Study On The Growth And Properties Of(D)ADP Crystals

Potassium dihydrogen phosphate (KH2PO4, KDP) and ammonium dihydrogen phosphate (NH4H2PO4, ADP) crystal, is the first materials exploited in nonlinear optical and electro-optic areas, and still widely used today. Study of the electro-optic effect could date back to 19th century, while the study on nonlinear optics started in 1960s, after the appearance of laser technique. The conversion efficiency of second harmonic has been dramatically increased by using the technique of phase matching in ADP crystal. Thereafter, due to the large nonlinear optical and electro-optical coefficient, high laser damage threshold and transmittance, KDP and ADP crystal has been widely used in the high power laser system, realizing the second harmonic, the third harmonic and the fourth harmonic. Besides, their easiness to grow large, high quality crystals has highly increased the studies on them, including both fundamental and application research. And also made KDP-type crystal the only nonlinear optical material used in the Inertial Confinement Nuclear Fusion (ICF).ADP crystal was earlier used in the study of second harmonic generation, but the subsequent study in the harmonic generation was focused on KDP crystal. One reason is that ADP crystal is less transparent in the near infrared region, affected by the N-H absorption band. With the rise of fourth harmonic generation, ADP crystal has gradually show better properties. In one side, ADP crystal is more suitable than KDP to grown by rapid growth method, with adequate homogeneity in crystal. In the other side, the conversion efficiency of fourth harmonic in ADP is higher than DKDP crystal. More important, the laser damage threshold of ADP is much higher than KDP, while the low LDT has become a bottleneck that restricting the development of ICF. Based on the above advantages of ADP, crystals were grown by different methods and then used in thermal and optical research. The growth characteristics and properties has been intensively studied to investigate the application prospect in laser harmonic area.DKDP crystal can effectively reduce the stimulated Raman scattering caused by high power laser, making it used as third harmonic generation crystal in ICF. For ADP crystal, deuterium can increase the infrared cut-off edge and the transmittance in the near-infrared region. So DADP crystals with different deuterium contents were grown by traditional lowering temperature method to investigate their structures and properties. The rapid growth 70% DADP crystals grown under different supersaturation were also studied for the influence of supersaturation on the deuterium distribution.In this paper, we have grown (D)ADP crystals by different methods and investigated the optical properties, thermal properties and electrical properties of them. Moreover, we have studied the effects of deuterium on crystal structures.1. After comparing the difference of KDP and ADP in the crystal growth process, a series of (D)ADP crystals were grown by tradition lowering temperature method and point seed technique to study the characteristic of ADP growth.The solubility and structure are all different for KDP and ADP, leading to the distinct growth characteristic that ADP crystal can not grown on a KDP seed.When growing ADP by traditional method using a Z-cut seed, the plate is hard to recover its cap wafer; Even if the recovery is completed, the cap is always prolonged or packaged with solution. This problem can be solved by using the tetragonal pyramid cap as growth seed, by this way the growth cycle is also shortened. When growing ADP by point seed technique, the solution made by high purity material C has better stability while the solution made by extra purity material B has a weaker stability. Materials with higher purity have less metallic ion impurities and smaller inhibition for crystal growth. In the growth of 70% DADP crystals with different supersaturation, the sharp of crystals become short with increasing supersaturation. Microscopic study on the (101) face of ADP crystal showed that large hollow core can easily formed on the center of growth hillock. Thus the solution can enter into it and form the inclusion, that can influence the normal growth on the pyramid face.2. Using the technique of measuring rocking curve, power XRD, power neutron and phase diagram, we have confirmed the crystalline perfection of these crystals and then studied the influence of deuterium on DADP crystal structure. The emphasis is putting on the distribution of deuterium over ammonium group and dihydrogen phosphate group.Lattice parameters are obtained from the power XRD and power neutron diffraction. Parameters along a direction increase with the deuterium content while parameters along c direction decrease with it. The c/a ratio decrease regularly with the deuterium content, as the difference between the two directions decreased. The deuterium content in NH4+ group is higher than that in H2PO4- group, and the difference also decrease with deuterium content. According to the deuterium content in solution and crystal, the distribution equation of DADP and two groups can be obtained. The Raman vibration peak of PO4 tetrahedron group at 925 cm-1 is shifted to lower wave number after deuterated, the intensity of it is also reduced. The Raman vibration peak of NH4 tetrahedron group at 3141 cm-1 is reduced, and a new peak revealed around 2245cm-1 with its intensity increased on deuterium content. Based on the shift number of 925 cm-1 peak and the distribution equation of H2PO4- group, we can find a simple and convenient way to estimate the deuterium content in crystal.3. The thermal properties of ADP, including specific heat, thermal expansion, thermal diffusion and conductivity, are measured and compared with KDP crystal. The electrical properties (electrical conductivity and dielectric constant) with its dependence on temperature are also studied.The measured specific heat value of ADP crystal is higher than KDP, and both of them is consistent with theoretical calculated values. Debye temperature of ADP is about 302 K while that of KDP is lower than room temperature. The thermal expansion ration of ADP along [100] direction is higher than [001] direction. The ration along [001] direction become negative as the temperature rises, making the difference between two directions increased. The thermal diffusion coefficient of ADP and KDP both decreased with temperature. The [100] direction value is higher than [001] direction while the value of KDP is higher than ADP. The variation tendency of thermal conductivity is in agreement with thermal diffusion.The electrical conductivities of DADP crystals increase slowly with temperature at beginning and suddenly increase after reaches the knee point. This variation attributes to two different conductive mechanism in ionic crystal, i.e. the intrinsic conductivity and impurity conductivity. The electrical conductivity increase with deuterium content while the activation energy decrease with it. The most important reason is that the tunneling frequency of deuterium among hydrogen bond is larger than hydrogen. The dielectric constant also increase gradually with temperature at lower temperature and dramatically increase after one temperature point.4. The optical properties of (D)ADP crystals such as refractive indices, nonlinear optic coefficient, transmission spectra, conversion efficiency of third harmonic generation and laser damage threshold, is studied systematically.Refractive indices of DADP and DKDP crystals decrease with the deuterium content and wavelength. The changing tendency of ordinary refractive indices and extraordinary indices diverged at 1529 nm. The ordinary indices increase with deuterium content while extraordinary indices unchanged at this wavelength. The birefringence of these crystals also decrease with the deuterium content and wavelength except for 1529 nm. Refractive indices and birefringence of DADP crystals are larger than DKDP, attributing to the different cations and environment. The calculated phase matching conditions based on variation of refractive indices changed with deuterium content and fundamental wavelength. Non-critical phase matching wavelength of these crystals increase with deuterium content. DADP crystals can realize the room temperature phasing with lower deuterium content. The measured nonlinear optic coefficient of ADP is higher than KDP, and decreased with deuterium content.The transmittance of ADP crystal grown by traditional method is better than that crystal grown by rapid method, while the sample in the cap region has higher transmittance than cylinder region, especially in the UV range. The cut-off edge of DADP crystals red-shifted after deuterated with the transmittance in the near infrafed region increased, affected by the change of N-H and O-H absorption band after deuterated. The increasement of transmittance for DADP crystals made it comparable to KDP crystal and dramatically enhanced its application as optical device.The third harmonic conversion efficiency and angular bandwidth of ADP crystal is comparable to that of KDP while the temperature bandwidth is narrower than KDP. The laser damage threshold of ADP is more than two times to KDP, and is decreased after deuterium while still larger than 70% DKDP. The laser damage threshold of ADP grown by traditional method is slightly high than rapid growth crystal. Crystals in different region have different thresholds:threshold for samples that close to the seed is relatively small while samples close to the cap have better value. The cut thickness and angle are also important for laser damage threshold. Samples with 8 mm thickness has larger thresholds than samples in 10 mm. Samples cut with the calculate angle has better performance than 59° cut one.