Anion Doped Kdp Crystal Growth And Its Performance Impact

KDP/DKDP crystals are non-linear optical crystal materials with excellent optical properties. The investigation of crystal growth has been lasting for more than 70 years. The type of crystals has been one of the lasting water-soluble crystals and has been widely applied in the high technology fields such as laser frequency conversion, electro-optic modulation and optical rapid switch etc.In recent years, with the development of Inertial Confinement Fusion (ICF), the investigation of large KDP/DKDP crystals with high laser damage threshold and their optical properties has entered a new phase internationally. Although many new non-linear materials have been turning up constantly, considering the excellent optical properties and the requirement of large crystal, so far KDP/DKDP crystals are the only kind of crystals that meet the requirement of high-power laser system such as ICF. So the investigation of the growth of large KDP crystals has been paid attention to at home and aboard at present. The requirements of KDP/DKDP crystals in ICF include not only large size, but also their excellent optical qualities such as high laser damage threshold, low optical absorption, high optical homogeneity and wide transparent wave band.In order to provide enough crystals for ICF and enhance the usage ratio of crystals, now many countries are trying to develop the rapid growth technique, which has higher requirement for raw materials, growth devices and growth conditions. Now large KDP/DKDP crystals are mainly grown by the traditional temperature- lowering method in our country. The growth period is much longer and the growth is more difficult. The optical qualities are affected by many factors such as impurities, supersaturation, stability of the growth solution, the growth rate and so on. So it is an important subject to judge the affection mechanism of different impurities on the KDP crystals and to find the method of decreasing or eliminating the influence of the harmful impurities. From the literature reports, the investigation of the effect of cations on KDP crystals is much more than that of anions. These impurity ions have great effect on the growth habit and optical qualities of crystals.In this paper, the crystals were grown from the aqueous solution by the traditional temperature-lowering method and a "point seed" rapid growth technique. Potassium sulphate (K₂SO₄) potassium nitrate (KNO₃) and potassium chloride (KCl) were chosen as additives. The effect of SO₄^2-, NO₃^- and Cl^- on the growth habit, light scatter, transparent spectra, optical homogeneity and electric conductivity of KDP crystals were studied. Many measurement means such as ultra-microscopy, XRD and ICP were applied to study the light scatter, the element content of crystals. Meanwhile the mechanism and reason of the affection were also analyzed.First, the effect of SO₄^2-, NO₃^- and Cl^- on the stability of growth solution, the growth habit, stress and defects were studied. The stability of the solution is the basis of the growth of KDP crystals, which decides not only the regular growth of the crystals, but also the optical quality. It's seen from the experiments that the spontaneous crystals appeared in the growth solution and the stability of the solution was destroyed when the dopant concentration of three ions exceeded some value. At high dopant concentration of suphate, the KDP crystals showed many defects such as mother liquid inclusions, parasite crystals and cracks. From the result of XRD, the structure of KDP crystal didn't change after the doping of sulphate. The high nitrate dopant made the crystals' capping become difficult and the expansion of the prismatic sectors occurred.Scatter particles is an important parameter to evaluate the properties of the KDP crystals. Appearance of scatter particles can cause the light loss of the crystals and decrease the optical homogeneity of the crystals. Some results have proved that scatter particles is the latent factor of the low laser damage threshold of KDP crystals. And scatters also affect the transmission of the crystals. SO₄^2- ions had little effect on the light scatter of KDP crystals and the scatters have low density. With the increase of the dopant concentration, the density of scatter particles increased a little. For KDP crystals grown by the traditional temperature-lowering method, the existent of NO₃^- and CI ions seriously aggravated the light scatter of KDP crystals and the scatter particles had larger scale with high density. The effect on the light scatter of KDP crystals grown by a "point seed" rapid method was not evident.Transparent wave band of KDP crystals is generally thought from 0.176μm to 1.5μm, which can achieve double frequency and thrice frequency for the laser with wavelength of 1.064μm. Through analyzing the transparency of the dopant KDP crystals, we found that SO₄^2- ions affected the transparency of the crystal evidently. At high dopant concentration, KDP crystals' transmittance decreased a lot in ultraviolet band compared with pure KDP crystals; at low dopant concentration, the transmittance didn't have evident change. The transmittance of the crystals by the rapid method in prismatic sectors decreased rapidly in the ultraviolet band and the cutoff wavelength was about 300nm. The dopants of NO₃^- and Cl^- ions didn't have evident effect on the transparency of KDP crystals.The stress inside KDP crystals can decrease the homogeneity of crystals. The doping of sulphate made the crystals form many defects, which caused the stress of crystals and decreased the homogeneity to some extent. The homogeneity of KDP crystals were studied by measuring the conoscopic figure. The distortion of black cross can prove of the decrease of the homogeneity of crystals.Electric conductivity is a key index to evaluate the KDP/DKDP crystals, which decides the application of electro-optic switches of crystals. For KDP crystals doped with sulphate, its electric conductivity improved with the increase of the dopant concentration, which is subjected to have relation to the defects of crystals caused by SO₄^2-ions. The dopants of NO₃^- and Cl^- ions had no evident effect on electric conductivity of KDP crystals.In this paper, the author systematically studied the effect of SO₄^<sup>2-, NO₃^- and Cl^- on the growth and optical qualities of KDP crystals for the first time. Experiments showed that SOn₄^2-ions had great effect on the growth and properties of KDP crystals. SO₄^2- has tetrahedron configuration and strong H-bond combinability ability, which make SO₄^2-ions easily join into the KDP crystal lattices through H-Bond and electron attraction. Furthermore, the existence of small-sized NO₃^- and Cl^- seriously aggravated the light scatter of KDP crystals. So, we think that it is necessary to control the concentration of SO₄^2-, NO₃^- and Cl^- in the growth solution to obtain KDP crystals of high quality. The work of this paper is important to the growth of large KDP crystals and the theory of crystals growth.