| dc.description.abstract | Photorefractive materials have been widely investigated in holographic data storage application for many years. One of the most popular photorefractive crystals is lithium niobate (LiNbO3) for the fact that it has large electro-optical and nonlinear optical coefficients, and it can be easily grown into a large size with excellent optical quality. However, un-doped congruent LiNbO3 crystals usually have relatively poor photorefractive properties. One of the most effective methods to enhance their performance is to add a small amount of transition metal ions. In this study, Ruthenium (Ru) doped lithium niobate (LiNbO3) single crystals with different Ru concentrations were grown by the Czochralski method from a congruent melt composition. The color of the Ru: LiNbO3 was red and darkened with increasing Ru concentration. The Ru concentration in the grown crystal gradually decreased along the pulling direction because the effective segregation coefficient of Ru in lithium niobate is greater than one, and also because RuO2 evaporated during the crystal growth period. However, a solubility limit does exist in Ru: LiNbO3 crystals, so the maximum amount of Ru doped into LiNbO3 single crystals is about 0.2 mol%. In addition, the lattice constants of Ru: LiNbO3 on the A- and C-axis decreased with the concentration of Ru in the crystal. An absorption spectra examination of Ru: LiNbO3 showed that there were two absorption peaks around 370nm and 530nm that is within the UV/VIS region. The absorption coefficients should increase, and the absorption edges shift toward longer wavelength as the Ru concentration increases. The OH- absorption spectra confirm that Ru ions may have substituted for both Li vacancies (V-Li) and antisite Nb ions ( ) as the Ru doping concentration in the present case is well below the threshold limit.
We investigated the photorefractive properties of Ruthenium (Ru)-doped LiNbO3 crystal with absorption peak around 530nm via 532nm solid-state laser. Maximum measurement reached 71% recorded for the 0.12mol%Ru:LiNbO3 crystal recording; the diffraction efficiency of the crystals increased with the Ru concentration. The dark decay time constant of Ru:LiNbO3 decreased with the Ru concentration and it could be deduced that in lithium niobate the energy level of Ru center was shallower than the Fe center. The erasing time constant of Ru:LiNbO3 crystals was short and decreased with the erasing beam intensity. Furthermore, both the sensitivity and dynamic range of the crystal could also be improved with Ru center into the lithium niobate.
The effect of post treatment on the photorefractive properties of Ru-doped lithium niobate was also studied. It was found that the oxidized Ru:LiNbO3 had smaller exponential gain coefficient and diffraction efficiency because the charges in the shallow level were exchanged to the deep level. On the other hand, the reduced Ru:LiNbO3 crystals had larger exponential gain coefficient and diffraction efficiency due to the increase of the Ru3+ which existed in the shallow level.
Ruthenium (Ru)-doped near-stoichiometric lithium niobate crystals (Ru: SLN) were prepared by the vapor transport equilibration (VTE) technique from Ru-doped congruent lithium niobate grown using the Czochralski method. Increasing the duration time of the VTE treatment would cause the ratio of [Li]/[Nb] in the crystal to increase. When the VTE treatment time reached 200 hours, Ru-doped near-stoichiometric lithium niobate crystals were obtained. Two-beam coupling examination with a 532nm laser showed that the Ru-doped near-stoichiometric lithium niobate crystals had a larger exponential gain coefficient, lower diffraction efficiency, faster response time, and higher sensitivity than did the Ru-doped congruent lithium niobate crystals.
Ru ions have three valences, Ru3+, Ru4+, and Ru5+, and these ions have different energy levels in crystal. An examination of the blue light-induced absorption change shows two peaks around the 480nm and 532nm. If the Ru:LiNbO3 crystals had deep and shallow levels, there would be light-induced absorption change peaks. So Ru:LiNbO3 can be considered a good candidate to be used for nonvolatile holographic storage. In the examination of the nonvolatile holographic storage experiment, the diffraction efficiency and the response of the crystal can be improvde by blue light illumination. In addition, the reduced Ru-doped lithium niobate crystal has larger diffraction efficiency than oxidized one. But only the oxidized Ru:LiNbO3 crystal can be successfully used in doing nonvolatile holographic recording. | en_US |