Study On Growth And Voltage Controlled Photorefractive Properties Of Paraelectric Potassium Lithium Tantalate Niobate

In recent years, paraelectric electro-optic crystals have attracted great attentions due to their potential applications in optical communication and optical information processing. Potassium lithium tantalate niobate (K1-yLiyTa1-xNbxO3, KLTN) has been considered as the best material for electroholographic applications. It exhibits enhanced photorefractive properties under external electric field, for combining the advantages of high diffraction efficiency, fast electro-optic response time (about 7 ns) and very large Kerr coefficients. However, the research on the physical properties and applications of KLTN has been hindered by the difficulty on growing high optical quality crystals. In this thesis, we investigated the growth, electro-optic and voltage controlled photorefractive properties of KLTN theoretically and experimentally.The top seed solution growth method was reviewed. The crystal growth system was designed and constructed according to the habits of crystal growth. The optimal growth procedure was investigated, and crystals with several compositions and dopant types were obtained. The crystals were with high composition uniformity, and no crack and striation was observed. The size of the as-grown crystal is up to 15×15×20 mm3. Basing on the experience of growing KLTN, we developed a new crystal sodium lithium tantalate niobate by substituting Na for Li. A solid/liquid interface diffusion model was introduced to explain the experimental results that some sodium lithium tantalate niobate crystals were blue.The physical properties of KLTN were investigated in detail. The structures of KLTN crystals were studied by using X-ray powder diffraction method. The crystals'lattice parameters were studied as a function of crystal composition. The dielectric coefficient of KLTN was studied; it changes with various temperatures obeying the Curie-Weiss law. The Curie temperatures of KLTN crystals were determined according to this law, and the relation between Curie temperature and crystal composition was obtained. The refractive indices of KLTN were measured accurately by the minimum deviation method. The Sellmeier's equation of refractive indices for KLTN was obtained in terms of least-squares fit. The optical transmission properties were investigated by a UV-Visible-NIR spectrometer, the absorption spectra of pure and doped KLTN were determined. Energy band gap of the crystals were calculated according to the crystals'ultraviolet absorbing edge.The refractive index changes under different external fields were calculated by using index ellipsoid. An auto scan Mach-Zehnder interferometer has been designed and built for measuring the quadratic electro-optic (QEO) properties of paraelectric KLTN. KLTN has large QEO coefficient s1 1 which is two orders higher than that of the other electro-optic materials. The reason of that QEO coefficient of KLTN changed with temperatures was explained by using the theory proposed by DiDomenico and Wemple. The polarization-optic coefficients of KLTN were calculated, and they didn't change with temperature within the experimental errors.The space-charge field of paraelectric KLTN was simulated using band transport model. The influence of the external field and grating period on the space-charge field was discussed. The diffraction behavior of volume phase grating in paraelectric KLTN was investigated under co-directional two-wave coupling configuration. The expression of two-wave coupling gain coefficient and diffraction efficiency was derived.The influence of transition metal dopant on photorefractive properties of KLTN was investigated using the two-wave mixing technique. The photorefractive properties of Mn doped and Fe doped KLTN were demonstrated for the first time, the dominant charge carrier was identified from the direction of two-wave coupling energy transfer. Comparing with the traditional Cu doped KLTN, Mn doped and Fe doped KLTN have higher diffraction efficiency, faster photorefractive response time and higher photorefractive sensitivity. Especially the Mn doped KLTN is a very promising voltage controlled photorefractive crystal.