【作者】 辛非非；

【导师】 张国权；

【作者基本信息】 南开大学 ， 光子学与光子技术， 2012， 博士

【摘要】 铌酸锂晶体是一种优良的人工晶体，具有很多重要的性质，如声光效应、电光效应、压电效应、热电效应、光折变效应等等，在很多方面具有广泛的应用前景。然而，由于铌酸锂晶体具有特殊并且复杂的内部缺陷结构，尤其是它的紫外深能级结构更加复杂，人们对于它的很多宏观效应对应的微观结构仍然处在猜测的阶段，严重影响了铌酸锂晶体在紫外波段的应用。因此，在紫外波段尤其是紫外带边附近的相关研究对于搞清其基本缺陷结构和性质具有重要的意义。本论文通过对掺杂铌酸锂晶体紫外带边附近的光折变效应的研究以及紫外带边光谱结构的测量和分析等手段系统地研究了掺杂铌酸锂晶体的深能级缺陷结构及其光学性质。论文的第一章综述了铌酸锂晶体的基本物理性质、目前关于铌酸锂晶体的缺陷结构模型以及铌酸锂晶体的非线性效应及主要应用。第二章着重介绍了光折变非线性效应的基本理论与表征方法，包括光折变效应的物理机制及探测手段，并且对于紫外吸收谱的研究方法也做了较系统的介绍。第三章系统介绍了二价、三价掺杂铌酸锂晶体在325nm紫外带边附近的光折变效应。过去对紫外光折变效应的研究大多使用的是351nm激光，研究发现铌酸锂晶体的紫外光折变效应呈现出与可见光截然不同的结果，引起了人们的广泛关注。更短波长的紫外激光可以激发更深能级上的载流子，通过对掺镁、掺锌以及掺铟铌酸锂晶体的325nm紫外光折变效应的研究，发现在325nm，这些高掺杂铌酸锂晶体的紫外带边光折变效应明显强于351nm的结果。例如，在掺锌9mol%样品中，我们得到了高达38cm^-1的二波耦合增益以及高达37.7cm/J的灵敏度；而掺镁9mol%样品的光折变记录响应时间只有73ms，也是目前铌酸锂晶体中测得的最短的响应时间。这些结果都说明，对于这些高掺杂样品，325nm是光折变全息存储的极为适宜的波段。通过对镁铁双掺铌酸锂晶体紫外光折变效应的研究，我们发现铁的掺入可以显著地增强掺镁铌酸锂晶体的光折变全息存储性能。第四章系统研究了四价掺杂铌酸锂晶体——掺铪和掺锡铌酸锂晶体在325nm处的紫外带边光折变效应。发现与可见光的情况相反，同成分纯铌酸锂晶体在掺锡以后紫外光折变效应有了显著的提高。另外，掺铪也起到了促进紫外光折变效应的作用。我们的实验结果说明，掺铪铌酸锂晶体和掺锡铌酸锂晶体是很好的紫外光折变材料，因为它们具有低掺杂阈值，快响应速度，很强的抗光斑畸变能力和较高的衍射效率。第五章对于同成分掺镁、掺铪和掺锆的铌酸锂晶体以及近化学计量比铌酸锂晶体的紫外带边结构进行了光谱研究，并且运用Bose—Einstein单声子模型以及Urbach定则对实验结果进行了理论分析。实验发现所有样品的光谱都具有很强的温度依赖性，紫外带边的位置随温度的升高会产生明显的红移。无论是Bose-Einstein拟合还是Urbach拟合的结果都表明，当掺镁、掺铪或掺锆浓度超过阈值以后，与吸收有关的有效声子的能量明显下降，并且电子—声子相互作用强度也在阈值浓度以上突然减弱，这些拟合结果很好地从微观上解释了为什么当这些“抗光折变掺杂离子”的浓度超过阈值以后铌酸锂晶体的光电导显著增强，进而抑制了可见光波段处的光折变效应。另外，随着晶体中锂含量的增加，发现在近化学计量比铌酸锂晶体中与吸收有关的有效声子的平均能量也相对于同成分样品有明显的下降，并且它的电子—声子相互作用也有所减弱。随着晶体温度的降低，所有铌酸锂晶体的吸收边发生紫移，而且在带边附近出现新的紫外吸收带。该紫外吸收带覆盖了我们紫外光折变效应的实验波段（325nm）。并且，这个紫外吸收带的峰值高度与掺杂铌酸锂晶体紫外光折变效应的变化规律十分相似，因此有理由认为，这个紫外吸收带对应的缺陷结构在铌酸锂晶体的紫外光折变过程中起到了重要的作用。第六章总结了本论文的主要研究成果及其意义，并对未来的关于掺杂铌酸锂晶体缺陷结构的研究工作进行了展望。更多还原

【Abstract】 Lithium niobate （LiNbO₃） is an excellent artificial crystal, which has manyimportant properties, such as acousto-optic, electro-optic, piezoelectric, pyroelectric,and photorefractive effects, etc. However, since the defect structure of LiNbO₃is veryunique and complicated, especially the ultraviolet （UV） deep level structures, manymicro-structure causes of the macro effects are still hypothesizes, which seriouslyhinder the development of the applications of LiNbO₃. Thus, the study in the UVregion especially that near to the UV band edge is very important in figuring out thefundamental defect structures of the crystal. In this dissertation, we systemicallystudied the optical properties of a variety of doped LiNbO₃samples and the relatedpossible deep level defect structures through the UV band edge photorefractivity andthe UV band edge absorption spectra structures.In chapter one, we introduced the basic physical properties of LiNbO₃. Then,presented several important models of defect structures of LiNbO₃, and gave a briefview of the nonlinear optical effects and the important applications of LiNbO₃.In chapter two, the fundamental theory of the photorefractive effect and itscharacterization were introduced, including the mechanism of photorefractive effectand the methods of detection. The detailed methods to study the absorption spectranear to the UV band edge were mentioned as well.In chapter three, the UV photorefractive properties of LiNbO₃samples doped withbivalent and trivalent dopants are systemically studied at325nm. In the past, most ofthe studies on the UV photorefractivity of LiNbO₃were carried out at the wavelengthof351nm, and people have found that the UV photorefractivity of doped LiNbO₃arequite different from the photorefractivity in the visible, which has attracted a lot ofattentions. Photons with a shorter wavelength are able to excite charge carriers ofdeeper levels. It was found that the UV photorefraction at325nm is enhanced muchmore signifcantly than that at351nm in LiNbO₃doped with Mg, Zn, and In. Forexample, two-wave coupling gain coeffcient as large as38cm1and a high photorefractive recording sensitivity of37.7cm/J were obtained in LiNbO₃dopedwith9.0mol%Zn. And a short response time of73ms was observed in LiNbO₃doped with9.0mol%Mg, which was the shortest response time reported in LiNbO₃so far. The results indicate that, in LiNbO₃doped with Mg, Zn, or In, the325nm is anexcellent wavelength for holographic storage. What’s more, doubly dopingMg:LiNbO₃crystal with Fe was found to improve the UV holographic storageproperties signifcantly.In chapter four, the UV photorefractive properties of LiNbO₃samples doped withtetravalent dopants such as Hf and Sn are systemically studied at325nm. On thecontrary of the result in visible, as the concentration of Sn increased, the UVphotorefractivity of LiNbO₃:Sn was improved significantly. On the other hand, theUV photorefractivity of LiNbO₃:Hf was also improved thanks to the tetravalentdopant Hf. Our experimental results suggested that LiNbO₃:Hf and LiNbO₃:Sn arepromising UV photorefractive materials with the advantages of low dopingconcentration, fast response speed, strong resistance to beam distortion, andacceptable diffraction efficiency.In chapter five, we studied the absorption spectra structure of the UV band edgeof LiNbO₃:Mg, LiNbO₃:Hf, and LiNbO₃:Zr with congruent composition andnominally pure LiNbO₃with both congruent and near-stoichiometric composition.The Bose-Einstein oscillator model and the Urbach law were used to analyze theexperimental results theoretically. The absorption band edge was found to bered-shifted significantly with the increase of crystal temperature in all LiNbO₃samples. As the results fitted by the Bose-Einstein expression and the Urbach lawsuggested, there was a significant drop in the average phonon frequency associatedwith the band edge absorption when the doping concentration of Mg, Hf, or Zrexceeds the threshold. The electron-phonon interaction was also weakened abovedoping threshold. The reduction in the electron-phonon interaction would lead to theincrease in the photoconductivity, which finally resulted in the suppression of opticaldamage in visible in LiNbO₃highly doped with these―anti-optical-damge dopants‖.What’s more, there was also a considerable reduction in the average energy of activephonons and electron-phonon interaction in near-stoichiometic LiNbO₃compared with the congruent ones. As the crystal temperature went down near to the cryogenictemperature, an absorption shoulder covering the wavelength of325nm showed up inall our samples. And the behavior of the height of the absorption peaks were quitesimilar to the observed UV band edge photorefractive properties, so it is reasonable todeduce that it is the defect structure corresponding to this absorption shoulder that isresponsible for the UV band edge photorefractive process in our LiNbO₃samples.In chapter six, we summarized our work in this dissertation, and then presentedour plans for the further research on the exploration of defect structures in dopedLiNbO₃.更多还原