Growth And Properties Of Series Re:YCOB Crystals

Function materials, as key role in new materials, play an important role in the development of national economy. With the development of lasers, energy, information, function materials are advancing towards low-dimension, miniaturization and composite. Crystal materials are an important component part of function materials. Since the anisotropy of crystal materials, crystal materials possess many advantages which could not compared by other kinds of materials. The properties of crystals are determined by the structure of crystals and the ions in them, so crystals with high symmetry have their own advantages, such as there is no need to orient the direction of one crystal belongs to cubic class, which could simplify processing and application. However, this high symmetry of structure leads to high symmetry of properties, which also exclude the crystals with high symmetry from the composite of functions.YCa4O(BO3)3 (YCOB) crystals belongs to monoclinic class, with Cm space group. Low symmetry of YCOB crystals leads to the application of them in composite of functions. Since YCOB crystals possess nonlinear property, so they could be used to obtain SHG (Second Harmonic generation). After we dope the laser active ions (such as Nd3+ and Yb3+) into it, we could acquire a SFD (self-frequency doubling) crystal. When Y3+ in YCOB crystals are substituted by Nd3+ completely, a novel self-activate laser material NdCa4O(BO3)3 (NdCOB) is achieved. At present, there is no report about self-activate properties of NdCOB still.There are already several kinds of crystals which have made the SFD laser come off, including series of doping lithium niobate, such as Tm:LiNbO3,Nd:LiNbO3,Nd:MgO:LiNbO3和Nd:Sc2O3:LiNbO3; yttrium aluminum borate with Nd3+ doped (Nd:YAl3(BO3)4); yttrium and gadolinium calcium borate with Nd3+ and Yb3+, such as Nd:YCa4O(BO3)3,Nd:GdCa4O(BO3)3,Yb:YCa4O(BO3)3. Because of their own properties and problem of crystals growth of former two kinds of crystals, widely applications are hardly carry out. Czochralski (Cz) method, which is used to grow series crystals of YCOB and GdCOB, plus the properties of them make the this kind of crystal attract many research focus on them, and they have good promising in becoming commercialized self-frequency-doubling crystal.The Czochralski method is the primary one among all the crystal growth methods for its convenient observation, controllability and short growth period to get crystals with large size and high optical quality. The crystals grown from melting are forced by temperature gradient of the environment in which they grow larger. The whole process of crystal growth could be seen as a transformation from liquid phase to solid phase and achieved by the regularly solidating of grow species at the growth interface. So, it is crucial to choose a proper temperature gradient to get an intact crystal with high optical quality. In addition, it is also critical to replenish the component deviation caused by volatilization of raw materials during the process of crystal growth.In this work, NdxY1-xCOB and YbxY1-xCOB crystals were grown by Czochralski method. Thermal properties, spectral properties and SFD laser properties were also carried out.1. Crystal growthNd3+ and Yb3+ doped YCOB crystals melt nearly congruently, so that it can be pulled directily from a melt reasonably close to the stoichiometric composition. The starting material was prepared by mixing CaCO3, Y2O3, H3BO3 and Nd2O3, Yb2O3 powders with high purity in proportions. Through the protection of N2, we successfully grew NdxY1-xCOB and YbxY1-xCOB crystals with different concentration of doping ions using iridium crucible. The diameter of crystals is 20mm and transparency through the whole bulk. Macro defects are eliminated by adjusting the temperature gradient. Also we discuss the influence of temperature gradient, growth atmosphere, excess of B2O3, parameters of growth on the crystals quality.2. Fundamental propertiesDensities and thermal properties of NdxY1-xCOB and YbxY1-xCOB were measured, and we analysis the relationship between the thermal properties, including thermal expansion, thermal conductivity and specific heat, and doping concentration. Thermal properties of NdCOB were analyzed in detail.Buoyancy method was adopted to measure the density of all the crystals at room temperature, and results shows that the densities are getting high with increasing of doping concentration.Thermal expansion coefficients of the crystals were measured by thermal-mechanical analyzer at a temperature range of 303.15K to 768.15K. We also calculate the thermal expansion coefficients along physical axes, ones of NdCOB to be 6.75189,6.37273和12.13381×10-6 K-1. The specific heat was measured by a Differential Scanning Calorometer (DSC). We find that it is not simply increasing or decreasing with the change of doping concentration. In NdxY1-xCOB crystals, the specific heat get maximum when doping concentration is 10at%, which is 0.71J/g·K at 330K. As for YbxY1-xCOB, when temperature is lower than 400℃, Yb0.15Y0.85COB has the max specific heat; with temperature increasing over 400℃, Yb0.3Y0.7COB contains the largest one. Thermal diffusion coefficient was measured by laser flash method and thermal conductivity of NdCOB was calculated to be 1.852512,1.8885 and 2.149088 W·m-1·K-1 along physical axes.3. Spectral propertiesRefractive index of Yb0.1Y0.9COB was measured and dispersion equations along different directions are fitted according to refractive index. We also measured non-polarized absorption spectra of different doping concentration with the range of 200～1500nm. The max absorption cross section of Nd:YCOB appears at 1060nm and 870nm; the max one of Yb:YCOB is at 900nm and 978nm. Absorption intensity is increasing with increasing of doping concentration. In Yb:YCOB, absorption intensity at 900nm is weaker than that at 978nm, however, with the increasing of doping concentration, absorption intensity at 900nm is getting higher than 978nm. We also analyzed the reasons that make the absorption peak deviation.4. Experiments of self-frequency doublingWe operate the experiments of SFD lasers on Nd:YCOB and Nd:GdCOB. We obtain 1.35W output SFD laser with the optical-optical convertion ratio is 17%, which is the best output SFD laser result as we know. We also analyzed the causations why two kinds of crystals show different central wavelength of output SFD laser.