| Potassium dihydrogen/deuterium phosphate(KDP/DKDP)crystal is a multifunctional crystal material with piezoelectric,dielectric,electro-optic,elastic-optic and nonlinear optical properties.The most remarkable feature of KDP/DKDP crystal is that it can grow very large single crystals,which can meet the requirements of inertial confinement fusion(ICF)device.However,in the process of practical application,under the condition of high power,the crystal may suffer from volume damage,further restricting the development of ICF engineering.The process of bulk damage in KDP/DKDP crystal is complex.Although damage models such as "defect assisted multiphoton absorption" and "nano particle induced absorption" have been proposed one after another,the mechanism of crystal damage has not been clearly understood.Fortunately,post-treatment such as thermal annealing and laser sub-threshold annealing will improve the laser-induced damage threshold(LIDT)of the crystal.At present,the relationship of micro defects,nonlinear absorption and laser damage has not been completely clarified.It is very necessary to systematically study,which will help to understand the mechanism of crystal damage and contribute to the practical application of large-size crystals.The structural information of rapid grown KDP/DKDP crystal was studied by infrared spectroscopy(IR)and Raman spectroscopy.The concentration of micro defects in the crystal was studied by photoluminescence(PL)spectroscopy,and the types of micro defects in the crystal were identified.The nonlinear absorption characteristics of the crystal were explored based on picosecond Z-scan technique at 532 nm.Combined with the LIDT results of the crystal,the structural changes of the crystal,the effect of micro defect concentration on the 532 nm laser damage of the crystal,and the relationship between the possible nonlinear absorption and the crystal damage properties have been analyzed and discussed.The related research contents hope to provide assistance for the understanding of damage mechanism and practical application of KDP/DKDP crystal.The main contents of this paper are as follows:a)Type Ⅱ samples were obtained from the same KDP single crystal grown by the "point seed" technique under different temperature range and supersaturation.Some samples were annealed at 160℃ for 24 hours.The results of PL emission spectra show that the fluorescence intensity increases with the rise of supersaturation and growth temperature range,which indicates that the concentration of micro defects in the KDP is abundant.The damage threshold conducted using 532 nm nanosecond laser pulses.The result shows that LIDT decreases with the rise of PL intensity,showing a negative correlation with micro defects concentration.The LIDT has been improved with the decrease of growth temperature range and supersaturation,and its variation has the highest correlation with the concentration of hydrogen vacancy defect.In addition,thermal annealing can effectively reduce the concentration of micro defects in the sample,and LIDT of the samples can be enhanced further after laser sub-threshold annealing.b)KDP single crystals doped with Zn2+were prepared.The content of Zn of KDP crystal was detected using inductively coupled plasma-optical emission spectrometer(ICP-OES).Small differences were found in Zn content of pyramidal and prismatic samples.The PL spectra analysis indicate that the concentration of micro defects of all doped pyramidal samples is lower than that of un-doped samples as well as doped prismatic samples.After thermal annealing,the intensity of PL spectrum of doped sample decreases not obviously compared to un-doped samples.The LIDT results show that the un-doped prismatic sample has relative the lowest LIDT of 3.59 J/cm2 under the condition of 1-on-1.The LIDT of prismatic sample doped with 1000 ppm Zn2+reaches 13.88 J/cm2 in the test mode of R-on-1 after heat treatment,which mainly is attributed to the relative low concentration of micro defects.c)KDP single crystals doped with trace dichromate(Cr2O72-)and their 532 nm nonlinear absorption and laser damage properties were systematically grown and studied.The IR spectra suggest that the crystal network becomes compact with trace of Cr2O72-while the lattice of crystal also can be destroyed with excessive Cr2O72-doping.The UV-Vis spectra show that the transmittance percentage is descending for the doped KDP crystals.Z-scan analysis demonstrates that with increasing of Cr2O72-concentration,a gradual raise has been observed for the nonlinear absorption coefficient.The LIDT of grown crystal to 532 nm laser is found to be 28.29 J/cm2 of sample doped with 3000 ppm Cr2O72-which is higher than that of pure one under the R-on-1 model.However,as the doping concentration continues to increase,the LIDT decreases significantly.The variation of LIDT of doped pyramidal sample is negatively correlated with micro defects concentration.The LIDT of pyramidal sample is higher than that of prismatic one,suggests that the laser damage mechanism of the sample may be due to synergistic effect of the concentration of micro defects and nonlinear absorption.d)Filtration is one of the common methods to improve the optical quality of KDP single crystals.The experimental design grows poly(styrene)microspheres into KDP single crystals,and it is found that transmittance of tested crystal in the ultraviolet region is reduced,especially for the prismatic samples.IR tests showed that the crystal structure became loose.The results of PL and 1-on-1 damage test showed that with the rise of size of microsphere,the concentration of microdefects in crystal increased,and the corresponding LIDT decreased.As the pore size of the filter membrane decreases,the concentration of microdefects decreases,and the corresponding LIDT increases.The LIDT of the pyarmidal sample containing 300 nm microspheres before annealing was relatively low at 6.73 J/cm2 under 1-on-1.Under the condition of R-on-1,the LIDT of pyramidal sample grown under the unfiltered condition of the solution is 10.12 J/cm2,while the LIDT of pyramidal sample grown after the solution is filtered by a filter membrane with a pore diameter of 100 nm and thermally annealed is increased to 22.09 J/cm2.In the 1-on-1 test mode,the LIDT of pyramidal and prismatic samples has a similar change trend.In the R-on-1 mode,the LIDT changes of crystal samples are generally consistent with the regular pattern of 1-on-1.The structure and laser damage properties of the samples were improved after thermal annealing,which may be related to the decrease in the concentration of microdefects and the weakening of nonlinear absorption.e)0%,12%,30%,50%,70%and 87%-DKDP single crystals were grown by rapid grown technique.Based on spectral measurement and picosecond Z-scan technology,the structural changes and nonlinear absorption characteristics of crystals were studied.The results show that with the increase of deuterium content,the transmittance of crystal samples in ultraviolet region decreases,while the infrared cut-off edge increases gradually.At the same time,the crystal structure also becomes loose with the increase of deuterium content.PL spectrum analysis shows that the micro defect concentration in low deuterium DKDP samples increases with the rise of deuterium content,and 50%-DKDP reaches the maximum,while that in high deuterium DKDP samples is on the contrary.The nonlinear absorption results show that with the increase of deuterium content in the crystal,the nonlinear absorption coefficient of the sample increases gradually,but the increase is small.In the 1-on-1 mode,the LIDT of crystal samples decreases gradually with the increase of deuterium content,which may be related to the weak force of D-O bond.The micro defect concentration and d-o bonding force contribute greatly to the 532 nm laser damage of DKDP crystal,while the nonlinear absorption effect is relatively small.Laser sub-threshold annealing can significantly improve the LIDT of the annealed low deuterium pyramidal samples,especially for the prismatic samples of high deuterium crystals. |
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