Performance Study Of Nd:LuVO₄ And BaWO₄ Functional Crystal

Laser diode (LD) pumped solid-state laser is widely used in the fields of optical communications, benthic exploration, laser printing, medical treatment, etc., since it possesses many advantages such as, high efficiency, compactness, stabilization and low cost. Thus, to search more and more optical crystal that suit this kind of lasers becomes a hot spot in the field of materials. In recent years, the rare-earth (RE) vanadates laser crystals pumped by LD have attracted people's attention due to their excellent laser properties. Among these laser crystals, neodymium-doped vanadate crystals, such as Nd:YVO₄ and Nd:GdVO₄, have been proved to be excellent laser crystals for their favorable spectroscopic characteristics and have been commercialized at present. With the exception of LaVO₄, all rare-earth (RE) vanadates crystallize in the tetragonal space group I4₁/amd (D_4H¹⁹), and the activated RE ions are suitable in the strong tetragonalcrystal field with the site symmetry D_2d, which lacks inversion symmetry and thereby increases the probability of the parity-forbidden f→f transitions.In comparison with the Nd:YV04 crystal, Nd:LuVO₄ crystal has been proposed to be an ideal laser crystal with excellent properties of laser, stability of physics and chemistry and has been receiving considerable attention in recent years. The substitution of lutetium ions for yttrium in the host materials has already proved to be beneficial in a number of circumstances. As a solid-state laser material, the theoretical and experimental investigations of their thermomechanical properties such as the thermal expansion, machinability, and so on, and thermodynamics parameters such as the thermal conductivity, thermal diffusivity and specific heat, and so forth, are interesting. Similarly, the optical properties such as lattice vibration, optical parameters, absorption and fluorescence spectra are very important. These aspects have been systemically studied in this thesis.With the exploration of new laser material, the research and development works focused on crystals suit for Raman lasers become a hot spot in the field of materials. Generally, there are two methods to investigate new laser lights, one is to explore new laser materials, and the other is to convert the frequency of the laser lights that is known. Laser frequency conversion is a key technology in laser application. Solid-state Raman crystals suitable for stimulated Raman scattering are used to frequency shift of high-peak laser pluses with high efficiency and high optical beam quality. This technology is an important trend for laser frequency conversion especially with the development of Q-switching and mode-locking technology.Raman crystal, whose performance can effect the conversion of laser light, quality of laser beam, transform efficiency, is the hardcore part of the Raman laser. As the Raman-active host, gas and liquid materials are not to be compared with crystal materials. Especially, crystal has tremendous potential to be the all solid state Raman laser materials. Thereby, Raman-active crystal with high quality is the key of Raman lasers' development.BaWO₄ crystal has been proposed to be a efficient Raman-active crystal. In comparison with the Ba(NO₃)₂ crystal which is the most frequently used Raman-active crystal, BaWO₄ crystal is nonhygroscopic with a wide spectral transparency range, good thermomechanical properties. As the result of Raman spectra, the most intense Raman line is at 925cm^-1, which can possess a high Raman gain for pump pulses of picosecond and nanosecond duration. But the growth and studying its mechanism for Raman laser applications have not been entirely reportedIn this thesis, the character, structure, spectrum and thermal properties of Nd:LuVO₄ and BaWO₄ crystals are systemically studied, and the applications of it in all solid state lasers are also discussed. The outline of this thesis is as follows:1) The space group theoretical analyses and assignment of the lattice vibration modes of the Nd:LuVO₄ crystal with various geometries have been performed. The lattice vibration spectra of this crystal, which arise mainly from internal vibrations of the VO₄ anionic cluster, are studied by Raman scattering measurements. The highest lattice vibration frequency is observed at 903 cm^-1 corresponding to symmetric stretch of VO₄ anionic group. Because of its large birefringence, polarization mixing is strong in this material. The temperature depending of the lattice vibrations are reported in the temperature range 300-1400 K. The Raman mode frequency decreased and the line width increased as temperature is increased. The decreasing of Raman mode frequency is due to the thermal expansion of the lattice and the relaxation of the line width is due to the distortion of VO₄ tetrahedral group.2) The thermal properties of Nd:LuVO₄ single crystal was systemically studied. The specific heat were measured by the method of differential scanning calorimetry, which value is 0.442-0.498 Jg^-1K^-1 in the temperature range of 300-700K. The thermal expansion of this crystal was measured by using a thermal dilatometer and the average line thermal expansion coefficient for all three crystallographic directions were calculated in the temperature range 298-573 K, which values areα_a = 1.7×10^-6/K,α_b = 1.5×10^-6 /K,α_c = 9.1×10^-6/K. The small thermal expansion indicated that Nd:LuVO₄ single crystal is suitable for the solid state laser. The thermal diffusivities of this crystal were measured for the first time by the laser flash method in the temperature range from 300 to 563 K and the thermal conductivities were also calculated. The thermal diffusivity of Nd:LuVO₄ crystal is anisotropic and decreases with increasing temperature. At room temperature, the thermal diffusivities are 3.60 and 2.95 mm²/s along c and a directions respectively. Correspondingly, the calculated thermal conductivities are 9.77 and 7.96 Wm^-1K^-1, respectively.3) The absorption spectrum of Nd:LuVO₄ crystal was measured from ultraviolet to near-infrared wavelength, using a UV/VIS/NIR spectrophotometer. The fluorescence spectrum of Nd:LuVO₄ crystal, which was recorded at room temperature excited with a 808 nm xenon lamp as a light source in the wavelength range of 800-1500nm. The strongest emission peak is located at 1068 nm, which was widely studied in recent years. The measured absorption spectrum of Nd:LuVO₄ single crystal has been compared to J-0 theory. When applied, the J-0 theory of parity-forbidden electric-dipole transitions of rare earth ions on non-centrosymmetric sites demonstrates good agreement. The three intensity parameters areΩ₂ = 8. 235×120^-20 cm²,Ω₄ = 4.683× 10^-20 cm² andΩ₆ = 6.090×10^-20 cm², respectively. The absorption cross section at 810 nm is 66×10^-20 cm². The intergrated emission cross-ection is 13.896×10^-18cm, corresponding to ⁴F_3/2→4^I_11,2 translation.4) The growth of BaWO₄ crystal was simply introduced. The refractive indexes were measured by prism coupling method at 632.8 nm and 1539 nm. The values of n₀and n_e are close. This crystal is negative uniaxial crystal and its birefringence is small. The transmission spectra were measured using V/VIS/NIR spectrophotometer and FTIR spectrometer at room temperature and the results show that BaWO₄ crystal has a wide spectral transparency range of 256-5150nm. The wide optical transparency region makes it suitable for Raman laser applications for pumping further in the infrared.5) The thermal properties of BaWO₄ crystal were systemically studied. The specific heat, which was measured by the method of differential scanning calorimetry, is 0.317-0.358 Jg^-1K^-1 in the temperature range 300-573K. The thermal expansion of BaWO₄ crystal was measured by using a thermal dilatometer and the average line thermal expansion coefficient for all three crystallographic directions were calculated in the temperature range of 298-1423 K, which values areα_a = 1.10×10^-5 /K,α_b = 1.08×10^-5 /K,α_c = 3.51×10^-5 /K. The small thermal expansion indicated that BaWO₄ single crystal is suitable for the solid state Raman laser applications. The thermal diffusivities of this crystal were measured for the first time by the laser flash method in the temperature range from 300 to 563 K and the thermal conductivities were also calculated. At room temperature, the thermal conductivities are 2.59 and 2.73 Wm^-1K^-1, failing to 1.64 and 1.61 Wm^-1K^-1 from 297 to 563 K along [100] and [001] directions, respectively.6) The Raman spectra of BaWO₄ crystal were measured by Confocal Micro Laser Raman spectrometer with different configurations and all the internal vibrational modes were observed. The infrared absorption spectra of BaWO₄ crystal were measured using KBr pellets and all the eight infrared-active modes were observed. The space group theoretical analyses and assignment of the lattice vibration modes of this crystal have been performed. The Raman and Infrared results are consistent with the space group analyses. The highest-frequency mode at 925 cm^-1 can be unambiguously assigned to a v₁ vibration and the half line width is only 3.4 cm^-1. This highest-frequency mode is very attractive in Raman lasers because it can shift green laser beam such as 532 nm laser to access the yellow and orange regions or shift 1.3μm laser to 1.5μm eye-safe laser.7) The stimulated Raman scattering of BaWO₄ and LiIO₃ crystals were studied under 532nm picosecond pump pulses. For BaWO₄ crystal with the length of 50mm, the threshold pump energy of the first Stocks stimulated radiation was measured to be 250 MW/cm² and the Raman gain coefficient was calculated to be about 20 cm/GW. For the 54 mm LiIO₃ crystal, the two values are 153 MW/cm² and 30.3 cm/GW. Thus, our experimental study showed that the LiIO₃ and BaWO₄ crystals are efficient solid state materials for picosecond SRS.