摘要
在高密度光存储、红外探测、激光防伪、生物标记、短波长固态激光器、三维显示、医学诊断等领域的应用前景促进了稀土离子上转换发光的研究。四十年来,人们广泛研究了掺杂不同稀土离子的玻璃或晶体的上转换发光,取得了长足的进展。目前研究工作的重点仍然是寻找最佳的上转换发光材料和深入理解上转换发光的激发机理。由于稀土离子具有丰富的能级,仅依据光谱、上转换发光强度和激发强度的幂次关系以及能量匹配分析上转换过程的激发机理,往往只能罗列出各种可能的跃迁过程,而不能唯一地确定其中哪种过程是主要的。本论文工作的目的是通过对上转换发光动力学过程的研究,对一些体系中上转换发光的激发途径得到明确的认识。
本文研究的内容主要分为以下几部分:
1.研制了方波电源,主要由多谐振荡、分频、前置放大和驱动电路四部分组成。半导体激光器在方波电源的驱动下,以频率可变的方式发射方波激光。
2.用方波电源驱动的808nm、980nmLD激发Er~(3+)掺杂亚碲酸盐氟氧化物玻璃,测量上转换绿光、红光的上升和衰减曲线。建立了速率方程,通过分析上转换发光的上升和衰减曲线,确定其不同的中间能级,从而确定两种波长激发下Er~(3+)离子上转换发光的激发过程。分析了808nm LD激发下Er~(3+)掺杂亚碲酸盐氟氧化物玻璃的绿光出现的明显的“饱和”现象。利用速率方程建立的模型对实验结果进行拟合,得到饱和现象是源于基态和激发态吸收饱和的结论。
3.用方波电源驱动980nmLD激发Er~(3+):碲酸盐玻璃和Er~(3+)-Yb~(3+):碲酸盐玻璃,测量~2H_(11/2)、~4S_(3/2)、~4F_(9/2)能级的上升和衰减曲线,分析了敏化剂离子Yb~(3+)能量传递对荧光上升和衰减的影响。建立了系统的速率方程,分析了单掺Er~(3+)和双掺Er~(3+)-Yb~(3+)的~2H_(11/2)、~4S_(3/2)、~4F_(9/2)能级的上升与Er~(3+)中间能级寿命和Yb~(3+)上能级寿命的关系,以区分激发态吸收和逐次能量传递两种上转换激发机理,通过对实验曲线的分析,确定Er~(3+)-Yb~(3+)掺杂碲酸盐玻璃上转换发光的动力学过程。
4.研究了808nmLD激发下Er~(3+)单掺、980nmLD激发下Er~(3+)-Yb~(3+)共掺体系上转换发光动力学过程的微分方程数值解,通过改变参数来分析Er~(3+)的激发态吸收、Er~(3+)的交叉弛豫、Er~(3+)-Yb~(3+)间的能量传递上转换发光的过程。对每种机理起主要作用的条件进行了讨论。通过与实验拟合,对前面工作用弱激发近似得到的结论作了进一步的验证。
The study on upconversion luminescence of rare earth ions is important for the applications in the areas such as high-density optical storage,infrared detection, laser anti-forgery,biomarkers,short-wavelength solid-state lasers,three-dimensional display,and the medical diagnostics.Over the past four decades,up conversion luminescence of various crystals and glasses doped with rare-earth ions was widely studied.Research works recently still focus on looking for the best upconversion materials and on understanding the upconversion excitation mechanism in depth.There are usually many possible transition routes that might be listed by the analysis based only on the energy match and the excitation intensity dependence of the upconversion emission,because of rich levels in the 4f~N configuration of the rare earth ions.The purpose of this thesis is to show how the measurements of the upconversion dynamics can help to clarify the upconversion excitation mechanism.
The thesis is organized in the following 4 sections:
1.A square wave light source was built.Square wave light with tunable repetition rate was obtained from a laser diode driven by a square wave power supply,which consisted from four parts:multi-harmonic oscillator,frequency divider,preamplifier and driver.
2.Temporal behaviors of the green and red upconversion emissions in Er~(3+) doped oxyfluoride tellurite glass were measured with the square wave lights excitation from 808 nm and 980 nm laser diodes.Rate equations were established to describe the dynamical processes of the upconversion by excited state absorption.Through analyzing the rise and decay curves of the up conversion emissions,the intermediate states,consequently the excitation routs of the up conversion have been determined.The results clearly indicated that the green upconvertions under 980 nm and 808 nm excitations were with different intermediate states.Saturation of the green up conversion emission under the excitation of 808 nm was studied.By fitting the power dependence deduced from the steady state rate equations with the experimental results, it was concluded that the saturation of the upconversion emission was originated from the saturation of excited state and ground state absorptions.
3.Rise and decay curves of the upconversion emissions from ~2H_(11/2)+~4S_(3/2) and ~4F_(9/2) states of Er~(3+) in Er~(3+) doped and Er~(3+),Yb~(3+) codoped oxyfluoride tellurite glass excited by 980 nm square wave laser were measured and compared.Effects of the energy transfer by sensitizer,Yb~(3+),on the rise and decay behaviors were analyzed.Rate equations were established for the upconversions originated from both the stepwise energy transfer and the excited state absorption processes.Contributions from both processes were determined by analyzing the rise curves of the upconversions in Er~(3+)-doped and Er~(3+)-Yb~(3+)-codoped samples.Upconversion excitation routs in the studied samples were then clarified.
4.Rate equations of the upconversions in Er-doped and Er,Yb-codoped systems without the limitations of weak excitation and weak energy transfer were solved numerically.Excitation state absorption,energy transfer from Yb and cross relaxation between two excited Er~(3+) ions were considered.Fitting with the experimental results, the conclusions obtained from weak excitation approximation in the previous chapter were further confirmed.By varying the parameters,conditions for each upconversion excitation mechanism being dominant and the its reflections in decay and rise curves were determined.
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