Tungsten Barium Raman Laser Crystal Growth And Its Nature Study

In recent years, much attention has been paid to the designing and operation of all-solid-state Raman lasers based on the stimulated Raman scattering (SRS) of crystals. Such all-solid-state Raman lasers can substantially extend the spectral coverage of solid-state lasers from ultraviolet to near infrared including yellow, orange and eye-safe range, depending on the pump lasers and Raman active crystals used. Important applications of such lasers include medical treatment, rangefinder, LADAR measurements, remote-sensing and coastal bathymetry, etc. Thus, the research and development works focused on crystals suitable for all-solid-state Raman lasers become a hotspot in the field of materials.Barium tungstate (BaWO4) crystal has been proposed to be a reliable and efficient Raman-active crystal for all-solid-state Raman lasers. This crystal is nonhygroscopic with a wide spectral transparency range, good thermal and mechanical properties, can be grown by using the Czochralski method and it possesses a high Raman gain for pump pulses of picosecond and nanosecond duration in comparison with the Ba(NO3)2 and KGd(WO4)2 crystals which are the most frequently used Raman-active crystals. All these properties make the BaW04 crystal to be a very attractive Raman-active crystal for utilisation in practical all-solid-state Raman lasers. But the growth of large dimension BaWO4 single crystal (normally the crystal sample length should be 5 centimeters) and studying its basic properties for Raman lasers applications have not been reported according to our knowledge. In this dissertation, the growth, structure, defects and properties of BaWO4 crystal are systemically studied, and the applications of it in Raman lasers are also discussed. The outline of this dissertation is as follows: 1. Crystal GrowthBaWO4 polycrystalline material was synthesized by means of a solid-state reaction with 4N pure BaCO3 and WO3. Free-cracked BaWO4 single crystals with high optical quality and large dimensions (22mmx80mm) were successfully grown along a- and c-axis directions by using the Czochralski method.Since crystal growth is a complex process, the factors that affect quality of crystals during the growth were analyzed, according to the theories of thermodynamics and dynamics. High quality polycrystalline material and a suitable field of temperature are the precondition to grow an excellent crystal; the optimal technics is the key for crystal growth; using a good crystal seed and eliminating outside influence can ensure the quality of crystals. We also presented the resolvents according to the characteristics of these factors.2. Crystal Components, Structure and DefectsThe concentrations of Ba and W elements in the' different parts of as-grown BaWC?4 single crystal were measured by the X-ray fluorescence analysis method. The measured results indicate that as-grown BaWO4 single crystal is closed to stoichimetric BaWC"4 and has high homogeneousness in components.The structure of as-grown BaWC?4 single crystal was studied by X-ray powder diffraction combined with conoscopic diagram method. The X-ray powder diffraction confirmed that as-grown BaWC>4 single crystal belongs to the tetragonal crystal system, I4j/a space group with unit cell dimensions a=b=5.6041 A, and c=12.7080 A. The conoscopic diagram shows that as-grown BaW04 single crystal is uniaxial crystal and the optical axis parallels crystallographic axis C4.Defects in the as-grown BaW04 single crystal have been studied for the first time by using the chemical etching method, white synchrotron radiation topography and high-resolution X-ray diffraction. The results of the observations and discussion of growth defects and formation mechanisms in as-grown BaWO4 single crystal were presented in this dissertation. It is important for us, since the investigation can guide the growth of single crystal with high perfection. The main defects in the as-grown BaWC>4 single crystals are dislocations and sub-grain boundaries. These defects seriously affected the perfection of crystals. In order to avoid or decrease these defects, we take some corresponding steps according to the above results.3. Basic PropertiesThe density, hardness, refractive indices and thermal properties of the as-grown BaWC>4 single crystal were measured, and influences of these properties on crystalgrowth and applications were also discussed.The density of BaWC>4 crystal was measured by using the buoyancy method at room temperature and its value is 6.393 g-cm'3.The Mohs hardness of BaWO4 crystal was about 4. The microhardness of BaWC>4 single crystal was measured for the first time. The value of microhardness decreases as the load increases and the hardness of the (100) plane is higher than that of the (001) plane. Under a load of 0.050kg, the microhardness in the (100) and (001) planes is 1814Mpa and 1393Mpa, respectively. Generally speaking, the hardness of BaWC>4 single crystal is low, which means that the single crystal is soft and more easily processed.The refractive indices of BaWC>4 single crystal was measured by using V-prism method at Hg lamp e line (A.=546.1nm) and its value is n^l.8483, ne=l .8470.The thermal properties of BaWC>4 single crystal were systemically studied. The melting point, enthalpy of fusion, entropy of fusion and specific heat were measured by the method of differential scanning calorimetry, which values are 1775.10 K, 96913.80 J-mol'1, 54.60 J-IC'-mol"1 and 0.30 J-g'-K"1~0.34 J-g'-IC1 in the temperature range of 323.15 K~ 1173.15 K, respectively. The thermal expansion of BaW04 single crystal was measured by using a thermal dilatometer and the average linear thermal expansion coefficient for all three crystallographic directions were calculated in the temperature range from 303.15 to 1423.15 K, which values are aa =10.9526 X 10^/K,ab=10.8069 X 10-6/K and ac =35.1063 X W6fK. The thermal diffusion coefficient ofBaWO4 single crystal was measured for the first time by the laser flash method in the temperature range from 297.15 to 563.15 K and its value decreases with increasing temperature. At 323.15 K, the thermal diffusion coefficients are 1.214 mmV and 1.178 mmV along the [100] and [001] directions. The calculated thermal conductivities of BaWO4 single crystal are 2.324 W-m"'K'' and 2.256 W-m"'-K"1 along the [100] and [001] directions at 323.15 K. The influences of these thermal properties on crystal growth and optical applications were also analyzed.4. Spectral Properties and Raman Laser PerformanceThe transmission spectra were measured with Hitachi U-3500 and NEXUS670 FTIR spectrometer at room temperature and the results show that BaWO4 single crystal has a wide spectral transparency range which is 255nm~3700nm. The wide optical transparency region makes BaW(>4 crystal suitable for Raman laser applications for pumping further in the infrared.The infrared absorption spectra of BaWC>4 crystal were measured using KBr pellets and we registed all the eight infrared-active modes. The Raman spectra of BaWC>4 crystal with different configutations were investigated. In the Raman spectra, we detected nine Raman-active modes (3Ag+3Bg+3Eg) and registed them according to the group theory calculation. There is one very intense Raman line of Ag symmetry at 922 cm'1 for BaWO4 single crystal, which correspond to the internal symmetrical vibrations (vi) of WO42' tetrahedron group. This intense Raman mode is very attractive in Raman lasers because it can shift green laser sources such as frequency-doubled Nd:YAG to access the yellow and orange regions or shift 1.3um laser to 1.5|im eye-safe laser.The Raman laser properties of BaWO4 single crystal were studied under 532nm, 30 picosecond pump pulses. The pump radiation was generated by KTP crystal doubling the 1064 run of mode-locked and Q-switched Nd:YAG laser and single-passed through the a- or c-axis oriented BaWO4 crystal samples. Under 0.25mJ pump energy, we obtained 0.07mJ yellow (the first Stokes Iine559.7nm) and orange (the second Stokes line 590.3nm) Raman laser output with the 30-mm-long c-axis oriented BaWC>4 crystal. The conversion efficiency of the pump radiation into the Raman lasers was about 28%. During the SRS experiments, we found that the SRS threshold decreases with the crystal length increasing. Under the same pumping conditions, the 50-mm-long a-axis oriented BaWO4 crystal showed more high-order Stokes lines (first, second, third) and anti-Stokes lines (first, second). The threshold pump energy of the first Stokes stimulated radiation for 20mm and 50mm a-axis oriented BaWO4 crystal samples were measured to be 666 and 373 MW/cm2, and the Raman gain coefficients were calculated to be 19, 13 cm/GW. For 30mm and 50mmc-axis oriented BaWC>4 crystal samples, the threshold pump energy of the first Stokes stimulated radiation was measured to be 480 and 250 MW/cm2, and the Raman gain coefficients were calculated to be 17 and 20 cm/GW. Thus, our experimental study showed that BaWC>4 crystal is a very efficient Raman active crystal for picosecond SRS, which indicates BaWC>4 crystal having great potential of applications in all-solid-state Raman lasers.