The effect of thermal treatment on the photorefractive properties of Ru-doped lithium niobate was studied. First, lithium niobate (LiNbO₃) single crystals doped with Ruthenium (Ru) were grown by the Czochralski method. In this study, the six samples of Ru:LiNbO₃ crsytals, two as-grown (with 0.01mol% and 0.065 mol%RuO₂, respectively), two oxidized ones (with 0.01mol% and 0.065 mol%RuO₂, respectively), and two reduced ones (with 0.01mol% and 0.065 mol%RuO₂, respectively), were prepared. Ru ions have three valences Ru³⁺, Ru⁴⁺, and Ru⁵⁺. The concentrations of each Ru ions valence in lithium niobate would be altered with different conditions of thermal treatment. To get the oxidized crystal, the as-grown crystals were placed under the O₂ atmosphere at the 1000^oC for 12h. The oxidized treatment was changed the Ru valences in the as-grown crystal from Ru⁴⁺ to Ru⁵⁺ and increased the ratio of Ru⁵⁺/Ru⁴⁺. To get the reduced crystal, the as-grown crystals were placed under the 100ppm CO/CO₂ mixture atmosphere at the 1000^oC for 12h. The reduced treatment was changed the Ru valences in the as-grown crystal from Ru⁴⁺ to Ru³⁺ and increased the ratio of Ru³⁺/Ru⁴⁺. Fig. 1 shows the 0.01mol% Ru:LiNbO₃ crystals with as-grown, oxidization, and reduction respectively. Fig. 2 shows the 0.065mol% Ru:LiNbO₃ crystals with as-grown, oxidization, and reduction respectively. We can see that the color of the as-grown Ru:LiNbO₃ crystal is orange and darkened as the Ru concentration increased . In addition, the color of oxidized crystals are lighter slightly, the color of oxidized crystals are darkened and had a little puce. Fig. 3 and 4 show the absorption spectra of as-grown, oxidized, and reduced Ru-doped LiNbO₃ crystals with different Ru concentrations. We can observe that there is a absorption peak centered around on 530nm in the Ru:LiNbO₃ crystals, and the absorption coefficient increase as the RuO₂ dopant concentration increases. Furthermore, the absorption coefficients at 530nm decrease after the oxidized treatment and increase after the reduced treatment.

In addition, the photorefractive properties of the Ru: LiNbO₃ crystals will be changed by different valences of Ru dopant. They were investigated with the two-beam coupling method. The diffraction efficiency, holographic writing time constant, erasing time constant, the sensitivity, and the dynamic range of Ru-doped LiNbO₃ crystals after different thermal treatment conditions would also be discuss in this study.

Fig. 1 Photography of 0.01mol%Ru:LiNbO₃ crystals with different thermal treatment conditions; 1-1:As-grown; 1-2: Oxidization; 1-3: Reduction.

Fig. 2 Photography of 0.065mol%Ru:LiNbO₃ crystals with different thermal treatment conditions; 2-1:As-grown; 2-2: Oxidization; 2-3: Reduction.

Fig. 3 Absorption spectra of 0.01mol%Ru:LiNbO₃ crystals with different thermal treatment conditions.

Fig. 4 Absorption spectra of 0.065mol%Ru:LiNbO₃ crystals with different thermal treatment conditions.

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