Study On The Growth And Properties Of DKDP Crystals

Inertial Confinement Fusion (ICF) is a kind of heat nuclear explosion that can be controlled in the laboratory, which will be one of the most promising methods for generating environmentally safe energy in the future. In the long tern, the ICF has the practical applied prospect in overcoming the shortage of energy for human being. The frequency conversion crystal is an important optical element in the high power laser system for ICF. At present, a large number of novel nonlinear optical crystals are constantly emerging. However, KDP and DKDP crystal is only one crystal with high laser induced damage threshold and large size that could meet the requirement of ICF research. The advantage of DKDP crystal used as third harmonic generation (THG) crystal in ICF project is for minimizing the stimulated Raman scattering caused by high power laser. Whereas the intrinsic breakdown of pure material is one order of magnitude above the maximum laser energy considered in these facilities, experiments showed that laser damage actually occurs at fluences below the maximum energy delivered. This problem greatly limits the fluence of output laser and the crystal’s useful life. The way to improve the laser induced damage resistance of as-grown crystals is by far the most challenging problem for DKDP performance.Laser induced damage of optics has a very complex process, which decided by both laser parameters and properties of material. The physical properties of optical material have an obvious influence on the laser damage. Although the relationship between the properties of material and laser damage has not been cleared yet, some results have been widely confirmed. The study on the properties of material and the physical parameters of optical element is important for forecasting possibility of laser induced damage. In present work, a series of DKDP crystals with different deuteration levels and80%DKDP crystals grown with different KH2PO4raw materials by two growth methods were prepared. The mechanical, thermal, electrical and optical property of DKDP crystal were measured and analyzed with its structure for investigating the effect of material on laser damage of DKDP crystal for THG. The behavior of laser damage with specified laser parameter was investigated to provide references for users.1. A series of DKDP crystals with different deuteration levels were rapidly grown by point seed technique. In addition,80%DKDP crystals were grown by both traditional method and rapid method with different raw material. The growth phenomenon of these crystals was described and summarized. The cell parameters and crystalline perfection of DKDP crystals with different deuteration levels were measured.The stability of growth solution prepared with high pure raw material is poor with respect to other two low pure materials. The content of metalline ion impurities in rapid growth crystal is obviously higher than that of crystal grown by traditional method. The distribution of metalline ion impurities in crystal grown by traditional method is uniform. The content of impurities in prismatic sector of the rapid crystal is higher with respect to pyramidal sector. The crystalline perfection of crystals with low deuteration level and high deuteration level is inferior compared to that of the crystals with in-between deuteration level.2. The Vickers hardness of (100),(001), and so called "tripler" faces for DKDP crystals with different deuteration levels were measured with load in the range from5to100g. The indentations made by different loads or indented on different faces were observed. The formation of cracks was analyzed. Initially the hardness number of (001) face for each crystal increases with the increase of the applied load until it reaches25g. With further increase in load, the hardness number decreases gradually. The hardness numbers decline with the increase in deuterium content. These composition dependences are expected since the bond strength is weakened by the substitution of deuterium for hydrogen. The hydrogen bond is considered to play the key role in effecting the crystal’s hardness. The visible hardness anisotropy of the different faces is attributed to the inhomogeneous distribution of the oxygen-hydrogen bond on these faces.3. The thermal parameters such as specific heat, thermal expansion and thermal conductivity of DKDP crystals with different deuterium contents were measured. The results show that the effect of deuteration level on the thermal property of DKDP crystal is obscure influenced by growth conditions. In test temperature range, the measured specific heat of K(H1-xDx)2PO4crystals is in accord with the value calculated on the basis of Dulong-Petit law and Neumann-Kopp law. With the increase of deuterium content, the phase transition temperature and the phase transition latent heat of DKDP crystal show a decreasing trend. Moreover, the thermal expansion ratio (c/a) is increasing. It means that the mechanical anisotropy of DKDP increases with increasing deuteration level.4. The electrical conductivity, dielectric constants, elastic constants, piezoelectric constants of DKDP crystals with different deuteration levels were measured.The electrical conductivities of these crystals were measured along a and c directions at room temperature. The electrical conductivity increases with the increase for deuterium content. Also the electrical conductivities of certain crystals were measured at various temperatures ranging from20to130℃. The values of activation energy decrease as the increase of deuterium content. The present study indicates that the deuterium tunneling frequency is smaller than that of hydrogen, which may be the reason why the variation of electrical conductivity happens after the substitution of deuterium for hydrogen in KDP crystal.With the increase of deuterium content, the dielectric constants of these DKDP crystals increase gradually. The dielectric constants along a direction are larger that those along c direction. The dielectric constant along a direction increases more rapidly than that along c direction. Moreover, the dielectric constant increases with the increase of frequency of applied electrical field. Similarly, the dielectric constant along a direction increases more rapidly.The elastic compliant constant s11of KDP crystal is measured and certified approximately as31pm2N-1by many tests, which is different with the values reported previously. In all elastic constants, the s11, s44, s66are larger than the others. With the content of deuterium increasing, s11, s44, s66gradually increase respectively, whereas s12, s13, S33have no demonstrable variation. The d36of DKDP crystal increases with the increase in deuterium content, and this trend for d14is slight. The length of the P-O bond increases with increasing deuterium content, which may be the cause of the variation of piezoelectric properties with the increase in deuterium content. The piezoelectric stress constant e14has no change, while e36increases with increasing content of deuterium. The electromechanical coupling coefficients kij follow a similar rule.5. The transmission spectra of DKDP crystals with different deuteration levels and80%DKDP crystals are measured. With the increase of deuterium content, the transmission spectra become wide and the infrared regions shift to longer wavelength. This variation arises from the substitution from Hydrogen to Deuterium in bonds with oxygen atoms. For80%DKDP crystal, the transmission in the UV region of crystal grown by traditional method is clearly higher than that of crystal grown by rapid technique. Moreover, the transmission in the range from800to1900nm of crystal grown by traditional method is wider than that of crystal by rapid method.The polarized Raman spectra of DKDP crystals with different deuterium concentrations were measured. With the increasing deuterium content, the red-shift of the Raman peaks which are assigned as the internal vibrations of the (H/D)2PO4-anion is contributed to the decrease in the bonding force of P-O bond influenced by the substitution of deuterium for hydrogen. Moreover, the intensity of the strongest peak of these crystals decreases first, and increases with the increasing deuterium concentration. It reaches its minimum value while the mole percentage of the deuterium content in the crystal is about74%.The laser induced damage thresholds of z cut and tripler cut samples for DKDP crystals with different deuterium contents were measured by1-on-1test at355nm. Laser induced damage threshold of z cut sample is higher than that of tripler cut sample. The intrinsic mechanical anisotropy of DKDP crystals and the existence of non-spherical absorbers are speculated to analyze the orientation dependence of laser induced damage threshold. In addition, the laser induced damage threshold decreases with the increase of deuterated degree. The dependence of laser induced damage threshold on deuterium content is mainly due to the decline of bond strength in primary crystallographic directions with the increase of deuterated degree. At constant growth temperature, damage resistance in DKDP is fairly independent of raw material with the mass content of main metallic ionic impurity below1ppm. However, DKDP crystal grown with high pure material has an excellent homogeneity. The samples cut from pyramidal head region have higher laser induced damage threshold with respect to those cut from other regions.The1064nm laser damage morphology consists of three distinct regions:a core, some oriented cracks spreading from the core, and a region of modified material surrounding the core. The size of the core is about2-30μm. Most of the355nm laser damage sites consist of two distinct regions:a core and a region of modified area surrounding the core. A few of355nm laser damage sites contain the oriented crack, which occurred frequently in1064nm laser damage sites. Moreover, the morphology of laser damage sites in single crystal is dependent on the crystallographic symmetry. The Raman tests show that the material in the damage site has been modified from the bulk material. However, the results are not clear and need to be studied further.