| Nowadays, Laser has been widely used in many fields such as industry, military, communication, medicine etc. and drives the development of many emerging disciplines. It now in our daily life plays an increasingly important role. Laser generally consists of three’parts:material, excitation energy, optical resonance cavity. Laser material is the core part of the laser. Laser materials are divided gas, liquid and solid etc. Solid laser materials mainly are laser crystals, the rare earth-doped crystals have been investigated widely. With the development of the laser diode (LD), LD pumped solid state laser has the characteristics of high pumping efficiency, good directionality and monochromaticity etc. So the search for laser crystals suitable LD pumped has become a hotspot. At first, the study mainly was focused on the laser crystals which belong to high symmetry and Intermediate symmetry such as cubic and tetragonal, the Nd:YAG and Nd:YVO4have been commercialized, but Nd3+ion can not be obtained greater doping concentration in the two kinds of crystals, and has a narrow bandwidth in the LD pump wavelength of808nm. Other laser crystals such as Nd:GGG, Nd:YVO4, Nd:GdVO4, Nd:LuVO4and Yb:YVO4were also investigated deeply which are not better than the two crystals above mentioned from the view point of growth cost and comprehensive performances.In recent years, people have began to explore and study the low symmetrical crystals, such as Nd:YCOB, Nd:GdCOB, Nd:KLu(WO4)2etc. The absorption spectra of the crystals are broadening due to the low symmetry, which indicates the crystals are suitable for LD pumped, but the thermal properties of them are poor. LaVO4crystal is special in vanadate crystals, it belongs to monoclinic monazite structure with low symmetry at room temperature, it must have its own unique optical properties due to the anisotropic structure. Therefore, it is worthy of our in-deep investigation on LaVO4crystal.LaVO4crystal was firstly grown by flux method in1976, and the structure of crystal was analyzed, the crystal belongs to monazite structure and is same with CePO4. There has been no reports about LaVO4crystal for a long time. Nd:LaVO4crystal was grown firstly by Czochralski method in2004, it has large FWHM of20nm at808nm, longer fluorescence lifetime of137μs, which is available for laser diode pumped. The structure of LaVO4was analyzed by the X-powder diffraction in2006, which proved that the crystal space group is P21/n(14). Yb:LaVO4crystal was grown firstly by Czochralski method in2006. In2009, the spiral growth encountered during crystal growth and crystal morphology were studied. In2010, energy band and optical properties of tetragonal and monoclinic structure of LaVO4were studied by the first-principle, and specific heart and thermodynamic functions of LaVO4crystal were investigated by Gavrichev. Nd:LaVO4crystal was grown by floating zone method in2012, the Nd-concentrations is5at.%, the fluorescence lifetime of Nd3+ion is80μs, The optic elasticity axes were determined by the conoscopic figures with a polarizing microscope.There were not many reports about growth and research for LaVO4crystal. Maybe, because it is monoclinic crystal, and difficult for growth, of course there were not systemic research about it. So, the growth, optical properties, thermal properties and laser performances of Nd:LaVO4crystal have been investigated in this work, and can be overviewed as follows:1. The growth and structure of Nd:LaVO4crystalThe Nd:LaVO4crystals was grown by the Czochralski method, and the influencing factors of the crystal growth and spiral growth encountered during crystal growth were discussed. By using X-ray powder diffraction, the structure of crystal and the lattice parameters were gained, the density of crystal with a buoyancy method was measured, result for5.07g/cm3and was similar to theoretical density (5.061g/cm3). The relationship between the crystallographic axis coordinate system and other different coordinate systems in monoclinic crystal was discussed in detail.2. Measurement of refractive indices of Nd:LaVO4crystalThe minimum deviation angle method was used to measure the refractive indices of the crystal. The relationship between the crystallographic axis coordinate system and the refractive principal axis coordinate systems was calculated. And the change of relationship with different wavelengths was fitted. There was a rotation of2.70during the different wavelengths. The optic axial angles were calculated which indicated the crystal belongs to positive biaxial crystal and the change of optic axial angles with wavelengths was calculated. The variation of the refractive indices with wavelengths was fitted with Sellmeier’s equations, and the differences between experimental and calculated values were very small. 3. Optical properties of Nd:LaVO4crystalIt has been proved with Fresnel equation that for hexagonal, tetragonal, trigonal and orthormbic crystals, the continuous wavelength polarized absorptions can be measured along refractive principal axes. But for monoclinic crystal, the real part of complex permittivity is diagonal in the dielectric frame, the imaginary of complex permittivity is not diagonal. There is a certain angle between absorption principal axes (and fluorescence principal axes) with refractive principal axes. So, the imaginary of complex permittivity is diagonal only in absorption principal axes for absorption, and in fluorescence principal axes for fluorescence. In addition, the refractive principal axes change with the different wavelengths. As a consequence, the continuous wavelength polarized absorptions can not be measured in the dielectric frame for monoclinic crystal. The absorption spectra of Nd:LaVO4were measured along different directions, it has large FWHM of17nm at808nm, The wider width may be generated by the following reasons: in the Nd:LaVO4crystal, the La possesses oxygen coordination number of nine. However, Nd has oxygen coordination number of eight in NdVO4phase. When the Nd ions are doped in LaVO4, a part of La ions are replaced by Nd, therefore, for Nd ions, there is a tendency from oxygen coordination number of nine to eight, which would induce the variation of crystal field and inhomogenously broadening in the spectra. Spectral parameters of Nd:LaVO4were analyzed using Judd-Oflet (J-O) theory, and were in comparison with other crystals, the three intensity parameters were2.142×10-20cm2,3.704×10-20cm2and2.948×10-20cm2respectively, the absorption cross section was1.67×10-20cm2. The fluorescence spectrum and lifetime were measured. The fluorescence lifetime of Nd3+was154.90μs, the fluorescence efficiency was80.1%which indicated that the nonradiative transition rate was very low in this crystal.4. Raman spectra of Nd:LaVO4crystalThe irreducible representations of the lattice vibration in crystal calculated with meth-ods of factor group and coordination symmetry, were18Ag+18Bg+18Au+18Bu.There were72lattice vibration modes, which were same with vibration freedom degrees of24atoms. The Raman spectra were measured for different polarized geometries Z(YY)Z and Y(ZX)Y. The observed peaks were assigned in detail based on internal vibration and external vibration. The frequencies of internal vibrations of VO43-ion were different in crystal and in solution which indicated that the tetrahedron VO43-ion has a distorted structure in crystal. Two most intense lattice vibration frequency were857cm-1and819cm-1, respectively for geometries Z(YY)Z and Y(ZX)Y, the FWHM of the two peaks were7.6cm-1and8.6cm-1corresponding to the relaxation time of1.39ps and1.23ps, respectively. The results indicated the crystal can be used as Raman laser crystal. The FWHM was smaller which meaned the crystal had good degree of crystallinity. It was not in contradiction with distorted structure in crystal, because the Raman band positions are very sensitive to the short-range order, whereas the Raman widths are more sensitive to the degree of crystallinity, defects and particle size etc.5. Thermal properties of Nd:LaVO4crystalThe thermal properties of Nd:LaVO4crystal including specific heat, thermal expansion, thermal diffusivity, thermal conductivity, were investigated systematically. The change of density with temperature was calculated. The principal coordinates for thermal diffusion and thermal conductivity were calculated and the changes of them with the change of temperature were calculated. The three principal thermal conductivity were2.731W/m·K,2.966W/m·K and3.388W/m·K at room temperature. There was a anticloc-kwise rotation angle19.82°between the most largest thermal conductivity and c coordination. The angle changed with different temperatures. The change of principal thermal conductivity with Nd concentration was calculated by the thermal conductivity model, and the change was small because the mass variance was small.6. Laser performance of Nd:LaVO4crystalThe continuous-wave(CW) laser experiments were carried out along Y-cut and Z-cut, For Z-cut crystal, The pump threshold was0.21W, the maximum output power was3.05W, the optical conversion efficiency was33.3%,the slope efficiency was34.3%. For Y-cut crystal, The pump threshold was0.14W, the maximum output power was3.56W, the optical conversion efficiency was40.3%,the slope efficiency was41.4%. The pulse laser was carried out in passively Q-switched laser operation for Y-cut crystal, the maximum repetition rate was14.6kHz, the minimum pulse width was10.9ns, the maximum pulse energy was38.3μJ and the highest peak power was3.52kW. |
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