Study On The Growth And Properties Of BaAlBO₃F₂ (BABF) Crystal

BaAlBO₃F₂ (BABF) crystal is a new nonlinear optical crystal. It has a potential use as a frequency conversion device, especially in third harmonic generation (THG) of Nd:YAG laser radiation. The present dissertation focuses on the growth and properties of BABF crystal. The main work and achievements are listed below:1. Crystal GrowthA new flux system B₂O₃-LiF-NaF was found through the experiment of flux selection. The suitable molar ratios of BaF2 : H3BO3 : Al2O3 : LiF : NaF for the growth of BABF crystal turned out to be 2 : 6 : 1 : 5 : 0.6 through plenty of experiments. The new flux system has many advantages compared with the original NaF flux system. The new system has a wide crystallization and the favorable concentration of BABF in solution is in the range of 35⁶5 %. The growing temperature of the new flux system is lower than that of the original NaF flux. The volatility of the solution is effectively reduced and the volatilization is less than 5%. The layer growth habit can also be avoided in this flux system.The parameters of crystal growth are as follows: [001] seed orientation, rotation rate 20-40rpm, cooling rate 0.2-5℃/day, growth periods 20-50days, anneal rate 10℃/hour.The inclusion-free BABF single crystal with sizes up to 50×40×25 mm3 has been successfully grown by the middle-seeded solution growth technique, using B₂O₃- LiF -NaF system as fluxes, and the optical homogeneity of the crystal is⊿n≈4.048×10^-6/cm.In the crystal-growing experiments, the morphology of BABF was also studied. It was found that BABF crystals grew along a-, c-axes exhibited a similar morphology. It was observed that the predominant faces were (001) and (100) faces. This is in good agreement with the ideal BABF morphology composed of the simple form of (001) hexagon prism and (100) rectangle prism.2. Properties StudyThe physical properties of BABF crystal were studied systematically, which were closely related with the crystal growth, the crystal processing and its application. The thermal expansion coefficients along [100] and [001] directions over the temperature range of 30– 800℃were measured. The specific heat 30℃and 400℃were determined to be 0.533J/g·K and 0.686J/g·K, respectively. The thermal diffusion coefficients were determined by the flash method, based on which, the thermal conductivity coefficients were calculated for the directions along [100] and [001]. Judging by all of the above physical properties, we can draw a conclusion that BABF has a good thermal properties and mechanical properties, which makes the crystal easy to cut and polish.The nonlinear optical properties and the related linear optical properties of BABF were studied. The optical transmittance spectrum was determined on a Lambda 900 UV-VIS-NIR spectrophotometer at room temperature. By the Maker fringes method, the nonlinear optical coefficient d11 = 3.172d36(KDP) = 1.24pm/V and d22 = 3.178d36(KDP) = 1.24pm/V were determined. Laser-induced damage threshold was about 6.256GW/cm2 using 1064nm Nd: YAG laser with 7.5ns pluses.THG energy conversion efficiency of BABF crystal and LBO crystal were also evaluated, respectively, under the same experimental condition. The BABF crystal with dimensions of 3×3×5.8mm3 was cut for Type-â… PM, while the LBO crystal with dimensions of 4×4×12.1mm3 was cut for Type-â…¡PM. The fundamental light source was a diode-pumped Nd:YAG laser (λ=1064nm & 532nm) with a pulse duration of 25 ps and a repetition of 10 Hz. The experimental result demonstrated that THG energy conversion efficiency for BABF was higher than that of LBO. Since the energy conversion efficiency strongly depends on the length of the crystal, it is expected to be further increased when a larger crystal is offered in the near future. 3. ConclusionThe experimental results demonstrate that BABF possesses good chemical, mechanical, thermal, and nonlinear optical properties. BABF crystal is therefore an excellent candidate for THG.