Study On Growth And Properties Of Nd_xLu_1-x VO₄ Series Laser Crystals

The laser diode pumped solid state lasers (abbreviated DPSSLs) possess the properties of high pumping efficiency, stable output power and all solid state, and have been developed very fast in recent years. Nowadays, DPSSLs have been widely used in most of important fields of modern technology such as medicine, material processing, optical communication, laser display, guidance and laser nuclear fusion, etc.The laser materials are one of the key factors for designing all solid state lasers, and their properties have a great impact on the output performance and applications of the lasers ultimately. Among the laser gain media, laser crystals especially the neodymium doped ones are the most commonly used in DPSSLs. For the past decades, Nd:YAG crystal has been taken the dominant position in all laser materials. Nd:YV04 crystal with excellent characters has been attracted many attentions with the development of laser diode since 1980s. On the basis of fundamental researches for decades, Nd:YVO₄ crystal has been commercialized for years, and it is the first candidate used in the low and medium power lasers. Analogous crystals to Nd:YVO₄ have also been grown and investigated to expand the crystal varieties. Nd:LuVO₄ crystal was synthesized successfully in 2002, and it was found that Nd:LuVO₄ possesses some preferred advantages over Nd:YVO₄. Up to now, there has been many reports on researching of the continuous-wave laser, Q-switching, mode-locking, frequency mixing and self-Raman frequency shifting based on Nd:LuVO₄ crystals. The Nd:LuVO₄ lasers is going to be commercialized. It is worth noting that the previous research work is mainly focused on the growth, characterization and laser performance of Nd:LuVO₄ crystal with low Nd³⁺ concentration but few on the highly doped ones. However, the commercialized laser crystals are usually with the high doping contents, it is necessary to carry out the study on growth and properties of the highly doped Nd:LuVO₄ crystal. Furthermore, the composite crystal consisted of low doping laser crystal together with host crystal is one of the hot spots currently, which is used to improve the laser output by releasing thermal energy of the lasers. However, the research on Nd:LuVO₄/LuVO₄ composite crystal is still in a initial stage, and one can also study as well as on Nd:LuVO₄/LuVO₄ composite crystal. Based on the motivation for improvement of Nd:LuVO₄ and its composite crystal, the growth, thermal properties, optical spectra and laser performance of Nd_xLu_1-xVO₄ crystals with different Nd³⁺-doped concentrations have been investigated in this work, and can be overviewed as follows:I. Growth, structure and quality of Nd_xLu_1-xVO₄ crystalsBy using LuVO₄ (also NdVO₄) powder with the purity of 99.99%as the raw materials, Nd_xLu_1-xVO₄ crystals with different Nd³⁺concentrations have been grown successfully by the Czochralski method, where x=0,0.002,0.005,0.02,0.023,0.03 and 0.05. The factors related to the growth process and quality of crystals were analyzed systematically based on the theories of thermodynamics and dynamics. The suitable temperature field, reasonable growing procedure, crystal seed and polycrystalline materials with excellent quality are referred to such factors. The corresponding measures related to these factors appeared in crystal growth process were also presented for high quality crystal growth.The structures of the as-grown Nd_xLu_1-xVO₄ crystals were determined by X-ray powder diffraction (XRPD). The XRPD results confirm that the as-grown crystals belong to the tetragonal crystal system with a space group of I41/amd. The lattice constants of these crystals were calculated by the DICVOL programme, and the dependence of these constants (a, c) on Nd³⁺concentration (x) would be fitted to be a(x)=7.02625+0.24224x (A) and c(x)=6.23286+0.17732x (A) based on the Vegard linear law. The density (p) of Nd_xLu_1-xVO₄ was measured by the buoyancy method at room temperature, and the functional relation between p and x was found to be P=1/(-1.12383x2+0.10804x+0.15873). The results of the Vickers microhardness measurement show that the microhardness of Nd_xLu_1-xVO₄ decreases with increasing x value and the microhardness of c-cut wafer is lager than that of a-cut wafer. The quality of the as-grown crystals was characterized by conoscopic image, high-resolution X ray diffraction and chemical etching methods. The results show that the Nd:LuVO₄ crystals with high doping contents are of excellent quality and the main defects in these crystals are dislocation and grain boundary. The measures for improving the crystal quality were given out based on the defect formation mechanism.II. Thermal properties of Nd_xLu_1-xVO₄ crystalsThe thermal properties of the Nd_xLu_1-xVO₄ series crystals were investigated systematically, the influence of these properties on crystal growth and applications was also discussed.The specific heat of Nd_xLu_1-xVO₄ was measured from room temperature to 573 K by differential thermal scanning calorimeter. It has been found that the molar heat capacity of Nd:LuVO₄ at room temperature is larger than that of Nd:YVO₄, and the optical damage threshold of Nd:LuVO₄ is expected to be larger than that of Nd:YV04. The molar heat capacities of Nd:LuVO₄ crystals with different Nd3+-doped concentrations at room temperature were measured to be 27?30.3 cal·K^-1·mol^-1. The thermal expansion of NdxLui_xVO₄ was measured by thermodilatometer in the temperature range of 303.15-770.15 K. The thermal expansion coefficient along c-axis of Nd_xLu_1-xVO₄ is several times larger than that along a-axis, and the thermal expansion coefficient along a-axis is changed greatly with different x values. The thermal diffusion coefficients of Nd_xLu_1-xVO₄ with x=0,0.005,0.02,0.03 in the temperature range of 303.15-563.15 K were measured by the laser flash method, and their principal thermal diffusion coefficients at room temperature are?1=2.832,2.827, 1.903,1.814 mm²/s and A3=3.395,3.293,2.140,2.172 mm2/s, respectively. The thermal conductivity of Nd_xLu_1-xVO₄ was calculated, and it was decreased with increasing temperature and x value. III. Optical properties of Nd_xLu_1-xVO₄ crystalsThe refractive index, transmission/absorption spectrum and fluorescence spectrum of Nd_xLu_1-xVO₄ crystals were measured. The refractive indices of Nd_xLu_1-xVO₄ series crystals were measured at 633 and 1539 nm by using a prism-coupler. It has been found that the refractive indices of Nd_xLu_1-xVO₄ with different x value are similar each other and also comparable to those of Nd:GdVO₄. The transmission spectrum of LUVO₄ was recorded with a Hitachi U-3500 spectrophotometer in the wavelength range of 200-3200nm at room temperature, and the tranmittance of LUVO₄ maintains around 80% in the wavelength range of 540-3010 nm. The non-polarized absorption spectra of Nd:LuVO₄ around 808 nm were recorded, and the relation of absorption coefficient a at 808nm on Nd³+ -doped concentration N_d in units of atomic% can be fitted to be a=7.649 N_d. The polarized absorption spectra were measured for Nd:LuVO₄ in the wavelength range of 400-1200 nm. Based on these spectra and J-O theory, the optical spectral parameters of Nd:LuVO₄ have been calculated. The results show that the difference among the parameters of Nd:LuVO₄ with different Nd3+-doped concentration is insignificant. In addition, it has been deduced that the concentration quenching effect on is serious. The fluorescence spectra of Nd_0.02Lu_0.98VO₄ and crystals were measured, and their emission cross-section at 1064 nm were obtained to be 14.4*10^-19cm² and 12.8*10^-19cm², respectively.IV. Group theoretical consideration and Raman spectra of Nd_xLu_1-xVO₄ crystalsThe symmetry species of Nd_xLu_1-xVO₄ series crystals were calculated by group theory, and the Raman-active optical phonon modes of zero wave vector were found to be T_Raman=5A_1g+7B_1g+2B_2g+10E_g. There are no more than 34 Raman bands can be recorded according to above theoretical analysis. Raman spectra of LUVO₄, Nd_0.02Lu_0.98VO₄, Nd_0.03Lu_0.97VO₄ and Nd_0.05Lu_0.95VO₄ were obtained with scattering geometry X(ZZ)X, X(YY)X, Z(XY)Z and Y(XZ)Y, corresponding to the symmetry species modes of A₁g, A₁g+B₁g, B₂g and Eg, respectively. The number of Raman bands observed in our experiments is less than expected number. By comparing with the Raman spectra between of LUVO₄ and Nd:LuVO₄, it can be found that there are some discrepancies among these spectra. For example, there are some peaks located in 441,500 and 739 cm^-1 in the spectra of Nd:LuVO₄, and however, such peaks are not observed in the spectra of LUVO₄. In addition, the peak located about in 380 cm^-1 (v2) for LUVO₄ is shifted towards to low wavenumber for Nd:LuVO₄ (red-shift), and this may be attributed to the lanthanide contraction effect. Such discrepancies are not significant yet, which implies that the Nd³⁺ dopants do not give rise to great changes in the host lattice and LUVO₄ can provide a suitable crystal field for Nd³⁺ions.V. Laser performance of Nd_xLu_1-xVO₄ crystalsThe continuous-wave (CW) laser experiments of Nd_xLu_1-xVO₄ crystals operating at 1.06?m were carried out with a planar-planar resonator by using a LD pump source centered at 808 nm, where x=0.005,0.02,0.023,0.03 and 0.05. It has been found that there is not any laser output for Nd_0.05Lu_0.95VO₄. The experimental result for Nd0.02Lu0.98VO4 is the best one among the highly doped Nd:LuVO₄ crystals, where the pump threshold is 0.489 W, the maximum output power is 1.09 W and the optical conversion efficiency is 16.6%. This is the first time for reporting the CW laser output. of highly doped Nd:LuVO₄ crystals.The Nd:LuVO₄/LuVO₄ composite crystals with two different styles were prepared through the diffusion bonding technique, which were (LuV0₄+Nd_0.002LU_0.998VO₄) and (LuV0₄+Nd_0.005LU_0.995VO₄+LUVO₄) respectively. The CW laser performance of these composite crystals operating at 1.06?m were investigated based on a planar-concave resonator by using a LD pump source centered at 808 nm. It can be found that the experimental result for the composite crystal with double-end bonding style is superior to that for the composite crystal with single-end bonding style. The maximum output power was obtained to be 17.2 W, optical conversion efficiency was 47.3%and the corresponding slope efficiency was 48.0%. As we have known, it is the first time that such lasers be investigated, and the output power of 17.2 W is the highest CW output power achieved in Nd:LuVO₄ crystal by far. LD end-pumped intracavity frequency-doubled (LuVO₄+Nd_0.005Lu_0.995VO₄+LuVO₄)/ KTP CW green laser, formed with a planar-concave resonator, were also demonstrated. The maximum green output power was obtained to be 457 mW.