laser-induced damage.The process can be described as follows:2KH2PO4?K2H2P2O7+H2O K2H2P2O7?2KPO3+H2OThe total thermal decomposition process occurs as follows:KH2PO4?KPO3+H2OThe K2H2P2O7 and K2H4(PO4)2P2O7 crystals were produced in retired components.It can be described as follows:2KH2PO4?K2H2P2O7+H2O,The crystal structure of K2H8(PO4)2P2O7 is K2[H2PO4-H-H2P2O7-H-H2PO4].The result shows that KDP crystals irradiated by high fluences were dehydrated completely but the retired components were incompletely decomposed.It is consistent with that the retired samples have long chains in the XANES spectra.Photoluminescence(PL)bands of KDP crystals are studied by time-resolved PL spectroscopy.Results show that the probability of phosphorus-oxygen hole center(POHC)defect in KDP crystals irradiated by 11.50 J/cm2 is 85.50%,whereas the probabilities of the defect in retired components and KDP crystals irradiated with 9.00 J/cm2 are 54.10%and 52.00%,respectively.By contrast,the probability of phosphorus-oxygen electric center(POEC)in KDP crystals irradiated by 11.50 J/cm2 is 2.40%,and the probabilities of the defect in retired components and KDP crystals irradiated with 9.00 J/cm2 are 13.50%and 29.60%,respectively.These defects may result from two different pathways,and can be transformed into each other at specific conditions.The bridging oxygen can be easier formed in long-chain structures.These bridges increased the lengths of P-O bonds and thus facilitated the formation of POHC defects.POECs occur primarily in short-chain structures.Thus,KDP crystals irradiated by 11.50 J/cm2 have the highest probability of POHC defects and the lowest probability of POECs.This is in contrast with that is found for KDP crystals irradiated with 9.00 J/cm2.The laser induced damage of retired components is inhomogeneous,and the samples irradiated by low fluence below 400 pulses.This led to the differences of the defects between the KDP crystals irradiated by high laser fluences and the retired components.We investigate the defects of KDP crystals irradiated with different fluences by vacuum ultraviolet photoluminescence(PL)emission spectra.Each KDP crystal spectrum is measured from 200 to 400 nm and 400 to 800 nm.Gaussian fitting is performed on the emission spectra of KDP crystal irradiated with different fluences,and each Gaussian curve represents a kind of defects.Comparing with the retired components and the KDP crystal irradiated by 11.50 J/cm2,the new band at 231.55 nm emerges in the spectra of KDP crystal irradiated by 9.00 J/cm2.The intrinsic luminescence band is assigned to the radiative annihilation of self-trapped excitons(STEs).According to our previous work,the short chain structures mainly exist in the KDP crystal irradiated by 9.00 J/cm2,and the medium chain structure mainly existed in the KDP crystal irradiated by 11.50 J/cm2.The retired components have short,medium and long chains compounds.The length of P-O bond in the short chain is shorter than that in the long chain structure.The overlap between phosphorus 3s orbitals and oxygen 2p is increasesd,and thus the radiative annihilation of STEs is enhanced.Therefore the band at 231.55 nm appeared in the spectrum of KDP crystal irradiated by 9.00 J/cm2.It suggests that the structure of the retired component and KDP crystal irradiated by 9.00 J/cm2 are different.In the theoretical calculations,the electronic structure and optical properties of KDP crystal with oxygen vacancy are investigated with the first-principles density functional theory calculations.The optimized structural parameters of the defective KDP crystal are analyzed.The structure of PO4 tetrahedron with O vacancy is distorted and its symmetry is broken.The band gap of the defective KDP crystal with O vacancy is about 2.75 eV which is lower than the perfect KDP crystal.It is because of that the O vacancy defect states located near the Fermi level.Moreover,more peaks appear in the low-energy region(lower than 5 eV)in the curves of the optical properties for the defective KDP crystal.And the defect KDP with oxygen vacancy has new optical adsorption within the wavelength region from 200 nm to 400 nm.As a result,the oxygen vacancy can decrease the damage threshold of KDP crystal.These work is summarized and the future of research is prospected in the end.