摘要
随着科技的发展,利用稀土离子掺杂材料上转换性能实现短波长蓝绿激光输出在高密度光学存储、彩色显示、光电子,光刻等领域有广泛应用前景和巨大发展潜力,设计研发稀土掺杂晶体材料便成为实现短波长激光输出的关键问题。因为具有大非线性光学系数、宽非临界相位匹配范围和高抗光伤阈值等优点,钨青铜型钽铌酸钾锂单晶(K_(1-y)Li_yTa_(1-x)Nb_xO_3,KLTN)是一种优良的蓝光二次谐波产生(SHG)晶体。通过研究稀土离子掺杂KLTN单晶的上转换荧光性能来分析其作为激光晶体的可行性具有实际研究价值。本论文对Er~(3+)及Er~(3+)/Yb~(3+)掺杂四方钨青铜型KLTN单晶的生长、基本物理性质、晶体吸收性能和光致发光性能做系统的实验和理论研究。
采用逐步冷却法摸索适合进行Er~(3+)掺杂KLTN单晶生长的组分配比和生长温度,利用提拉法并通过改进晶体生长工艺生长出Er~(3+):KLTN和Er~(3+)/Yb~(3+):KLTN单晶。详细地描述晶体生长过程和各种工艺参数对晶体生长的影响。研究Er~(3+):KLTN单晶的基本物理性质,利用电感耦合等离子体发光光谱分析(ICP)和能量色散谱分析(EDS)测试晶体组分;利用X射线衍射技术分析晶体结构;利用差热分析法测试晶体的居里温度;利用椭偏法测试晶体的折射率,并利用赛尔迈耶尔方程拟合Er~(3+):KLTN晶体的折射率色散关系。
使用分光光度计测试晶体的紫外-可见-近红外吸收光谱,描述晶体样品Er~(3+)吸收规律,并确定晶体紫外吸收边。根据吸收光谱结合Judd-Ofelt理论计算晶体JO强度Ω_t (t=2, 4, 6),并由此计算Er~(3+)在晶体中的自发辐射系数、能级辐射寿命和荧光分支比等多项光谱参量。
系统地研究800 nm和980 nm波长激光激发Er~(3+):KLTN单晶上转换荧光性能。Er~(3+)掺杂浓度升高能够提高上转换发射强度,并且红光发光强度随Er~(3+)掺杂浓度升高而明显增强。通过功率曲线确定上转换为双光子过程。通过发光强度衰减曲线确定绿光发射由激发态吸收(ESA)和能量转移上转换(ETU)过程完成;800 nm激发红光发射由交叉驰豫(CR)过程完成,980 nm激发红光发射由ESA和ETU过程完成,给出详细上转换跃迁机制。结合980 nm激发Er~(3+)上转换发光机理建立微分速率方程模型,推导上转换发光强度随着泵浦功率和掺杂浓度的变化关系。通过比较400 nm和800 nm波长激光泵浦下Er~(3+)/Yb~(3+):KLTN单晶上转换发光强度的不同,发现由Er~(3+)到Yb~(3+)有一个高效能量背向传递过程起减弱548 nm绿光发射同时增强红光发射的作用。
利用固相反应法制备Er~(3+)掺杂和Er~(3+)/Yb~(3+)共掺KLTN陶瓷,研究其发光性能,分析荧光光谱随着Er~(3+)和Yb~(3+)掺杂浓度增加而产生的变化。建立速率方程推导并解释上转换发光强度随Yb~(3+)浓度变化规律。根据陶瓷和晶体的荧光光谱的不同研究稀土离子在四方钨青铜型KLTN晶格中的占位情况。
综上所述,本文系统研究了Er~(3+)和Er~(3+)/Yb~(3+)掺杂KLTN单晶和陶瓷生长、基本物性表征和光谱性能。研究结果表明Er~(3+)和Er~(3+)/Yb~(3+)掺杂KLTN单晶具有优良的吸收光谱性能和绿光上转换发光性能,在绿光上转换激光晶体方面有一定的潜在应用。
In recent years, the short wavelength blue and green solid state laser based on frequency upconversion (UC) rare-earth-ions-doping materials has attracted so much attention due to its wide applications and potential in the field of high density optical storage, color displays, photoelectron, and photoengraving, etc. The development of rare-earth doping single crystal materials has consequently become the foremost issue. The tungsten bronze-type potassium lithium tantalate niobate crystal is an excellent blue second harmonic generation crystal because of its broad non-critical phase-matching range, large nonlinear coefficients and photorefractive resistance threshold, etc. Therefore, it is of practical significance to grow the RE ions doped KLTN single crystal, and investigate its feasibility as laser crystal based on the upconversion luminescent properties.In the present dissertation, we have systematically studied the growth, basic physical properties, absorption, and photoluminescence in rare-earth (RE) doped KLTN crystals.
By step-cooling method, we explored the compatible raw materials concentration ratio and growth temperature for Er doped KLTN crystal. Furthermore, we improved the crystal growth crafts and used the Czochralski method to grow the Er:KLTN and Er/Yb:KLTN crystals. The details of growth process and effects of growth craft were described in this thesis. The constituents of KLTN crystal were determined by means of the inductive coupled plasma emission spectrum (ICP) and energy dispersive spectrum analysis (EDS). The crystal structure was determined by the X-ray diffraction technique. The Curie temperature of crystal was measured by the differential thermal analysis (DTA) technique. Furthermore, the refractive indices of crystal were measured using the ellipsometry method, and the refractive indices dispersion was fitted using the Sellmeier equation.
The ultraviolet-visible-near infrared optical absorption spectra were measured using the spectrophotometer. The absorption rule of crystal was described, and the ultraviolet absorption edges were determined. The Judd-Ofelt (JO) intensity parametersΩ_t (t=1, 2, 3) of crystal were calculated using the absorption spectra and JO theory. Moreover, the spontaneous emission probability, excited state radiative lifetimes, and fluorescence branching ratio of the Er~(3+) in the crystal were calculated.
UC luminescence properties under 800 nm and 980 nm laser excitation were investigated systematically in Er~(3+):KLTN single crystals. The UC luminescence would be enhanced by the Er~(3+) concentration increase; moreover, the red emissions were enhanced more obviously than the green emissions. The UC processes were two-photon processes determined by pump energy dependence of the Er~(3+) UC luminescence intensities. The decay profiles of luminescence intensity investigations indicated that the green emissions were accomplished by ESA and ETU processes; the red emissions were accomplished by CR processes under 800 nm excitation, and ESA and ETU processes under 980 nm excitation, respectively. Furthermore, the UC mechanisms were provided. The UC emission rate equation model under 980 nm excitation was built and deduced to prove the change of pump power dependence. By comparing the fluorescence emission intensities of Er~(3+)/Yb~(3+):KLTN single crystal under 400 nm and 800 nm excitations, a effective energy back transfer process from Er~(3+) to Yb~(3+) played the role in enhancing the red emission and weakening the 548 nm green emission.
The Er~(3+) doped and Er~(3+)/Yb~(3+) codoped KLTN ceramics were prepared by solid state synthesized method, and their fluorescence properties were investigated. The variations of the fluorescence spectra of KLTN ceramics with different Er~(3+) and Yb~(3+) concentrations were analyzed. The rate equation model was built and deduced to prove that the UC emission intensity changed with the increase of Yb~(3+) concentration. Furthermore, the arrangements of RE ions in tungsten bronze-type KLTN crystal lattice were investigated by the difference of the Er~(3+) fluorescence spectra in ceramics and crystal.
In conclusion, the growth, physical properties, and spectroscopic properties of Er~(3+) doped and Er~(3+)/Yb~(3+) codoped KLTN crystals are studied in the paper. The resultes show that Er~(3+) and Er~(3+)/Yb~(3+) codoped KLTN crystals have excellent absorption and green upconversion propterties, they maybe have potential applications as a green upconversion laser crystal.
引文
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