| Nonlinear optical (NLO) materials are indispensable functional materials in laser technology. They have important applications in modern science and technology, especially in photonics technology. Rubidium titanyl phosphate (RbTiOPO4. abbreviated as RTP) crystal is an excellent nonlinear optical crystal material, which has large NLO and electro-optical coefficient, high optical damage threshold and good physical and chemical stability. The devices made of RTP crystals such as frequency conversion and electro-optical modulators are widely used in electronics, communications, medical and other fields. However, at present, the conductivity of the as-grown RTP crystal in our country is high, and domain structure can not be eliminated in RTP crystal, which limits the application of RTP crystals. To seek more superior performance of RTP crystal materials and expand their scope of application, on the one hand we try to explore the high-quality RTP crystals by improving the growth process, on the other hand we study the Nb/Yb doped RTP crystals. The electronic structure of the crystals was studied by X-ray photoelectron spectroscopy (XPS), and the thermal and optical properties were also characterized. The reason for the color change of Nb/Yb doped RTP crystal after heating was analyzed. The Rb2Ti1.9sYb0.05(P04)3 crystals with langbeinite-type structure were found when we grew Yb doped RTP crystals, and the structure and properties of Rb2Ti,95Yb0.05(pO4)3 crystals were characterized.Main contents and results of this work are as follows:(1) The effect of various parameters on the growth of pure RTP crystal was studied in the process of high-temperature solution growth. RTP crystals with colorless and free of macroscopic defects were obtained by optimizing the growth parameters, and the crystal structure, thermal and optical properties have been characterized. X-ray rocking curve analysis shows that the crystals have good integrity and quality. Thermal analysis shows that RTP is a non-uniform melting crystal and its decomposition temperature is 1070℃. The specific heat of the crystal increases linearly with increasing temperature. The composition and electronic structure were studied by X-ray photoelectron spectroscopy, and the result indicates that Ti exists in form of Ti4+ion. The density of state of the crystal was calculated by MS software, and the result is consistent with XPS studies.(2) Nb (or Yb) doped RTP crystals were grown by using top-seed of high temperature solution method, and the electronic structure, absorption and luminescence properties of the crystals were characterized. After doping, the growth rate of a-direction of RTP crystal becomes slower, and the crystal shape is flat. Absorption peaks at 973nm and 971nm appear in the absorption spectrum of 3%Yb:RTP crystal, the two peaks are the characteristic absorption peaks of Yb3+ indicating that the Yb3+ ions have entered the lattice of RTP crystal. The strong emission peaks at 1071 nm、1051nm、1023nm and 1002nm are attributed to the transitions from (0’)'(3), (1’)'(3), (0’)'(2) and (0’)'(1) of Yb3+. The photoelectron peak of Nb was observed in the survey XPS spectrum of Nb:RTP crystal, which indicates that the element Nb also enters the RTP crystal.(3) Exploring the growth of Nb/Yb double-doped RTP crystals by using high temperature solution top-seed method. The influence of cooling rate, seeds and the flux systems on crystal growth was studied, and more transparent Nb/Yb:RTP crystals were obtained by improving the growth parameters. We determined the composition, structure and performance of the crystals.The concentrations of Nb and Yb in RTP crystals were determined by using EPMA, the results show that introduction of Nb into Yb:RTP crystals can greatly increase the content of Yb in RTP. Addition of WO3 and MoO3 into self flux is not helpful to the incorporation of Nb and Yb. especially for adding MoO3， the content of Yb in the crystal is too small to detect by EPMA. With the increase of doping content of Nb/Yb, the quality of the crystals significantly decreased, the FWHM of X-ray rocking curve increased. XPS studies show that the chemical bonding properties of Nb or Yb single-doped RTP crystals did not change when compared with pure RTP crystal. For Nb/Yb codoped RTP crystals, when the doping content of Nb/Yb in RTP is low (Yb%, Nb%≤3%), the covalency of P-O bonds becomes stronger with the increase of doping content. The absorption and luminescence performance of Nb/Yb double-doped RTP crystals were measured, the results show that the double-doped RTP crystals grown from self-flux show strong absorption peaks, indicative of the greater content of Yb3+. When Yb:RTP crystals were codoped with Nb5+ can improve the emission intensity of Yb3+ However, when the doping content of Yb3+ is more than 5%, the emission intensity don’t change significantly.In the process of growing Nb/Yb double-doped RTP crystals, when the doped crystals was used as the seed to measure their saturation point, we found that the color of the seed changed from pale yellow to black after the seed was removed from furnace. XRD result shows that the phase of the crystal is unchanged. After annealing, the intensity of absorption increases, which may derive from the formation of oxygen vacancies. The relative content of oxygen in crystals and the valence of Ti were determined by XPS and EPR, the content of oxygen decreases and Ti3+ ions are present after annealing. The color center caused by oxygen vacancy defect may be the main reason for the color changing.(4) In an attempt to obtain ytterbium-doped RTP crystal, a small number of blue crystals of langbeinite-type structure were found at the bottom of crucible. The composition of the crystal is determined as Rb2Ti1.95Yb0.05(PO4)3 by XRD and inductive coupled plasma atomic emission spectroscopy (ICP-AES). The electronic structure, thermal and optical properties of the as-grown crystals were characterized. XPS results show that two valances of titanium Ti3+and Ti4+were coexisting in the Rb2Ti1.95Yb0.05(PO4)3 crystal. It was confirmed that Ti-O bonds have stronger ionicity and the bonds of P-O have stronger covalency compared with RTP crystal. The specific heat of the crystal was increased with increasing temperature. The crystal shows strong Yb3+ absorption bands at about 910 and 972 nm. The SHG intensity of Rb2Ti1.95Yb0.05(P04)3 is 5-6 times greater than that of KDP crystal when the particle size is between 38 and 74 μm. |
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