Study On Ultraviolet Band Edge Photorefractivity And Defect Structures Of Doped Lithium Niobate Crystals

Lithium niobate （LiNbO₃） is an excellent artificial crystal, which has manyimportant properties, such as acousto-optic, electro-optic, piezoelectric, pyroelectric,and photorefractive effects, etc. However, since the defect structure of LiNbO₃is veryunique and complicated, especially the ultraviolet （UV） deep level structures, manymicro-structure causes of the macro effects are still hypothesizes, which seriouslyhinder the development of the applications of LiNbO₃. Thus, the study in the UVregion especially that near to the UV band edge is very important in figuring out thefundamental defect structures of the crystal. In this dissertation, we systemicallystudied the optical properties of a variety of doped LiNbO₃samples and the relatedpossible deep level defect structures through the UV band edge photorefractivity andthe UV band edge absorption spectra structures.In chapter one, we introduced the basic physical properties of LiNbO₃. Then,presented several important models of defect structures of LiNbO₃, and gave a briefview of the nonlinear optical effects and the important applications of LiNbO₃.In chapter two, the fundamental theory of the photorefractive effect and itscharacterization were introduced, including the mechanism of photorefractive effectand the methods of detection. The detailed methods to study the absorption spectranear to the UV band edge were mentioned as well.In chapter three, the UV photorefractive properties of LiNbO₃samples doped withbivalent and trivalent dopants are systemically studied at325nm. In the past, most ofthe studies on the UV photorefractivity of LiNbO₃were carried out at the wavelengthof351nm, and people have found that the UV photorefractivity of doped LiNbO₃arequite different from the photorefractivity in the visible, which has attracted a lot ofattentions. Photons with a shorter wavelength are able to excite charge carriers ofdeeper levels. It was found that the UV photorefraction at325nm is enhanced muchmore signifcantly than that at351nm in LiNbO₃doped with Mg, Zn, and In. Forexample, two-wave coupling gain coeffcient as large as38cm1and a high photorefractive recording sensitivity of37.7cm/J were obtained in LiNbO₃dopedwith9.0mol%Zn. And a short response time of73ms was observed in LiNbO₃doped with9.0mol%Mg, which was the shortest response time reported in LiNbO₃so far. The results indicate that, in LiNbO₃doped with Mg, Zn, or In, the325nm is anexcellent wavelength for holographic storage. What’s more, doubly dopingMg:LiNbO₃crystal with Fe was found to improve the UV holographic storageproperties signifcantly.In chapter four, the UV photorefractive properties of LiNbO₃samples doped withtetravalent dopants such as Hf and Sn are systemically studied at325nm. On thecontrary of the result in visible, as the concentration of Sn increased, the UVphotorefractivity of LiNbO₃:Sn was improved significantly. On the other hand, theUV photorefractivity of LiNbO₃:Hf was also improved thanks to the tetravalentdopant Hf. Our experimental results suggested that LiNbO₃:Hf and LiNbO₃:Sn arepromising UV photorefractive materials with the advantages of low dopingconcentration, fast response speed, strong resistance to beam distortion, andacceptable diffraction efficiency.In chapter five, we studied the absorption spectra structure of the UV band edgeof LiNbO₃:Mg, LiNbO₃:Hf, and LiNbO₃:Zr with congruent composition andnominally pure LiNbO₃with both congruent and near-stoichiometric composition.The Bose-Einstein oscillator model and the Urbach law were used to analyze theexperimental results theoretically. The absorption band edge was found to bered-shifted significantly with the increase of crystal temperature in all LiNbO₃samples. As the results fitted by the Bose-Einstein expression and the Urbach lawsuggested, there was a significant drop in the average phonon frequency associatedwith the band edge absorption when the doping concentration of Mg, Hf, or Zrexceeds the threshold. The electron-phonon interaction was also weakened abovedoping threshold. The reduction in the electron-phonon interaction would lead to theincrease in the photoconductivity, which finally resulted in the suppression of opticaldamage in visible in LiNbO₃highly doped with these―anti-optical-damge dopants‖.What’s more, there was also a considerable reduction in the average energy of activephonons and electron-phonon interaction in near-stoichiometic LiNbO₃compared with the congruent ones. As the crystal temperature went down near to the cryogenictemperature, an absorption shoulder covering the wavelength of325nm showed up inall our samples. And the behavior of the height of the absorption peaks were quitesimilar to the observed UV band edge photorefractive properties, so it is reasonable todeduce that it is the defect structure corresponding to this absorption shoulder that isresponsible for the UV band edge photorefractive process in our LiNbO₃samples.In chapter six, we summarized our work in this dissertation, and then presentedour plans for the further research on the exploration of defect structures in dopedLiNbO₃.