Growth Of Gray Track Resistant KTP And Study Of The Formation Mechanism Of Gray Track

KTiOPO4 (KTP) has excellent nonlinear optical properties, which is widely used in the field of frequency conversion, electro-optical modulation and optical waveguide. However, when it is used as 1064 nm double frequency or OPO pumped at 532 nm at high power or high-repetition frequency, a macroscopic harmful absorption regionwill produce named "gray track". The formation of gray track will decrease nonlinear output power and generate lots of heat in the crystal, leading to serious distortion of laser beam. The existence of transition metal impurities and vacancy defects will lower the gray track resistant property of KTP, however, some ions such as Pb2+can increase the gray track resistant property. Therefore, we calculated the defect models with vacancy defects, beneficial ions Pb2+, Ce4+ and harmful transition metal ions Fe3+, Co2+, No2+, Cr3+, and Rh3+ substitutation. Based on the theoretical analysis and literature researches, we had grown gray track resistant KTP in the flux of PbO-self-flux system, and studied the mechanism of gray track formation from both views of Ti3+ and Vo. However, hydrogen annealing was also a method to produce gray track, and we studied the mechanism of hydrogen treatment in KTiOPO4 crystals at high temperature from experimental and first-principles. In addition, KTP is not only a nonlinear optical crystal, but also a piezoelectric crystal. There were not many reports about its piezoelectric property, and we measured and calculated temperature dependence of full set tensor properties using RUS method. Main contents and conclusions are as follows:(1) We studied the influence of the contents of PbO and the ratio of K/P for crystallization quantity using spontaneous nucleation method. When [PbO/K6]=0.3 and the ratio of K/P was about 1.5, the quantity of crystallization were the highest. High gray track resistant KTP crystals had been grown from a flux containing PbO using the top seed method, and a comparison of the properties was made between the crystals grown by us and the ones grown in self-flux.(2) A large number of potassium vacancies are produced during the process of growth, and some oxygen vacancies emerge in order to compensate the charge.These vacancies and imputies defects will affect the property of gray track resistance and application seriously. We studied the impact of vacancy defects and impurities on state density and optical absorption using the first-principles calculation.We calculated three structure models with different potassium vacancy concentration. The states in the conduction band became broader and the absorption increased gradually with the increase of potassium vacancy concentration; The researches showed that the flux with Pb2+ could improve the property of gray track resistance, and thus we calculated three defect models (Pb2+ substitutes K+, Ti4+ and P5+, respectively). The influence to the density of states of Pb2+ subsitituting for K+was very small, and thus it didnot induce an increasing optical absorption. After substitution, the edge of absorption moved to a lower wavelength; The previous studies showed that Ce4+was also a beneficial ion, and thus we calculated the desity of states and optical absorption of defect models with Ce4+ substitution. Ce4+ will substitute Ti4+ and shift the edge of absorption shorter; It is possible that there are transition metal impurities in the starting materials. In addition, Rh is also a common impurity existing in the Pt crucible. All the impurities will bring a bad impact to optical properties. We studied the influence to optical properties of impurities substitution using theoretical computation, such as Fe3+, Co2+, Ni2+, Cr3+ and Rh3+. Based on formation energy, valence state and ionic radius, we conclude that transition metal impurities will substitute for Ti4+ and bring in defect states between conduction band and valence band, and increase the optical absorption. Among them, the absorption edges moved to a higher wavelength of Fe3+ and Cr3+ substituting for Ti4+, and moved to a lower wavelength of Co2+, Ni2+ and Rh3+ substitution.(3) Based on theoretical analysis, we had grown gray track resistant KTP from a flux with PbO, and studied the mechanism of gray trak formation from two views of Ti3+ and Vo.Gray track was not easy to form in the crystal grown in the flux containing PbO. Gray track was produced in the crystal grown in K6flux with 532 nm CW laser. We studied the gray track region in situ using XANES spectrum, and confirmed the existence of Ti3+. EPR was used to further confirm the results of XANES.We had grown gray track resistant KTP crystal in a solution of [KTP]/[K6]=0.75 [PbO]/[K6]=0.3. The absorption of this crystal at 532 nm and 1064 nm and gray track resistant absorption were all far below the one grown in K6, and LIDT was higher than the one grown in self-flux.Positron annihilation lifetime spectroscopy and Doppler broadening were used to measure the concentration and type of vacancy in the three KTP samples. The crystal grown in the flux of K4-PbO had a lower concentration of vacancy. The shape parameters of S and W were nearly a straight line, which demonstrated that there was only one kind of vacancy defect (VK) in the crystal. However, the concentration of VTi and Vpwas very low.In addition, judging from the values of positron lifetime in defect states, we concluded that there were complex defects in our crystals. The dielectric measurements also confirmed the existence of oxygen vacancy. The crystal with a higher Curie point has a lower concentration of oxygen vacancy (closer to the stoichiometry), and also has a better property of gray track resitance.(4) It is reported that hydrogen annealing can also produce gray track defect. We studied the influence of annealing temperature, time and hydrogen concentration, and showed that temperature was the primary fator affecting the results. Only when the annealing time was long enough or the hydrogen concentration was high enough did the color change and absorption become obvious. There are two types of oxygen sites in KTP:O(1)-O(8) (in Ti-O-P chains) and O(Til)+O(Ti2) (in Ti-O-Ti chains). We considered four defect models:Vo from two kinds of oxygen atoms and OH by adding H atom bonding with two kinds of oxygen atoms. From the formation energy and the comparision between the experiment and calculation, oxygen vacancy from O(1)-O(8) is well identified. The oxygen vacancy cause a defect level between conduction band and valence band, and the number of levels are in proportion to the number of oxygen vacancies. With the increase of oxygen vacancy concentration, the lattice parameters donnot have a regular change, and the absorption in visible and near infrared increases rapidly which is in great agreement with the experimental results. Hydrogen annealing at high temperature doesnot cause a phase transition, but makes crystallization quality worse. However, the crystallization quality after annealing doesnot depend on annealing temperature. Experimental analysis shows that under high temperature and hypoxic conditions, oxygen ions in KTP diffuse from the interior to the surface, and the high temperature provides enough energy for the chemical reaction. Most of the oxygen atoms on the surface react with H2 to produce H2O and a small portion is released in the form of O2. The KTP crystal is the only oxygen source, so the elemental O in O2 and H2O can only come from KTP. Oxygen vacancies emerge due to the diffusion process, and some Ti4+ ions change to Ti3+ ions to balance the charge.(5) We measured the temperature dependence of full set tensor properties of KTP with one only sample using RUS method. However, in general, at least 7 crystals with different cut must be used to measure the full set tensor properties of piezoelectric crystal with mm2 point group. We used 109 resonance modes (almost 8 times of independent material contants) during our inversion. The elastic constants (c11E, c13E, C22E, C33E) and piezoelectric constants (d15, d24, d32) depend strongly on temperature, and other contants only have little fluctuation. Using these single domain data, we evaluated the orientation dependence of elastic, dielectric and piezoelectric properties. The elastic constant C33 reaches its maximum value at β=90°, while the maximum values of d33 and ε33 are all at β=0°. The temperature dependence of four selected mode frequencies Au-2, Bg-1, Ag-15 and Bu-13 are in excellent agreement, and the self consistency of data is also very good.