摘要
掺铒光波导放大器(EDWA)是未来微光子系统的重要组件之一，Er:LiNbO_3晶体作为EDWA的基体材料具有稳定性高和集成性能好等优点，并可以将Er~(3+)离子的激光性能和LiNbO_3晶体的非线性光学性能结合在一起。Er:LiNbO_3晶体发射的1.54μm波段近红外发光对应现代光纤通信的标准“窗口”波长。然而，Er:LiNbO_3晶体的光损伤效应限制了晶体的实际应用。本论文选择不同价态Zn~(2+)，In~(3+)和Zr~(4+)离子提高晶体的光损伤性能，研究了不同价态抗光损伤离子对Er:LiNbO_3晶体缺陷结构和光学性能的影响，为获得高效率1.54μm波段近红外发光提供了理论依据和实验指导。
采用第一性原理的方法，研究了Zn~(2+)，In~(3+)和Zr~(4+)离子掺杂LiNbO_3晶体的几何结构。结果表明，Zn~(2+)，In~(3+)和Zr~(4+)离子在晶体中均优先占据Li位。研究了Zn~(2+)，In~(3+)和Zr~(4+)离子在Li位上的稳定性，认为在Er:LiNbO_3晶体中，Zn~(2+)离子会促使Er~(3+)离子进入Nb位，形成Er~(3+)离子团位束。In~(3+)离子对Er~(3+)离子占位影响不大。Zr~(4+)离子在Li位上稳定性较低，易于进入Nb位，使Er~(3+)离子继续占据Li位。
采用Czochralski方法生长了Er:LiNbO_3晶体，通过研究不同浓度Er~(3+)离子掺杂同成分LiNbO_3晶体的缺陷结构和不同激发波长下的近红外发射光谱，上转换发射光谱和上转换布局机制，提出了Er~(3+)离子占位与光学性能的关系，即Er:LiNbO_3晶体中Er~(3+)离子团位束的形成增大了交叉弛豫的发生概率，在980nm激发光源下，Er~(3+)离子团位束会抑制1.54μm波段近红外发光。研究了不同[Li]/[Nb]比(分别为0.94，1和1.25)对Er:LiNbO_3晶体缺陷结构和光学性能的影响。结果显示，当[Li]/[Nb]=1时，Er:LiNbO_3晶体中的Er~(3+)离子团位束被解离，发射高效率的1.54μm波段近红外发光。
研究了Er~(3+)(3mol%)/Zn~(2+)(3,6和7mol%):LiNbO_3晶体的缺陷结构和光学性能，结果显示，Er~(3+)离子并未影响Zn~(2+)离子在晶体中的阈值浓度；当Zn~(2+)离子掺杂浓度低于其阈值浓度时，晶体中的Er~(3+)离子团位束被解离，引起了1.54μm波段近红外发光的增强；当Zn~(2+)离子掺杂浓度高于其阈值浓度时，晶体中会再次形成Er~(3+)离子团位束，导致1.54μm波段近红外发光降低。结合第一性原理计算结果和Er~(3+)离子占位和光学性能的关系，生长了Zn~(2+)/Er~(3+)摩尔比例为3/1.5(mol%/mol%)，6/1(mol%/mol%)和7/1.5(mol%/mol%)的Zn/Er:LiNbO_3晶体。光学测试结果表明Zn/E(r6mol%/1mol%):LiNbO_3晶体具有最强的1.54μm波段近红外发光。Zn/Er:LiNbO_3晶体的OH-吸收峰Lorentz三峰分解和上转换绿光寿命测试结果表明，Zn~(2+)离子促使Er~(3+)离子在低于其阈值浓度时从Li位进入Nb位，在晶体中形成了Er~(3+)离子团位束。
研究了In/Yb/Er:LiNbO_3晶体(In~(3+)离子掺杂浓度为1，2和3mol%)的缺陷结构和光学性能。研究发现，Yb3+和Er~(3+)离子共掺降低了In~(3+)离子的阈值浓度；当In~(3+)离子达到阈值浓度时，晶体中形成Er~(3+)离子团位束。由于Yb3+离子的敏化作用，In/Yb/Er:LiNbO_3晶体中Er~(3+)离子团位束的形成有利于1.54μm波段近红外发光。In~(3+)离子掺杂浓度为阈值浓度时，晶体1.54μm波段发射最强。
Zr/Er:LiNbO_3和Zr/Yb/Er:LiNbO_3晶体近红外发射光谱测试结果显示Zr~(4+)离子提高了1.54μm波段近红外发射效率。通过晶体Raman光谱和Er~(3+)离子4I11/2→4I15/2跃迁在1020nm处的衰减曲线测试结果分析了Zr~(4+)离子增强近红外发光强度的机理是晶体最大声子能量的增大。不同价态Zn~(2+)，In~(3+)和Zr~(4+)离子掺杂Yb/Er:LiNbO_3晶体的1.54μm波段发射性能结果显示，Zn~(2+)离子降低了1.54μm波段发射，In~(3+)离子对此波段发光没有影响，Zr~(4+)离子大幅度增强了1.54μm波段发射。因此Zr/Yb/Er: LiNbO_3晶体将成为最有前景的优良波导器件基质材料。
采用Judd-Ofelt理论研究了Zn~(2+)， In~(3+)和Zr~(4+)离子掺杂Er:LiNbO_3和Yb/E r:LiN bO_3晶体的光谱性质，Zr/Yb/Er:LiNbO_3晶体具有最大的光学品质因子值。通过McCumber和Füchtbauer-Ladenburg理论分析了抗光损伤离子掺杂Er:LiNbO_3和Yb/Er:Li NbO_3晶体1.54μm波段的发射性能，结果显示，晶体均具有大的发射截面积，适合作为EDWA的基体材料。
Er-doped wavelength amplifiers (EDWAs) will be one of the key componentsof futher microphotonic systems, and Er:LiNbO_3crystals, used as the host materialsfor EDWAs, have excellent advantages such as the high stabilization and integration.Furthermore, Er:LiNbO_3crystals could combine the laser performances of Er~(3+)ionwith the nonlinear characteristics presented by LiNbO_3crystal. Er:LiNbO_3emits1.54μm near infrared emission, which corresponds to the standard communication“window” wavelength. Howerve, the practical applications of Er:LiNbO_3crystalsare limited by the optical damage effect. Different valence Zn~(2+), In~(3+)and Zr~(4+)ionsare chosen to enhance the optical damage of LiNbO_3crystals. We performinvestigations on the effect of different valence optical damage resistant ions on thedefect structure and optical characteristics of Er:LiNbO_3crystals, which provide thetheoretical and experimental direction for us to obtain the high efficient1.54μmnear infrared emission.
The geometry structure of LiNbO_3crystals doped with Zn~(2+), In~(3+)and Zr~(4+)ionsare calculated by the first principle theory, which shows that Zn~(2+), In~(3+)and Zr~(4+)ionsoccupy Li sites firstly. The stuies on the stability of Zn~(2+), In~(3+)and Zr~(4+)ionsoccupied Li sites show that Zn~(2+)ions will push Er~(3+)ions into Nb sites, and the Er~(3+)clusters sites are formed in Er:LiNbO_3crystals, and In~(3+)ions have little effect on theoccupancy of Er~(3+)ions, and the low stability of Zr~(4+)ion makes Er~(3+)ions stilloccupy Li sites.
The Er:LiNbO_3crystals are grown by Czochraski method. The defect structureof Er:LiNbO_3with different Er~(3+)ion concentration, as well as the near infraredemission, upconversion emission and mechanism under different excitationwavelength are studied. It has been proposed that the formation of Er~(3+)cluster sitescould increase the rate of cross relaxation processes. Under980nm excitation, Er~(3+)cluster sites will suppress the1.54μm near infrared emission. Studies on the effectof [Li]/[Nb] ratio (is equal to0.94,1and1.25, respectively) on the defect structureand optical characteristics of Er:LiNbO_3crystals show that as for [Li]/[Nb]=1, theEr~(3+)cluster sites are dissociated and the strongest1.54μm emission is observed inEr:LiNbO_3crystal.
The studies on the defect structure and optical characteristics of Er~(3+)(3mol%):LiNbO_3crystals doped with3,6and7mol%Zn~(2+)ions show that thethreshold concentration of Zn~(2+)ion is not affected by Er~(3+)ions in Zn/Er:LiNbO_3crystal. When the concentration of Zn~(2+)ions is lower than its threshold, the Er~(3+)cluster sites are dissociated by Zn~(2+)ions, which results in the enhancement of1.54 μm near infrared emission. When the concentration of Zn~(2+)ions is higher than itsthreshold, the reformantion of Er~(3+)cluster sites are observed, which decreases the1.54μm near infrared emission. Combined the first principle theory with the relationbetween the occupancy and optical characteristics of Er~(3+)ions, the Zn/Er:LiNbO_3crystals with the3/1.5,6/1and7/1.5mol ratio of Zn~(2+)/Er~(3+)are grown. The strongest1.54μm emission is found in Zn/Er (6mol%/1mol%):LiNbO_3crystal. Stuies on theOH-absorption Lorentz three decomposition and the decay curves of upconversiongreen emission show that Zn~(2+)ions would push Er~(3+)ions from Li sites into Nb sites,and the Er~(3+)cluster sites are formed at lower concentration.
The defect structure and optical characteristics of Yb/Er:LiNbO_3crystal dopedwith1,2and3mol%In~(3+)ions are studied. The experiemtal results show that Yb3+and Er~(3+)ions decrease the threshold concentration of In~(3+)ions. When theconcentration of In~(3+)ion is equal to its threshold, the Er~(3+)cluster sites are formed.The formation of Er~(3+)cluster sites is favor for the1.54μm near infrared emissiondue to the sensization effect of Yb3+ion. The strongest intensity of1.54μmemission is observed in Yb/Er:LiNbO_3crystal codoped with the thresholdconcentration of In~(3+)ions.
Zr~(4+)ion doping enhances the1.54μm emission in Zr/Er:LiNbO_3andZr/Yb/Er:LiNbO_3crystals. Raman spectra and the decay curves of Er~(3+)4I11/2→4I15/2transition at1020nm indicate that the enhancement of the1.54μm emission arisesfrom the increased maximum phonon energy of host material. The near infraredspectra of Yb/Er:LiNbO_3crystals doped with the different valence Zn~(2+), In~(3+)andZr~(4+)ions suggest that Zn~(2+)doping decreases the1.54μm emission, In~(3+)ions havealmost no effect, and Zr~(4+)ions lead to a drastical increase of1.54μm emission.Zr/Yb/Er:LiNbO_3crystal could be considered as the most outstanding host materialsfor waveguides.
The spectral characteristics of Er:LiNbO_3and Yb/Er:LiNbO_3crystals dopedwith Zn~(2+), In~(3+)and Zr~(4+)ions are studied by Judd-Ofelt theory, and Zr/Yb/Er:LiNbO_3crystal has the largest spectroscopic quality factor. The emission performance of1.54μm emission is analyzed by McCumber and Füchtbauer-Ladenburg theory. Allthe crystals have large emission cross section, which is suitable for the hostmaterials of EDWA.
引文
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