| 摘要: | 多年來，光折變材料運用在全像記憶儲存的應用上受到廣泛的研究，而鈮酸鋰晶體因為擁有良好的光電及非線性光學係數，且可利用柴氏長晶法生長大尺寸且高品質的晶體，因此成為最熱門的光折變材料之一。然而，未摻雜鈮酸鋰晶體的光折變性質相對地較差，通常可透過摻雜一些過渡金屬離子增強晶體的光折變性質，因此本研究是利用自行架設的柴氏長晶系統生長各種不同濃度的摻釕共熔配比鈮酸鋰（Ru:LiNbO3）晶體。摻釕鈮酸鋰晶體的顏色為紅色，且隨著摻雜濃度的增加，晶體顏色也會慢慢的加深。而在晶體生長過程中，由於釕離子在鈮酸鋰晶體中的偏析係數大於1，且RuO2會揮發的因素，釕濃度會沿著晶體生長方向慢慢的減少。然而，在Ru:LiNbO3中存在一摻雜濃度極限值，晶體中最大的釕濃度大約為0.2mol%。此外，摻釕鈮酸鋰的A軸與C軸的晶格常數隨著摻雜濃度的增加而減少。透過紫外可見光吸收光譜的檢測顯示了摻釕鈮酸鋰有兩個明顯的吸收峰值位於370nm和530nm附近，隨著濃度的增加，吸收係數也會增加，而且吸收邊會往較長波長偏移。另外，透過OH-吸收光譜顯示在我們目前的摻雜濃度下， Ru離子在晶體當中同時取代Li空缺（V-Li）及反位鈮( )的位置。 由於Ru：LiNbO3存在一吸收峰位於可見光波段的530nm附近，因此我們利用532nm固態雷射去瞭解晶體的光折變特性。我們在0.12mol%的Ru:LiNbO3中發現了最大的繞射效率約為71％，而且繞射效率隨著摻雜量增加而提升。透過黑暗衰減的實驗發現Ru離子的能階低於Fe離子的能階，且摻雜量越多，衰減速率越快。我們也發現Ru:LiNbO3有一較短的消除時間，且隨著消除光的能量增加而遞減，此外晶體的光敏感度與動態範圍也將由於Ru摻雜到晶體中而提升。 透過氧化及還原處理改變釕離子的價數，我們也將瞭解熱處理對於Ru:LiNbO3晶體的光折變特性的影響。當晶體經過氧化處理過後的有較小的增益係數和繞射效率，因為在淺能階的載子都往深能階移動，造成淺能階的載子無法被532nm雷射所激發。另外，晶體經還原處理過後的有較大的增益係數和繞射效率是因為存在比較淺能階的Ru3+離子的濃度增加所造成的。 利用氣相傳輸平衡法（VTE）可將利用柴氏法生長出的共熔配比摻釕鈮酸鋰（Ru:CLN）轉換成化學計量配比（Ru:SLN）。當我們增加VTE的反應時間時，晶體內部的[Li]/[Nb]的比例也將慢慢增加，經過200小時的VTE處理後即可得到近化學計量配比摻釕鈮酸鋰。透過532nm雷射的雙波混和實驗檢測其光折變性質可知，Ru:SLN晶體相對於共熔配比晶體有較大的增益係數，較小的繞射效率，較快的反應時間，以及較高的光靈敏度。 通常釕離子有三種不同的價數，且各存在不同的能階上。透過藍光的照射，在晶體的光致吸收光譜中可發現位於480和530nm處有兩個吸收峰值產生，因此證明Ru:LiNbO3晶體中有深淺能階的存在，所以摻釕鈮酸鋰晶體可用於非揮發性的全像儲存上。經由非揮發性實驗檢測中發現，當晶體經過藍光的照射之後，可提升晶體的繞射效率與反應時間，而還原晶體比氧化晶體有較高的繞射效率，但只有在氧化的晶體中才可實現非揮發性全像記憶。 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. |
