Effect Of Sc³⁺ Concentration On Defect Structure And Holographic Storage Properties In Sc:Fe:Cu:LiNbO₃ Crystal

The volume holographic storage technology has gradually developed into a popular storage technology in today’s society because of the advantages of high storage density,high transmission rate and low energy consumption.Lithium niobate（LiNbO₃）crystal is regarded as an excellent volume holographic storage medium material as a result of its excellent photorefractive properties.In this thesis,LiNbO₃crystal co-doped Fe³⁺ions and Cu²⁺ions was selected as the study object,and the two kinds of ions could be used as the double photorefractive center of the crystal to improve the photorefractive performance of the crystal.On this basis,Sc³⁺ions doping with low threshold concentration were selected to improve the ability of optical damage resistance of the crystal.In order to further explore the modification effect of Sc³⁺ions doping,the influence of Sc³⁺ions on the crystal defect structure and holographic storage performance was systematically studied.High-quality Sc:Fe:Cu:LiNbO₃crystals doped with different concentrations of Sc³⁺（0,1,2,3 mol%）and co-doped Fe³⁺and Cu²⁺ions（1 mol%）were grown by Czochralski method.After a comprehensive summary from the crystal pretreatment process to processing process,the optimal process parameters which were suitable for crystal growth were determined.The crystals processed by polarization,directional processing,cutting,grinding and polishing would be used for the next stage of testing.The effective segregation coefficients of Sc³⁺,Fe³⁺and Cu²⁺ions were measured by ICP-AES.The test results showed that with the increasing of Sc₂O₃concentration in the melt,the effective segregation coefficient of Sc³⁺ions decreased gradually,and the effective segregation coefficients of Fe³⁺and Cu²⁺ions were close to 1 at different growth rates.According to the infrared absorption spectrum test,Sc³⁺ions preferentially replaced anti-Nb defects(Nb_Li⁴⁺)after entering the lattice.When the Sc³⁺ions concentration reached 3 mol%,Sc³⁺ions completely replaced Nb_Li⁴⁺defects and occupied the normal Li and Nb sites in the lattice,forming Sc_Li²⁺-Sc_Nb^2-defects groups,and the absorption peak appeared blue shift phenomenon.It illustrated that the Sc³⁺ions reached the threshold concentration at this time.It could be seen from the X-ray diffraction test that there were no new diffraction peaks after Sc³⁺ions entered the lattice,which proved that the unit cell structure had not changed.After the calculation of lattice constants,it was found that the unit cell volume increased first and then began to shrink,which could indicate that the change of Sc³⁺ions concentration would cause the change of ions occupation in the lattice and form different types of defect groups.The photorefractive properties of crystals were tested by two-wave coupling experiment,and the influence of the different Sc³⁺ions concentration on the photorefractive property was talked out.The experimental results showed that when the Sc³⁺ions concentration increased from 1mol%to 3mol%,the writing time decreased from 705.62 s to 250.45 s,and the dynamic range increased from 1.06 to7.21.The saturated diffraction efficiency increased from 52.1%to 68.1%and the time was shortened by nearly half.The photorefractive sensitivity increased from0.02 cm/J to 0.066 cm/J,the saturation refractive index modulation increased from4.72×10^-5to 5.36×10^-5,and the photoconductivity increased from 0.35×10^-12cm/Ω·W to 0.99×10^-12cm/Ω·W.The above results showed that the photorefractive property of the crystal was significantly improved with the increasing of Sc³⁺ions doping concentration.In this process,Sc³⁺ions were not involved in carrier transport and grating formation,mainly by changing the occupation mode of doped ions and forming new defect groups to improve the holographic storage performance of crystals.Finally,the refractive index of the crystals doped with different concentrations of Sc³⁺ions in all directions was tested by the birefringence gradient method,and the optical uniformity of the crystals was characterized by calculating the birefringence gradientΔR.It could be seen from the experimental results thatΔR was inversely proportional to the doping ions concentration,and the smaller theΔR value was,the better the optical uniformity of the crystal was.When the Sc³⁺ions concentration was 3 mol%,Sc:Fe:Cu:LiNbO₃crystal had the minimum birefringence gradientΔR value of 3.3×10^-5cm^-1,which was holographic storage material with the best optical uniformity.