Equipment Development And Single Crystal Growth Of Micro-Pulling-Down, And The Synthesis And Investigations Of Functional Crystal TbCOB

As a kind of key materials in the areas of modern industry and military, synthetic crystals have been taken more and more attention by governments and scientists worldwide. Different from traditional natural crystals, the concepts of scientific design and synthesis can be applied in the synthesis of crystals, to realize the interaction and transformation between different forms of energy, such as light, electricity, sound, magnetism, heat, etc. Toward the crystal materials with great application-oriented functions, on the one hand we should manufacture and totally evaluate these critical crystals with obvious potential application, avoiding the research gap in the related field; on the other hand, the independent development of new crystal growth equipment in China, exploration and research on novel crystal materials, should be persisted and enhanced.Single crystal fiber (SCF) is a new kind of 1D laser material, which is made of traditional excellent laser crystal and has high ratio of length to diameter, and large surface area as glass fibers. The laser based on SCF illustrate the advantages of good thermal management, high efficiency and small nonlinear effect coefficient. In 2005, U.S. Army/Department of Defense released a Small Business Innovation Research (SBIR) award on SCF to improve the performance of high power fiber lasers. SPIE Photonic West host the topic conference on Single Crystal Fiber Lasers every year from 2011 to now, which shows its values in basic research and industrial processes, and other important commercial fields. However, China lags far behind the international level in the field of this critical material.High-quality single crystal fibers can be grown by using micro-pulling-down (u-PD) method, through which Yb:YAG SCF has been obtained and realized "hundreds of Watts" laser output for the first time in the world. μ-PD has several advantages of less material needed, high growth rate, short growth period and high efficiency, and can be developed for the exploration of novel materials and the optimization of single crystal properties. However, there is no related research on μ-PD in our country for a long period, although it is critically important for application and is worthy for fundamental research. This thesis firstly makes a review of the μ-PD method and its crystal growth, including the principle schemes, equipment development, crystal growth of μ-PD, and novel materials exploration and performance optimization. Based on the tradition of crystal growth equipment development in our group, we designed and manufactured μ-PD equipment by ourselves, which was supported by the National Natural Science Fund (special program for basic research-scientific instrument). Meanwhile, we realized the YAG SCF growth by using the μ-PD method, and have handled the experience on the controllable SCF growth and its characterizations and measurements.In order to design special compound with the multifunctional properties, we have chosen the low-symmetric ReCa4O(BO3)3 compound as structure host, with Tb3+ ions which exhibit strong fluorescence and paramagnetism due to the transition 4f8â†'4f75d1. The inspiring compound TbCa4O(BO3)3, a combination of the noncentrosymmetric structure of ReCOB with magnetism and fluorescence of Tb3+, can be used to investigate the relationship between the chemical constituent, crystal structure, and the functional properties. This thesis reported the process in solving the difficulties of TbCOB crystal growth, and got large-sized bulk crystals, following the material characterizations of its crystal structure, thermal properties, linear and nonlinear optical properties, fluorescence, piezoelectric and high-temperature piezoelectric performance, and magnetic anisotropy. Moreover, by combining new method (μ-PD) and new material (TbCOB), we picked up the idea that using μ-PD to grow spatial optimum phase matching (PM) crystal device in a high-efficiency way for the first time. Our experiments realized the direct TbCOB crystal growth for NLO devices through μ-PD, and the devices have been used to generate frequency-doubling of laser.Main contents and conclusions in this thesis are as follows:I. Self-developed μ-PD equipment and researchWe have developed the first μ-PD furnace in China, and modified the crucible, thermal field, mechanical system and CCD observation setup during the process of different crystal growth. The research and development of μ-PD have been supported by the National Science Foundation of China, which will fill the research gap in China.A method and setup made from graphite, molybdenum or tungsten has been picked up and designed for different crystal materials. Specific temperature distributions near the solid-liquid interface or along the pulling-down path, can be realized through the corresponding structure and shape. Compared with the previous μ-PD technique, the device is made of relatively cheap materials (compared to platinum, iridium and other precious metals) to provide the desirable temperature gradient distribution of high-quality single crystal growth, through a low-cost and simple way.II. Crystal growth and SCF fabrication through μ-PDBased on the self-developed μ-PD furnace, we have grown LiNb.03 (LN) and Y3Al5O12(YAG) crystals, which shows the situation and stability of our furnace. At the same time, Y3Al15O12 and Lu3Al5O12 single crystal fibers with diameter of 400 μm-1 mm and length up to 400 mm, can be obtained directly by using μ-PD method. SCFs are confirmed to be a total single crystal from top to tail, respectively, through Laue Back-Reflection of X-Rays. In addition, the emission and lifetime spectra of Nd:YAG SCF were measured and analyzed.in. Crystal growth, structure, and thermal properties of TbCOBThe pure polycrystalline of TbCOB can be synthesized through optimization of B2O3 amount and Tb3+valence, and the melting behavior was confirmed through thermal analysis and crystal growth, with a melting point of 1498℃. Thus, TbCOB bulk crystal can be grown by using melting crystal growth methods including Czochralski (Cz), Bridgman, and μ-PD, etc. In this thesis, YCOB and TbCOB crystals were used as seeds to carry out bulk crystal growth, during which the necking art and thermal field had been modified and optimized, and the size and quality of bulk crystals have been improved with the diameter up to Φ 40 mm. TbCOB belongs to monoclinic system with Cm space group, and its structure was resolved by X-ray single crystal diffraction. There are three types of coordinated polyhedron of Ca/Tb and O, connected by edge or corner sharing, also linked by BO3 triangles to form a 3D framework. Compared with TbCOB single crystals grown by flux method, the structure of crystals obtain by Cz has more disorder occupations.Furthermore, the thermal properties of TbCOB single crystal, including thermal capacity, thermal expansion, thermal diffusivity and conductivity coefficients, were measured and discussed. It has a specific heat value of 0.616 J·g-1·K*1 at 25℃, which is comparable to that of GdCOB and YCOB. The thermal expansion anisotropy ((αc-αa)/αc is set for reference) for TbCOB is lower than the above two crystals, with the value of 0.17, which means TbCOB crystal can bear large thermal gradient during the period of crystal growth, processing, and application.IV. Linear and nonlinear optical properties of TbCOBThe transparency range of TbCOB crystal is from 490 nm to 1500 nm. The UV-visible absorption spectrum of crystal samples recorded at 10 K contains seven known group bands, corresponding to 7F6â†'5H6,5D0+5H7,5L9+5G4,5D2+5G5,5L10, 5D3+5G6 and 5D4 transitions, respectively, and the infrared range contains three bands corresponding to 7F6â†'7F6,7F1,7F2 transitions. TbCOB is a negative biaxial optical crystal with a birefringence Δn=0.034 at 1064 nm, which between that of YCOB and GdCOB. The orientation of the optical axis (X, Y, Z) and the crystallographic reference axis (a, b, c) was determined as:b//Y, (a, Z)=26.12°, (c,X)=14.86°,β=101.26°.As far as frequency doubling at 1064 nm is concerned, only type-â…  second harmonic-generation (SHG) phase matching can be realized, where the phase-matching angle was found to be (90°,44.35°) in the XY principal plane, and (22.56°,0°) in the ZX principal plane, respectively. The all set of six independent second-order nonlinear optical (NLO) coefficients, d11, d12, d13, d31, d32, and d33 were determined by the Maker fringe (FM) technique. The largest NLO coefficient (deff=0.86 pm/V) is along (22.56°, 180°), which is the optimum PM direction in the principal planes. The outer-cavity (SHG) performance with 1064 nm pumping source was conducted along (22.56°,180°) PM direction, with the efficiency up to 57.1%. In a word, TbCOB has comparable NLO properties as YCOB and GdCOB, and easier to get bulk crystal since its lower thermal expansion coefficients and anisotropy.V. Piezoelectric performance and magnetic anisotropy of TbCOB functional crystalThe complete set of elastic, dielectric, and piezoelectric constants were determined for the first time through the 16 different sets of crystal cuts. The relative dielectric permittivities were calculated to be εT11/ε0=10.34, εT22/ε0=12.92, εT33/ε0= 10.04, and εT33/ε0=-0.36, respectively, which are close to those of GdCOB and higher than YCOB. The largest piezoelectric coefficient of TbCOB is 10.09 pC/N, obviously higher than that of YCOB, and the largest coefficients for TbCOB, YCOB and GdCOB are all d26, respectively. Moreover, TbCOB has a relatively high shear electromechanical coupling factor k26, being on the order of 21.9%.The electrical resistivities p of TbCOB crystals decrease with the increment of temperature, and are found to be on the order of 5×006 Ω·cm at 600℃. Although measured to be slightly lower than those of YCOB and GdCOB, the p of TbCOB possess greatly low anisotropy of the electrical resistivity. The elastic compliances 5^33 and SE66, and electromechanical coupling factors k26, can keep stable with a variation of 10%, as temperature increased from room temperature to 600℃. Even when the temperature raised up to 600℃, the value of d26 still remain in the order of 10.67 pC/N, exhibiting good thermal stability.The susceptibilities along three crystallographic axis keep on increasing as the temperature is decreased, under conditions of zero field cooling (ZFC) and field cooling (FC), which indicates no magnetic correlation among Tb3+ ions, combined with the magnetism behavior at 5 and 300 K. In addition, the susceptibility anisotropy is large being larger than theji, in the whole Trange, and χc/χb=23.93 at 5 K.VI. Directional growth of SHG crystal device by μ-PDCamparing with traditional Cz method, μ-PD can be used to realize the directional crystal growth with designed shape, which saves both raw material and fabrication time. For the first time, this thesis sugguests that crystal device can be directly obtained by using μ-PD in an effective and low-cost way.The spatial nonlinear optics anisotropy was studied based on theoretical calculation and the complete set of six second-order NLO coefficients, and the detailed spatial distribution of |deff| for TbCOB crystals at 1064 nm was obtained. The optimum space phase matching (PM) angle (113°,46°) was found to possess the largest deff value (type-â… ) being on the order of 1.39 pm/V, which is comparable to YCOB and GdCOB. Along the optimum PM direction, we designed and conducted the TbCOB crystal growth through μ-PD method, following the modification of thermal field and growth parameters. The obtained TbCOB crystal device has a fine shape and transparancy, and no scattering particles were found inside the crystal. We also found no B2O3 evaporation on the surface of the obtained crystal.Along the optimum SHG direction, a rod-shape TbCOB crystal with 3 mm in diameter was successfully grown by using the μ-PD method. After simple cutting and polishing of the ends for μ-PD grown crystal, a high SHG efficiency (stable above 55%) was realized through a Nd:YAG pico-second laser at 1064 nm, which was comparable to that of crystal samples from Cz growth. TbCOB crystals also exhibit a high laser damage threshold of >15 GW/cm2.In summary, direct crystal growth for NLO devices is feasible using p.-PD, and these results will also be useful for further studies and potential applications of NLO crystal devices, as well as for μ-PD crystal growth of other borates.