【作者】 孙尚倩；

【导师】 张怀金；

【作者基本信息】 山东大学 ， 材料学， 2014， 博士

【摘要】 激光现在已经广泛应用于工业、军事、通信和医疗等众多领域,并带动了许多新兴学科的发展,现在它已经在我们的日常生活中发挥越来越重要的作用。激光器一般由三部分组成：工作物质、激励能源、光学共振腔。激光工作物质是激光器的核心部分,产生激光的介质有气体、液体和固体等。固体激光物质主要以激光晶体为主,以稀土为激活离子的晶体被广泛研究。随着激光二极管(LD)技术的发展,LD泵浦的固体激光器具有泵浦效率高,方向性和单色性好等特点。因此寻找适合LD泵浦的激光晶体成为研究热点。开始主要集中在对高对称性立方晶系和中对称性四方晶系的激光晶体的研究,其中Nd:YAG和Nd:YVO4现在已经商业化。但是Nd3+离子在这两种晶体中对LD泵浦波长的吸收带较窄,又难以获得更大的掺杂浓度。后来人们又对Nd:GGG, Nd:YVO4, Nd:GdVO4, Nd:LuVO4和Yb:YVO4等激光晶体进行了深入研究,从生长成本和综合性能上都不如上面两种晶体。近些年人们开始探索研究低对称激光晶体,像Nd:YCOB, Nd:GdCOB, Nd: KLu(WO4)2等,由于晶体的对称性降低,使得晶体的吸收光谱展宽,适合LD泵浦,但是它们的热学性质较差。LaVO4晶体在钒酸盐晶体中比较特殊,常温下是单斜的独居石结构,具有低对称性,其各向异性结构在光学上必然有其独特的性能。因此值得我们对LaVO4晶体进行深入的研究。1976年用熔盐法生长了LaVO4晶体,并对晶体结构进行了分析,发现晶体是与CePO4相同的独居石结构。此后很长时间没有关于LaVO4晶体的相关报道,至到2004年首次采用提拉法生长了Nd:LaVO4晶体,发现晶体在808nm处的半峰宽(FWHM)为20nm, Nd3+离子的荧光寿命为137μs,适合作为激光晶体。2006年用X粉末衍射对LaVO4结构重新进行了分析,证明晶体的空间群确实为P21/n(14)。同年用提拉法生长出Yb:LaVO4晶体。2009年又对Nd:LaVO4晶体生长中遇到的螺旋生长,以及晶体形貌进行了研究,2010年利用第一性原理对四方和单斜结构的LaVO4的能带和光谱进行了计算。同年Gavrichev等研究了LaVO4的比热和热力学函数。2012年又用浮区法生长出Nd:LaVO4晶体,掺杂浓度为5at.%,Nd3+离子的荧光寿命为80μs,并对折射率进行了定向。2013年用固相反应合成LaVO4晶体,对其结构进行了分析,测量了拉曼光谱。2010年首次报道了在Nd:LaVO4晶体中实现了1.064μm的连续激光输出,最大输出功率为45mW。可见对于LaVO4晶体的生长和研究不多,这主要是它是单斜晶体,而且较难生长,也没有对其进行很好地系统的研究。基于此,本论文系统的研究了Nd:LaVO4晶体生长、光学、热学和激光等性能,主要工作如下：1、Nd:LaVO4晶体的生长和结构采用提拉法生长出Nd:LaVO4晶体,并对影响晶体生长的因素和晶体生长过程中出现的螺旋问题进行了讨论。利用X射线粉末衍射得到晶体的结构,获得其晶胞参数,用浮力法对晶体的密度进行了测量,结果为5.07g/cm3与理论密度(5.061g/cm3)相近。对单斜晶体中不同的坐标系与晶体的结晶学轴之间的相互关系进行了详细的说明。2. Nd:LaVO4晶体折射率的测量利用最小偏向角法测量了晶体的折射率,计算了折射率主轴与结晶学轴之间的关系,拟合了随波长的变化关系：18.69643-7.39838λ+2.63972λ2,在测量波长范围内,有2.7。左右的旋转。计算了光轴角,晶体为正光性双轴晶,计算了光轴角随波长的变化关系。用Sellmerier方程拟合了折射率随波长的变化,拟合计算结果与测量结果比较吻合。3、Nd:LaVO4晶体光谱性质研究利用菲涅尔方程证明对于六方,四方,正交和三方各向异性的晶体可以在折射率主轴下测量连续波长偏振光的吸收。但是对于单斜晶体,复介电常数的实部ε二阶张量在折射率(介电)主轴下是对角化的,而虚部8’是非对角化的,吸收主轴和荧光主轴与折射率主轴存在一定夹角,复介电常数ε的虚部对于吸收来说只有在吸收主轴下是对角化的,对于荧光来说只有在荧光主轴下是对角化的,而且单斜晶体的折射率主轴有可能随波长发生变化。因此对于单斜晶体,不能像四方、正交等晶系的晶体一样在介电主轴下测量连续波长的偏振吸收。测量了Nd：LaVO4晶体沿不同方向的吸收光谱,在808nm处的半峰宽(FWHM)达到17nm,可能是由于La离子周围有9个配位O离子,而Nd离子在NdVO4晶相时周围有8个配位O离子,当Nd离子掺杂到LaVO4时,部分La离子被Nd离子取代,对于Nd离子,有从9配位向8配位转化的趋势,从而导致晶格场的振动和光谱的非均匀展宽。用Judd-Oflet (J-O)理论分析了光谱参数,晶体的三个唯象参数分别为：Ω2=2.142×10-20cm2, Ω4=3.704×10-20cm2,和Ω6=2.948×10-20cm2,808nm处的吸收截面为1.67×10-20cm2,并与其它晶体进行了比较。测量了Nd：LaVO4晶体荧光光谱和荧光寿命,测得的荧光寿命为154.90μs,荧光效率为80.1%。大的荧光量子效率说明晶体中的非辐射跃迁几率小。4、Nd:LaVO4晶体的拉曼光谱研究从因子群方法和位置对称性方法两个方面计算了Nd:LaVO4晶体的简正振动模式的对称性分类为：18Ag+18Bg+18Au+18Bu。共有72个简正模,与24个原子的运动自由度相等。测量了不同偏振配置Z(YY)Z和Y(ZX)Y下的拉曼光谱,对观测到的拉曼峰分内振动和外振动进行了详细的指认,钒酸根离子VO43-的内振动在晶体场中其晶格振动峰与溶液状态下有所变化,表明在晶体中四面体结构有畸变。不同配置下最强峰857cm-1和819cm-1的半峰宽分别为7.6cm-1和8.6cm-1,弛豫时间分别是1.39ps和1.23ps,说明Nd:LaVO4可作为拉曼激光晶体。半峰宽小说明晶体具有良好的结晶度,与有畸变并不矛盾,因为峰的位置与短程有序有关,而半峰宽与结晶度,缺陷,粒子尺寸等有关。5、Nd:LaVO4晶体的热学性质研究系统的研究了晶体的热学性质：比热、热膨胀、热扩散和热导率。计算了密度随温度的变化关系。对热扩散和热导率进行了主轴化,并计算了它们随温度的变化关系,室温下三个主轴热导率系数分别为：2.731W/m-K,2.966W/m·K,3.388W/m-K,具有最大热导率的主轴方向为沿c方向逆时针旋转19.82°。并且随温度的升高这个角度是不断变化的。利用热导率随浓度变化模型,计算出Nd:LaVO4晶体的主轴热导率随掺杂浓度变化关系,发现随着浓度的增加热导率变化较小,与质量方差变化较小有关。6、Nd:LaVO4晶体的激光研究实现了Nd:LaVO4晶体沿Y和Z切晶体的连续激光输出,Z切晶体的阈值功率是0.21W,最大输出功率是3.05W,斜效率是34.4%,光光转化效率是33.3%。Y切晶体的阈值功率是0.14W,最大输出功率是3.56W,斜效率是41.4%,光光转化效率是40.3%。利用被动调Q技术在Y切晶体实现了最大重复频率为14.6kHz,最小脉冲宽度为10.9ns,最大脉冲能量为38.3μJ,最高峰值功率为3.52kW的脉冲激光。更多还原

【Abstract】 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.更多还原