Study On Electro-Optic Properties And Growth Kinetics Of K(H_1-xD_x)₂PO₄ Crystals

Potassium dihydrogen phosphate（KDP）and potassium deuterium phosphate（DKDP）are excellent functional crystals with piezoelectric,dielectric,electro-optic,and nonlinear optical properties,which have been extensively used since 1930s.Owing to their subnanosecond switching capabilities and large electro-optic coefficient,they have widely application in modern optics and laser technology as high-speed optical shutter,optical intensity and phase modulators with the emergence of laser.Large aperture KDP is employed in Pockels cell of Inertial Confinement Fusion（ICF）device as electro-optic crystal.It has the advantages such as large electro-optic coefficient and laser damage threshold,high transmittance and optical uniformity,and large size single crystal easily grown.High deuterated DKDP has larger electro-optic coefficient and higher transmittance at fundamental frequency than KDP.However,amount of heavy water with high cost is consumed for highly deuterated crystal via metastable phase growth.The growth solution synthesized by raw materials through reactions contains many types of impurity ions,which increases the growth difficulty and reduces crystal optical quality.Partially deuterated DKDP can lighten the above issues.Moreover,its electro-optic coefficient is larger and transmittance at fundamental frequency is higher than those of KDP.However,there are few reports on growth kinetics and variety of electro-optic coefficient with deuterium.Study on electro-optic properties of K(H_1-xD_x)₂PO₄ crystals with varied deuterium can provide a reference for the feasibility of partially deuterated DKDP as ICF electro-optic crystal.The growth kinetics research can provide guidance for large size K(H_1-xD_x)₂PO₄ crystal growth.The prismatic face growth kinetics of K(H_1-xD_x)₂PO₄ crystals with varied deuterium have been studied using laser polarization interferometry.The voltage dispersion of KDP generated by end-plate ring electrodes and variation of K(H_1-xD_x)₂PO₄ half period voltage and electro-optic coefficient with deuterium have been studied in 763 longitudinal electro-optic modulation.The study on transmittance,Raman spectra,laser induced damage threshold,conductivity and dielectric constant provides assistance for application of crystals.The main contents of this paper are as follows:1.K(H_1-xD_x)₂PO₄ crystals with varied deuterium were grown by conventional and rapid methods,respectively,which providing the basis for electro-optic properties and related investigation.The growth profile of deuterated crystals is consistent with that of KDP,but the growth habit at different stages varies with deuterium.The growth rate of highly deuterated DKDP crystals by conventional and rapid methods is slow at the initial stage.The crystals are easy to expand in the later stage of conventional growth,and heterocrystals are generated easily in the middle stage of rapid growth.The growth rate of crystals with medium deuterium grown by rapid method is fast,but the inclusion easily forms in crystal which reduces the growth success rate.The conventional growth crystals are easy to tapering at low temperature.The above phenomena indicate the necessity of study on growth kinetics of crystals with varied deuterium.2.The prismatic face normal growth rates of K(H_1-xD_x)₂PO₄ crystals with varied deuterium were measured using laser polarization interferometry.The results show that there exists obvious dead zone,transition and linear region in growth rate curve.The dead zone and growth rate vary with deuterium and temperature.The dead zone decreases and the growth rate increases,respectively,with the increasing of temperature at fixed deuterium.The dead zone of KDP is relatively large at 35℃,which may be due to the difficulty in breaking hydrogen bonds in（H₂PO₄）^-·H₂O hydrated ions at low temperature.The hydrogen proportion in 12%and 30%DKDP lattices with low deuterium is large,and the lattice constant is relatively small.The growth units containing deuterium require a large driving force to form the lattice,and the dead zone is also relatively large.For 87%DKDP crystal,the dead zone is large and the growth rate in linear region is slow relatively,which is limited by diffusion velocity.There are many growth units in diffusion boundary layer,and the diffusion velocity is fast when growing 45%-75%DKDP crystals,therefore,the dead zone is small and the growth rate in linear region is fast relatively.The kinetic coefficients of crystal with varied deuterium are small and the growth rate is relatively slow at low temperature.3.The voltage-transmittance curves of KDP crystals with end-plate ring electrode configuration were measured between two crossed or parallel polarizers using γ63 longitudinal electro-optic effect.The variation of refractive index ellipsoid caused by electro-optic effect,photoelastic effect and residual stress has been analyzed.The difference of voltage parameters between theoretical and experimental voltage-transmittance curves has been discussed.The dispersion of half period voltage has been investigated.A method for quantitative characterization of residual stress has been proposed on the basis of deviation voltage combined with theoretical analysis.The residual stress moves voltage-transmittance curve along horizontal ordinate.Half period of theoretical and experimental curves corresponds to longitudinal half wave voltage Uπ and half period voltage Ui,respectively.Half period voltage Ui is always less than half wave voltage Uπ due to nonuniformity of electric field distribution.The experimental voltage transmittance curve is transverse stretched from theoretical curve.The results demonstrate that loading voltage is close to the integration of electric field intensity in crystal along optical path when the ring electrodes have large outer radius R,small inner radius r and long-distance d.The half period voltage Ui is also close to longitudinal half wave voltage Uπ in these circumstances.The unclamped electro-optic coefficient γ63σ of KDP crystal at room temperature has been measured to be 10.24±0.05 pm/V at the wavelength of 632.8nm.4.The voltage-transmittance curves of K(H_1-xD_x)₂PO₄ crystals with varied deuterium grown by conventional and rapid method were measured in crossed and parallel polarization states using 763 longitudinal electro-optic effect.The results demonstrate that half period voltage decreases and electro-optic coefficient increases with the increasing of deuterium in crystal.The half period voltage of 98%DKDP crystal is about 44%of that of KDP crystal.The unclamped electro-optic coefficient γ63σ of 98%DKDP crystal is about 2.3 times greater than that of KDP crystal.The half period voltage of K(H_1-xD_x)₂PO₄ crystals has been fitted with deuterium linearly and parabolically.The unclamped electro-optic coefficient has been fitted in same way.A method for quantitative charactering deuterium in crystal via electro-optic effect has been proposed.The residual stress of prismatic sector in rapid growth crystal is higher than that of pyramidal sector,and is generally higher than that of conventional growth crystal with same deuterium.The distribution uniformity of residual stress in prismatic sector is higher than that in pyramidal sector.5.The transmittance,Raman spectra,laser damage threshold,and electrical properties of K(H_1-xD_x)₂PO₄ crystal have been investigated,which can provide reference for the feasibility of partially deuterated crystals in ICF Pockels cells.The infrared absorption cut-off edge of K(H_1-xD_x)₂PO₄ crystal increases gradually with deuterium increasing.The transmission range broadens and transmittance at fundamental frequency light increases as deuterium increases,which is conducive to improving the laser conversion efficiency.The vibration peak of PO4 group in Raman spectra shifts to the direction of short wavenumber with the increasing of deuterium.The strongest scattering peak splits and the intensity decreases when the deuterium exceeds 45%,and thus the transverse stimulated Raman scattering effect weakens.The laser induced damage threshold of crystals at fundamental frequency light is higher than that of double frequency light.The laser induced damage thresholds of deuterated crystals at fundamental and double frequency light mostly exceeds those of KDP crystals.The pyramidal sector in rapid growth crystal has strongest resistance to laser damage at fundamental frequency light among different sampling location.The laser indued damage threshold of crystal in R-on-1 mode is higher than that in 1-on-1 mode due to laser subthreshold annealing effect.The dielectric constant of crystal increases with deuterium increasing.The conductivity of crystal with varied deuterium is lower than 55.7 GΩ^-1·m^-1.