Study On The Growth And Properties Of K(D_xH_1-x)₂PO₄Crystals

Potassium dihydrogen phosphate (KDP) and its (partialy) deuterated form DKDP and K(H1-xDx)2PO4are the most popular nonlinear optical crystals with good performances. With the development of laser technology, KDP-type crystals have been widely used. KDP crystal, characterized by high damage threshold, good nonlinear optical coefficients and high transmission of wide-spectrum, is adopted to generate second harmonic for the conversion process1064nmâ†'532nm. The Transverse Stimulated Raman Scattering (TSRS) in tripler with KDP of ICF is the most probable destructive nonlinear effects. Therefore, the copropagating second harmonic and residual fundamental beams that emerge from the doubling crystal enter the70%-DKDP crystal, where a third-harmonic beam is created by sum-frequency mixing of the fundamental and second-harmonic beams (1064nm+532nmâ†'355nm). DKDP, with high electro-optic coefficients, are widely used as electro-optical modulators and Q-switches. Type I12%-DKDP crystal of which retracing point (i.e., wavelength) occures at1054nm can support efficient SHG over20nm bandwidth of the fundamental pulses in the10-mm-long crystal. At the moment, the growth and propertites of KDP and DKDP was investigated widely in the world. Those studies are rarely conducted in partially deuterated K(H1-xDx)2PO4, especially for K(H1-xDx)2PO4with low deuterated level. Therefore, in this work we systematically study on the growth kinetics, growth technology, optical properties and high-temperature transition of K(H1-xDx)2PO4with different deuterium content. The primary coverage is as follows:1. The dependence of growth rate and dead zone on supersaturation of (100) face of K(H1-xDx)2PO4with different growth condition and deuterium content by using laser polarization interference technique. The results are analysized by the theory of mass transport effect within the boundary layer as well as screw dislocation and two-dimensional nucleus models.Growth kinetics of K(H1-xDx)2PO4with different deuterium content was studied. The experimental results on the (100) face growth rate were fitted with screw dislocation and two-dimensional nucleus models. It indicates that the latter is more suitable for higher supersaturation while the former dominates at lower region. The growth rate and growth kinetics coefficient of DKDP is slightly larger than that of KDP. Dead zone and step free energy of K(H1-xDx)2PO4(deuterium concentration of solution y<90%) slightly decrease with deuterated degree. For highly deuterated K(H1-xDx)2PO4(y≥90%), these parameters decreased when the deuterated degree reduced.The results of growth rate of (100) face of K(H1-xDx)2PO4with different saturation temperature show that low saturation temperature broaden the dead zone of KDP and partially deuterated K(H1-xDx)2PO4. Conversely, DKDP have greater growth rate and kinetics coefficient of (100) face at lower temperature solution. This can explain the phenomenon that tapping and fatting occurs in KDP and DKDP as the temperature drops, respectively.The growth rate and dead zone of (100) face of KDP were measured with different pH by adding phosphoric acid (H3PO4). The results showed that the magnitude of dead zone reduce and transition region became wider, while the dead zone decreased to0.1℃with an increased acid content (pH-2.3).According to these parameters of growth of these crystals, a series of K(H1-xDx)2PO4were grown with different growth condition by using tranditional temperature-lowering and rapid growth methods.2. From the results of powder X-ray diffraction (XRD), it shows that the no changes occurred on symmetry of structure in K(H1-xDx)2PO4crystals, while lattice parameter a super linearly increases and lattice parameter c changes little with a increase of deuterium content of crystal. High resolution XRD was taken to characterize crystalline perfection. It is indicated that crystalline perfection in KDP-DKDP mixed crystals are as well as pure KDP and DKDP. It is also show that crystals grown by tranditional method and crystals grown in high temperature range by rapid method have good crystalline perfection.3. Refractive indices of K(H1-xDx)2PO4crystals, in which the range of x was from0to1, were measured in the visible region by V-prism method. The measured data revealed that both refractive index and birefringence decreased with the increasing of the deuterium content in K(H1-xDx)2PO4crystals. Furthermore, there is a good linear dependence of refractive index on deuteration level. In terms of numerical fits to Sellmeier formula were made, permitting the interpolation consistent with the experimental data. Refractive indices of deuterated KDP crystal with arbitrary deuterium content at varying wavelength are provided to predict phase-matching behavior of frequency doubling according to our data and formulas. Meanwhile, our results provide a potential method to quantify the deuterium content of K(H1-xDx)2PO4crystals by measuring the refractive index. According to longitudinal electro-optic effect of γ63, half-wave voltage of K(H1-xDx)2PO4crystals were measured. Results of linear electro-optic coefficient K(H1-xDx)2PO4crystals, based on the experimental values of half-wave voltage and refractive index, show that there is a linear relationship betweenty63and deuterium content of crystals.4. Isotopic effects on the optical properties in KDP-DKDP mixed crystals have been researched and compared with pure KDP and DKDP crystal. Transmission spectra were recorded in the region190-2700nm for the Z-cut and X-cut plates of K(DxH1-x)2PO4crystals. It can be found that the X-cut plates have higher transmission efficiency especially in near-infrared region than that of Z-cut plate derived from the same crystal. The deuterated crystals have higher transmission efficiency and the infrared absorption edges of DKDP red-shifted by0.6μm in comparision to KDP. Absorption coefficient at1064nm and infrared absorption edge both show monotone increasing with the increase of deuterium content. No absorption are observed at532nm in all crystals. At355nm in the UV region, there are some absorption which is not sensitive to deuteration concentration. The O-H/O-D stretching vibrations were recorded in the infra-red spectra and proposed assignments for the infra-red bands in K(DxH1-x)2PO4have been performed. At the same time, this indicates that hydrogen modes and the deuterium modes in crystals are essentially independent with each other. Six Raman peaks, arise mainly from internal vibration modes of the H2PO4anion in KDP, are observed in the frequency range200-1200cm-1. New peaks of715and965cm-1are found in K(DxH1-x)2PO4 (x≥29%), which are also observed in infrared spectrum. Raman spectra show that the frequencies of internal vibrations of PO4tetrahedrons is red-shifted caused by replacement of hydrogen by deuterium atoms, especially for v1(PO4). Dependence of v1(PO4) shift on the degree of deutration perfectly linear. Meanwhile, the scattering cross-section of intense Raman line can be reduced by deuteration and minimum occurred in70%-DKDP. This means that70%-DKDP, preventing laser damage caused by Transverse stimulated Raman scattering (TSRS), is more suitable to be as tripler in ICF.5. Optical properties of K(DxH1-x)2PO4were measured by conoscopic interference, scattering particles and laser damage threshold. Lower degree of perfection were observed in K(DxH1-x)2PO4mixed crystals in comparision to pure KDP and DKDP crystals. The R-on-1bulk threshold of K(DxH1-x)2PO4crystals are1.5-4.3times of the damage threshold with1-on-1test. It is proved that laser conditioning is an efficient way to improve the damage resistance. However, laser conditioning at1064nm made the damage threshold raised3-4times. Test at355nm with R-on-lmode shows damage threshold of all K(DxH1-x)2PO4crystals reached more than30(J/cm2,@3ns), clearly demonstrating the presence of a threshold fluence for conditioning. There is little difference in the damage resistance of transparent crystals taken from the first-and last-grown materials, respectively. Damage threshold of Z-cuts is about1.6times of one of tripler cuts (type II), indicating that dependence of bulk damage on the light is with the orientation effect.6. Measurements of dielectric constants and thermogravimetric analysis (TGA), microscopy were used to study high-temperature transition in K(H1-xDx)2PO4crystals. The data of TGA demonstrate that the onset of thermal decompositions occurs at478-481K in K(DXH1-X)2PO4crystals with different deuterium content. Our results show the existence of two kinds of phase transitions Tp and Tι, both of which are no business of thermal decomposition. The Tp is dependent on a change in hydrogen/deuterium bond, and this temperature drops with the increase of deuterium content of crystals. At the same times, a large number of cracks appears in K(H1-xDx)2PO4observed from microscopy. The Tι transition associated with H2PO4 group rotation was observed in KDP and12%-DKDP crystals. In the KDP-DKDP Mixed crystals(12%<x<55%), a new anomaly occurred below Tι), assigned to be Tι’, seems to be responsible for D2PO4group. A long crack, traversing the sample was observed in KDP above transition temperature by using microscopy, then crack grew. But for DKDP, several cracks appeared first and much more cracks appeared after keeping for10min which show milky white in eyes. Microscopy conoscopic interference figures of KDP and DKDP changed above temperature each Tp.