【作者】 陆燕玲；

【导师】 孙宝德； 王俊；

【作者基本信息】 上海交通大学 ， 材料加工工程， 2007， 博士

【摘要】 2μm波段的光波具有被水强吸收、对人眼安全、可用石英光纤传输等优点,在军事、医疗和环境保护中有着广泛的用途。作为2μm波段激光器的工作物质,Tm: YAlO3 （Tm: YAP）晶体具有掺杂浓度高、生长速度快、转换效率高、偏振激光输出等优点,是具有应用潜力的新型固体激光材料。本研究围绕Tm: YAP晶体的生长工艺、缺陷类型及光学性质展开,通过X-射线衍射仪（XRD）、光学显微镜（OM）、扫描电镜（SEM）、同步辐射（SRWBT）等分析手段和热膨胀系数测量、分光光度计、干涉仪等测试手段,系统研究了Tm: YAP晶体的生长规律、着色机理、光学特性等,取得以下主要结果:采用中频感应加热提拉法生长Tm: YAP晶体,研究了掺杂浓度、籽晶取向、拉速和转速对晶体质量的影响规律,消除了晶体生长中出现的开裂、包裹物等缺陷,提出了适合Tm: YAP晶体生长的工艺参数。成功生长了不同掺杂浓度、不同晶轴取向的Tm: YAP晶体,晶体尺寸为φ（23～25）mm×（90～110）mm。部分晶体具有较好的结构完整性和光学质量。对制备的晶体进行XRD分析,确认主相为钙钛矿YAP相;测定了Tm³⁺在YAP晶体中的分凝系数为0.88,Tm³⁺沿晶体轴向分布较为均匀。详细研究了Tm: YAP晶体的常见缺陷。测量了沿晶体主轴方向的热膨胀系数,分别为αa=11.9458×10^-6 /K,αb=5.0012×10^-6 /K,αc=10.5439×10^-6 /K,定量解释了沿b轴方向生长晶体不易开裂的原因,揭示了引起Tm: YAP晶体开裂的主要原因是热膨胀系数的各向异性。确定了Tm: YAP晶体的腐蚀条件,对不同晶面的位错腐蚀坑进行了观察,发现蚀坑形貌符合晶体的结构对称性。在沿b轴生长的Tm: YAP晶体的（010）面上发现了呈十字分布、位错蚀坑密度不同的区域,经分析是晶体生长过程中径向应力在（1|-01）滑移面上分切应力作用的结果。在沿c轴生长的Tm: YAP晶体中发现了孪晶及消光现象,为了减少晶体生长过程中孪晶的产生,必须尽量减小温度梯度。在扫描电镜下观察到基体中弥散分布有一些尺度为几十微米、形状不规则的散射颗粒夹杂,EDS分析证实夹杂中含有一定的C元素;此外,还发现基体中存在形状规则的铱颗粒。在掺杂浓度为10at%的Tm: YAP晶体中发现了小面的存在,经XRD分析确认为（110）面。本研究首次采用同步辐射X射线形貌术对晶体的隐形缺陷进行了观察。同时,结合晶体的生长条件分析了缺陷的成因,提出了消除缺陷的工艺措施。通过对晶体进行退火和γ射线辐照处理,研究了Tm: YAP晶体的着色机理。研究表明晶体生长过程中偏离化学计量比、杂质Fe³⁺、O^-心、F⁺心以及它们之间的耦合组成的缺陷是造成晶体着色的重要原因,为深入研究YAP晶体的色心奠定了基础。测定了室温下不同掺杂浓度、不同取向Tm: YAP晶体的偏振吸收光谱。结果表明:该晶体在795nm波长附近有较强的吸收峰,对应于³H₆→³H₄的能级跃迁。在检测范围内,随着Tm³⁺掺杂浓度的增加,晶体吸收性能提高。对于a轴偏振方向来说,最强的吸收峰位于794nm,吸收截面为8.64×10^-21cm²;而对于b轴和c轴取向来说,最强的吸收峰均位于793nm,吸收截面分别为9.22×10^-21cm²和6.80×10^-21 cm²。因此,选择b轴取向的晶体作为激光工作物质,可以获得更高的激光输出效率。研究了偏光方向对Tm: YAP晶体吸收光谱的影响。对π谱来说,最强的吸收峰位于794nm,其吸收截面为7.30×10-20cm2,半高宽FWHM为29nm;而对于σ谱,最强的吸收峰位于793nm,其吸收截面为18.30×10^-20cm²,FWHM为25nm。与掺铥的其它激光晶体相比,Tm: YAP具有较大的吸收截面和FWHM,适于LD泵浦。研究了Tm: YAP晶体的荧光光谱。光谱中心波长位于1.86μm,发射截面为4.5×10^-21cm²,相应的荧光寿命为4.8 ms,表明该晶体具有较高的储能特征,有利于获得低阈值高效率的2μm波段激光输出。由于存在浓度淬灭效应,Tm: YAP中Tm³⁺的最佳掺杂浓度在4～6at%之间。根据Judd-Ofelt理论计算了Tm³⁺在Tm: YAP晶体中的各种光谱参数。Tm³⁺离子的唯象强度参数:Ω₂=0.93×10^-20 cm²,Ω₄=2.23×10^-20 cm²,Ω₆=1.12×10^-20 cm²。在此基础上,获得了多项其它光谱参数,并据此分析了晶体产生各向异性的原因。根据计算得到的光谱参数,预测了Tm: YAP晶体的激光输出特性。更多还原

【Abstract】 2μm light wave has widely application in laser radar, medicine and remote sensing due to its strong absorption by water and safety for eyes. As a main working medium of solid lasers, Tm: YAlO3 （Tm: YAP） is a promising laser material which possesses some advantages such as high doping concentration, high growth speed, sufficient conversion efficiency, and significant structural anisotropy.Concentrated on the improvement of the growth parameters, growth defects and spectral properties, the growth process, coloration mechanism, and optical characteristics of Tm: YAP single crystals were studied through X-Ray diffraction （XRD）, optical microscopy （OM）, scanning electron microscopy （SEM）, Synchrotron Radiation White-Beam Topography （SRWBT）, thermal expansion coefficient test, spectrophotometer and interference stripe measurement. The main results are listed blow:Based on the research on growth parameters and growth process, effect of seed orientation, pulling and rotation rates on the crystal quality was investigated. Crack-free and perfect Tm: YAP single crystals with the size ofφ（23～25）mm×（90～110）were successfully grown along the different crystalline axis by the Czochralski technique. The optimized pulling and rotation rates were proposed to be 1.0-1.5 mm/h and 15-25rpm, respectively.The as-grown crystals were analyzed by XRD method using its powder sample, which showed that only YAP phase existed and there was no TmAlO₃ phase in the crystal. The X-ray rocking curve （XRC） of as-grown crystal showed a full-width at half-maximum （FWHM ） 31.5″, indicating the as-grown Tm: YAP crystal has the good perfection. Effective segregation coefficient of Tm³⁺ in YAP crystal was determined to be 0.88 by X-ray fluorescence spectroscopy, which is smaller than that in YAG crystal.The defects commonly contained in Tm: YAP crystals were investigated in detail. Thermal expansion coefficients along three main crystallographic axes were measured in the temperature range of 30～1350℃, and the results showed that they are 11.9458, 5.0012 and 10.5439×10^-6 /K, respectively. Therefore, it is suggestive that the b-axis orientation as the growth direction is preferable in Czochralski method. Dislocations were observed by chemical etching method and optical microscope. It was found that there exist the areas with different dislocation etch-pit densities on the （010） plane of Tm: YAP crystal grown along b-axis. Such a distribution of pits could be attributed to the stress distribution during crystal growth. Twins were found only in the crystal grown along c-axis. The existence of main scattering centers in the crystals was confirmed by means of He-Ne laser beam and SEM technique. Meanwhile, the synchrotron radiation white-beam topography （SRWBT） was applied to study hidden defects of Tm: YAP crystals for the first time. The formation processes of defects were analyzed and the approaches to eliminate these defects were discussed.Investigation on annealing andγirradiation mechanism were carried out and discussed through absorption spectra. The results testified that the growth coloration is caused by deviation from the stoichiometric composition, the presence of uncontrolled impurities of the iron group, O–center, F+ center, and the complex of them. It is helpful to understand the coloration mechanism of YAP crystals doped with other rare earth elements.The polarized absorption spectra of Tm: YAP crystals were analyzed and the results suggest that the most interesting aspect is the intensive absorption peak at about 795nm, corresponding to the transition of Tm³⁺ from ³H₆ state to the excited state ³H₄. Absorption coefficient of Tm³⁺ ion in YAP crystal increases with increasing Tm³⁺ doping concentration. The strongest absorption peak for Tm: YAP crystal grown along a-axis is centered at 794nm and its absorption cross section is 8.64×10^-21cm². However, the peaks are both centered at about 793nm for crystals grown along b and c-axis, and their cross sections are 9.22×10^-21cm² and 6.80×10^-21 cm², respectively. These results showed that the anisotropic effect of optical characteristics exists in Tm: YAP crystal and b orientation is preferred.Investigation on the effect of polarized light on the absorption spectra of Tm: YAP crystal revealed that forπpolarization spectrum, the most intensive absorption at 794nm has a cross section 7.30×10^-20cm² and a 29nm FWHM, while forσpolarization spectrum, the most intensive absorption is situated at 793nm, with an absorption cross-section 18.30×10^-20cm² and FWHM≥25nm. Therefore, Tm: YAP crystal is suitable for LD pumping.Using a Ti sapphire laser that emits 795nm, the photoluminescence spectra were obtained. The strong emission peak is centered at about 1.86μm. The corresponding photoluminescence lifetime is about 4.8ms and emission cross-section is 4.5×10^-21cm². The results indicate that Tm: YAlO₃ is a potential candidate for efficient mid-infrared lasers with laser diode pumping and the optimized Tm concentration is 4～6at% due to concentration quenching effect.Based on Judd-Ofelt approach, the intensity parametersΩ_t（t=2,4,6） of Tm: YAlO₃ were calculated to beΩ₂=0.93×10^-20 cm²,Ω₄=2.23×10^-20 cm², andΩ₆=1.12×10^-20 cm². The spectral parameters such as experimental and theoretical oscillator strengths, radiative transition probability, radiative lifetime as well as the photoluminescence branching ratio were also obtained. The results showed that Tm: YAlO₃ crystal has large emission cross-section and long radiative lifetime. Therefore Tm: YAlO₃ crystal can be regarded as a high-efficient infrared laser material and favored for GaAlAs laser diode pumping.更多还原