| The laser inertial confinement fusion(ICF)engineering is one of the effective approaches for obtaining clean fusion energy and has attracted widespread attention due to its important value in nuclear explosion simulation,high-energy density physics,and other scientific frontiers.Potassium dihydrogen phosphate/deuterated potassium dihydrogen phosphate(KDP/DKDP)crystals,characterized by low half-wave voltage,large electro-optic coefficient,wide transmission band,and high laser induced damage threshold,as well as the ability to grow large-size single crystals with a diameter exceeding 500 mm,have become the only nonlinear optical crystal material used in ICF engineering.Currently,KDP crystals are used to fabricate electro-optic switches in ICF laser systems.With the development of ICF engineering,laser systems are evolving towards high power and high integration,which impose higher requirements on the performance of optical components.KDP crystals will meet the demand for electro-optic switch performance in ICF engineering.Compared with KDP crystals,high deuterated DKDP(98%DKDP)crystals exhibit superior electro-optic performance,and the latest generation of Laser Inertial Fusion Energy(LIFE)engineering in the United States explicitly proposes the use of high deuterated DKDP crystals to replace KDP crystals.At the same time,efficient fourth harmonic generation at 266 nm ultraviolet laser has been achieved using high deuterated DKDP crystals.However,large-size high deuterated DKDP crystals also face challenges of high growth cost and difficulty,making the development of rapid growth technology for large-size,high-quality high deuterated DKDP crystals an urgent need.(Unless otherwise specified,all DKDP crystals mentioned in the following text refer to high deuterated DKDP crystals.)This paper systematically studies three major problems faced by rapid growth DKDP crystals:low laser-induced damage threshold due to nonlinear absorption,performance heterogeneity caused by pyramid-prism boundary,and crystal cracking.Firstly,small-size crystals are grown in solutions with different pD values,and medium-size crystals with different sizes are grown under the same pD value conditions(For aqueous solutions,the acidity of the solution is usually characterized by the concentration index of H",named as pH value,which is calculated as pH=-lg[H+];while in high deuterium DKDP solution,the form of H element is D+,so the pH value is defined as pD value,and the corresponding calculation formula is pD=lg[D+]).The effects of different growth solution pD values on crystal growth and nonlinear absorption are investigated using single crystal X-ray diffraction(XRD),Raman spectroscopy,and Z-scan.The performance inhomogeneity of medium-size crystals is systematically studied from the perspectives of transmittance,dielectric constant,crystallinity,and deuterium content,confirming the process conditions for the growth of small and medium-size DKDP crystals,thereby providing reference for the optimization of rapid growth process of large-size DKDP crystals.During the study,it was found that some prepared samples also exhibited varying degrees of cracking.The cracking phenomenon and its causes are analyzed using scanning electron microscopy(SEM),Raman spectroscopy,high-resolution X-ray diffraction(XRD),macroscopic and microscopic mechanical property testing,and thermal properties,providing reference for the engineering application of crystals.The main research contents of this paper are as follows:1.The synthesis and crystal growth process of DKDP growth solution are described in detail,and the effects of solution pD value on solution composition,crystal growth,crystal structure,and deuterium content in crystals are investigated.Firstly,DKDP crystal growth solutions were synthesized,and the saturation point-concentration curve and densityconcentration curve of series deuterium content crystal growth solutions were determined.Based on the above results,the correlation between solution deuterium content,saturation point,concentration,and density parameters were confirmed.The relationship between pD value and D2PO4-component in the solution was studied by titration experiment,and the suitable pD value range for crystal growth was confirmed to be 2.9-4.5.Small-sized DKDP crystals with series pD values(2.9,3.3,3.8,4.3)were grown using the point-seed rapid growth method.Single crystal XRD results showed that the pD value does not affect the crystal lattice structure,while Raman spectroscopy results indicated that the sample with a pD value of 4.3 is more prone to hydrogen-deuterium exchange.Spectral and inductively coupled plasma mass spectrometry(ICP-MS)results showed that at a pD value of 3.3,the prismatic region are more prone to adsorption of impurity Fe ions.Comprehensive analysis suggests that a pD value of 3.8 is the optimal growth condition.2.The nonlinear absorption coefficient β of DKDP crystals was measured using the Z-scan technique with picosecond laser,and the effects of laser wavelength,laser fluence,sampling region,growth solution pD value,and crystal cut on the nonlinear properties of DKDP were studied.The results showed that the nonlinear absorption at 1064 nm is very weak,with a maximum decrease in normalized transmittance of only 2%,which can be considered as normal fluctuations caused by sample motion and laser stability and can be ignored.The nonlinear absorption at 266 nm is due to two-photon absorption,with a relatively large nonlinear absorption coefficient on the order of 10-1 cm/GW.The nonlinear absorption coefficient increases with decreasing laser fluence.For crystals with the same pD value,the nonlinear absorption coefficient of the prismatic region sample is larger than that of the pyramidal region sample.The crystal cut has a significant influence on the nonlinear absorption,with the ranking of the nonlinear absorption coefficients for samples in the same sampling position(referring to pyramidal and prismatic region)as Z-cut direction>fourth-harmonic generation cut direction.Based on the above results,the optimal sampling position for fabricating fourth-harmonic generation frequency conversion devices should be in the pyramidal region.3.The transparency and dielectric constant inhomogeneity of medium-sized DKDP crystals were investigated,and the performance inhomogeneity of pyramidal and prismatic region in the crystal and the reasons for its occurrence were analyzed from the perspectives of crystal structure and deuterium content using single crystal XRD,high-resolution X-ray diffraction(high-resolution XRD),conical interferometry,Raman spectroscopy,and other methods.This provides a reliable reference for the growth and selection of large-sized DKDP crystals.The main conclusions obtained are as follows:In the visible and infrared wavelength ranges,the transmittance of Z-cut samples of 3 mm and 10 mm thick DKDP crystals exceeds 90%,and there is no difference between the pyramidal and prismatic region.As the thickness increases,the influence of the pyramid-prism boundary gradually increases at 355 nm.The transmittance curve of the thick sample at 261 nm shows an absorption peak compared to the transmittance curve of the thin sample.This is because as the thickness increases,the accumulation of a small amount of metal ions adsorbed in the pyramidal region begins to have a noticeable effect on linear absorption.The transmittance in the inhomogeneity test shows similar characteristics for samples of different thicknesses,and the linear transmittance of the prismatic region at 261 nm is only 20%-40%,indicating that the prismatic region should be avoided when sampling for the preparation of quadruple frequency samples.The statistical results of the relative dielectric constant ε33 for the entire Z-cut sample show an average value of 56.56,and the distribution in the pyramidal and prismatic regions is uniform without any significant variation characteristics.Therefore,when preparing switches,sampling can be collected from both the pyramidal region and prismatic region.The high-resolution XRD test of the Z-cut DKDP crystal shows that the distribution range of the half-width along the X-axis is 19"-27",and the overall half-width of the pyramidal region is smaller.The interference fringes in the pyramidal and prismatic regions of the conical interferogram show no significant distortion,and the interference fringe density is consistent.These results indicate that the lattice integrity in the pyramidal and prismatic region is relatively high,and the crystal quality is good.The crystalline quality of the pyramidal region is higher than that of the prismatic region,which is caused by the adsorption of trivalent metal impurity ions on the prismatic region.XRD and Raman spectroscopy test results both show that the deuterium content of the Z-cut DKDP crystal is distributed in the range of 94.27%-95.41%along the X-axis,with slightly higher deuterium content in the pyramidal region compared to the prismatic region.4.A study on the phenomenon and causes of cracking in DKDP crystals was conducted.The microscopic morphology of the crystal cracks was observed,revealing that the cracks on the(001)plane are oriented at a 45° angle to the a and b axes,while on the(100)plane,they are distributed along the[001]direction.The depth of the cracks is typically in the nanometer range,with widths ranging from a few micrometers to tens of micrometers.Raman spectroscopy and nanoscale infrared spectroscopy were employed to investigate the causes of the cracks.It was found that there is a significant decrease in deuterium content along the crack propagation direction,with a more pronounced decrease closer to the crack.Furthermore,in the direction perpendicular to the crack extension,the deuterium content is lower near the crack endpoints.This indicates that hydrogen-deuterium exchange is one of the inducers of cracking.Additionally,high-resolution X-ray diffraction(XRD)results revealed that a higher defect concentration is also a contributing factor to the occurrence of cracking.The results of thermal expansion coefficient testing showed that the average thermal expansion coefficient a,for the pyramidal region samples is 2.32×10-5 K-1,while for the prismatic region samples,it is 2.36×10-5 K-1,with a difference of 1.72%.When the crystal temperature changes,the mismatch in thermal expansion coefficients between the pyramidal and prismatic regions leads to differential deformation,ultimately resulting in cracking in the prismatic region.Nanoindentation testing and macroscopic mechanical parameter testing were conducted to characterize the compressive strength,elastic modulus,and tensile strength of the DKDP crystal along the Z-axis,XY-plane,and fourth-harmonic generation direction.The results demonstrated that the tensile strength is significantly lower than the compressive strength,with the lowest tensile strength of 2.535 MPa observed in the XY-plane,indicating that the DKDP crystal is more prone to tensile failure,particularly in the XY-direction.When H-D exchange occurs,the O-D-O bonds in the prismatic region’s crystal lattice fracture when the tensile stress reaches the fracture strength,resulting in macroscopic cracking.Crystal damage testing results revealed that cracking leads to a significant decrease in the laser-induced damage threshold of the crystal,with most of the damage occurring on the surface.Surface damage also induces the formation of cracks.Analysis of the relationship between cracking and temperature and humidity confirms that cracking is influenced by both factors.However,the current results are not sufficient to resolve the issue of cracking,and further research is still needed to address this critical problem. |
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