| Lithium niobate (LiNbO3, or LN) is one of the most used synthetic crystals. It not only has excellent physical properties such as electro-optic, acousto-optic, photorefractive, piezoelectric and thermal-optic effects, etc, but also has many advantages such as stable mechanical properties, easy processing, high temperature and corrosion resistant, abundant raw material source, low price, easy to grow large crystals, etc. Meanwhile, adding different dopants into LiNbO3can make it obtain various properties. Thereby it can be applied for surface acoustic wave filter, optical waveguide, electro-optical modulation, frequency conversion and holographic storage, etc.Holographic data storage promises to be one of the next generation storage technologies. Despite many efforts, it is not yet mature for commercial application because the ideal material is lacking. Although the widely used photorefractive holographic storage material, namely iron doped LiNbO3(LN:Fe), performs well in some respects, it is still too slow and volatile. Even if LiNbO3is doped with iron and manganese (LN:Fe,Mn) which solves the volatility problem, the material still responds very slowly. On the other hand, LN:Fe crystals only have excellent photorefraction in blue and green region, but in ultraviolet range only have strong absorption. Besides, LN is a platform of optical integrated circuits. However, LiNbO3practically always serves as a passive material. Until today there is rare report on active component based on LN. The main problem is the lack of p-type LN.The doped LiNbO3crystals have multi-function and versatility, but currently the valences of dopants are all below5+, which is the valence of Nb. These doped ions preferably occupy Li sites. Then a crucial question is arisen:if LiNbO3is doped with ions of valence6+or more, whether these ions may occupy Nb sites and thereby induce new effects?In this thesis, we studied on the photorefractive properties of LiNbO3doped with the hexavalent molybdenum ion Mo6+(LN:Mo), co-doped with zirconium ion (Zr4+) and molybdenum ion (LN:Zr, Mo) and co-doped with magnesium ion (Mg2+) and molybdenum ion (LN:Mg, Mo). And we intend to control the photorefractive properties of LN:Mo and obtain p-type LiNbO3by polarization technology. The structure of defects and the relationship of the defects and properties are also investigated.In chapter one, we introduced the basic structure, physical properties and defect structural models of lithium niobate crystals, then gave a brief view of the doping engineering.In chapter two, we grew LN:Mo, LN:Zr,Mo and LN:Mg,Mo crystals, the effect of polarization was also investigated.In chapter three, the photorefractive properties of Mo-doped crystals are studied. The experimental results show that LN:Mo allows for holographic storage from UV to the visible. The optimal doping concentration of LN:Mo crystal is0.5mol%, which has fastest respond and good saturated diffraction efficiency. The photorefractive properties of LN:Mo can be improved by the enhancement of polarization current. Especially, when the polarization current was enhanced to145mA, the response time of LN:Mo can be shortened to as small as0.35s with a still high saturation diffraction efficiency of about60%at351nm, while the response time is also in the order of second in visible range. The results of X-ray Photoelectron Spectroscopy show that the valences of the Mo ions were4+,5+and6+. The results of X-ray single crystal diffraction analysis show that the Mo6+ions occupy Nb-sites. The excellent photorefraction of LN:Mo can be attributed to Mo6+ions occupying regular Nb-sites and forming new defects of Mo+Nb.In chapter four, we studied the photorefractive properties of LN co-doped with0.5mol%Mo and Zr or Mg. The experimental results show that LN:Mg,Mo and LN:Zr,Mo also allows for holographic storage from UV to the visible. The OH-spectra exhibit that6.5mol%Mg and2.5mol%Zr have exceed their thresholds in LN:Mo crystals. It is interesting that ZrO2cannot improve the photorefraction of LN:Mo even when its concentration is above the threshold. However, when the. concentration of MgO is6.5mol%, a very short photorefractive response time of0.22s,0.33s,0.37s and1.2s for351,488,532and671nm was obtained, respectively. Up to now, it is the only report that can realize holographic storage from UV to the visible with so fast response speed. In chapter five, the influence of the polarization condition on the type of photorefractive carriers in LN:Mo crystals were investigated. The experimental results showed that the color and properties of LN:Mo crystals were greatly influenced by the polarization current and polarization time. If LN:Mo crystal was polarized with the current of70mA for30minutes, the main carriers can be transformed from electrons to holes at351nm.In chapter six, we summaried the work of this dissertation, and gave an outlook of the further research on molybdelem doped LiNbO3was also presented. |
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