Study On The Effect Of Indium/Scandium Doping On The Micro Structure And Optical Properties Of Lithium Niobate Crystal

Lithium niobate（LiNbO₃）crystal is known as"optical silicon"in the optoelectronic industry.Due to its advantages in electro-optic,acousto-optic,nonlinear optics and photorefractive properties,LiNbO₃ material has attracted much attention in electro-optic modulation,surface acoustic wave devices,optical frequency conversion,holographic data storage and integrated optics.The rapid development of related fields proposed an urgent demand for iterative and performance optimization of LiNbO₃ crystal materials.Studies have shown that the optical properties of LiNbO₃ crystals are closely associated with the micro-defect structure in the crystal,and manipulating the micro-defect structure is an important measure to improve the properties and efficiency of LiNbO₃ crystal.At present,introducing doped ions is an effective method to regulate the defect structure in LiNbO₃ crystals.Therefore,studying the effect of ion doping on the defect structure and optical properties of LiNbO₃ crystals is of great significance for achieving new functions and new applications of LiNbO₃.In this dissertation,the defect structure,photorefractive properties,upconversion luminescence properties and resistance to photodamage of In/Sc trivalent ion-doped LiNbO₃ crystals were mainly studied.The effect of In/Sc doping ions on lattice defects of LiNbO₃ was analyzed.The results reveal the effect mechanisms of the corresponding micro-defect structure on the photorefractive properties,upconversion luminescence properties,and photodamage resistance,which provide an effective strategy for the doping modification of LiNbO₃crystals.In/Sc:Ru:Fe:LiNbO₃ crystals with no macroscopic defects and excellent optical properties were grown by the Czochralski method.The defect structure and photorefractive energy of In/Sc:Ru:Fe:LiNbO₃ crystal were studied.The influence of doping concentration of In³⁺/Sc³⁺dopants on the defect structure of birefringent Ru and Fe（transition/transition）doped LiNbO₃ crystals was studied by means of IR absorption spectrum,UV visible absorption spectrum and X-ray diffraction spectrum.The threshold concentration of In³⁺/Sc³⁺ions occupying the defect of Nb _Liwas obtained,and the occupation substitution model of In/Sc:Ru:Fe:LiNbO₃ crystals was proposed.By testing the performance parameters and exponential gain coefficient of photorefractive holographic storage,it is found that the diffraction efficiency and exponential gain coefficient decreased with the increase of In³⁺/Sc³⁺doping amount at the wavelength of 633 nm,while the two coefficients are decreased at 476 nm.In accordance with the microstructure of the crystal,the mechanism of the crystal performance change caused by lasers with different wavelengths is clarified.In/Sc:Yb:Ho:LiNbO₃ and In/Sc:Yb:Er:LiNbO₃ crystals were grown by Czochralski method using two kinds of rare earth ions,Ho³⁺and Er³⁺,as luminescent centers,Yb³⁺ions as sensitizers and In³⁺/Sc³⁺ions as modifiers.The crystal defect structure and upconversion luminescent properties were studied.It is shown that the spectrum changes induced by ion occupation in In³⁺/Sc³⁺doped LiNbO₃ crystals are the same in（rare earth/rare earth）and（transition/transition）ions.The green upconversion emission intensity of Ho³⁺/Er³⁺increases when the doping concentration is lower than the threshold concentration（3 mol%）.However,the green upconversion emission intensity of Ho³⁺/Er³⁺decreases significantly when In³⁺doping is equal to or higher than the threshold concentration,and the upconversion emission intensity of Ho³⁺/Er³⁺is not restrained when Sc³⁺doping is equal to the threshold concentration.Combined with the analysis of defect structure and energy level structure,it is revealed that the changes in Yb³⁺/Ho³⁺/Er³⁺site beam concentration,OH group energy and unit cell volume caused by In³⁺/Sc³⁺doping are the main reasons for the above-mentioned changes in upconversion emission performance.That is,when the concentration of In³⁺/Sc³⁺is lower than its threshold concentration 3 mol%(substitute Nb _Li⁴⁺),Yb³⁺/Ho³⁺/Er³⁺preferentially occupies the Li site and a small part occupies the Nb site,forming Yb³⁺/Ho³⁺/Er³⁺solitary defect centers and Yb³⁺/Ho³⁺/Er³⁺cluster beam defect centers.When the In³⁺/Sc³⁺doping concentration exceeds the threshold concentration,it replaces the normal Li site and Nb site,which causes the dissociation of the beam defect center of Yb³⁺/Ho³⁺/Er³⁺cluster,and induces the blue shift of OH vibration energy.As a result,the upconversion luminescence intensity is decreased.It is also found that Sc doping will reduce the unit cell volume,leading to an enhanced energy transfer rate between rare earth ions,which also counteracts the influence of the defect concentration of the cluster beam and the blue shift of OH energy,and the upconversion luminescence intensity is creased.Finally,the effects of different concentrations of In³⁺/Sc³⁺ions on the photodamage resistance of（transition/transition）and（rare earth/rare earth）triple-doped LiNbO₃ crystals were studied.It was found that the light damage resistance of the crystal increased with the increase of In³⁺/Sc³⁺ion concentration.The photodamage resistance is improved by more than two orders of magnitude compared with double-doped LiNbO₃ crystal,and the mechanisms of In³⁺/Sc³⁺ion doping induced defect structure change and the optimizing of the photodamage resistance are revealed.