Study On The Growth And Photorefractive Characteristics Of Uranium-doped Lithium Niobate Crystal

Lithium niobate crystal（LiNbO₃,LN）is a multi-functional and multi-purpose material,evaluated as"optical silicon"by Nature Materials Update.The excellent photorefractive characteristics of LN make it have broad development and application prospects in the field of holography,such as holographic data storage and holographic display.However,LN crystals still have problems such as low growth efficiency,poor crystal uniformity,slow photorefractive response,and limited bands for realization of holography,which limit their commercial applications.Aiming at the above problems,this work used the modified vertical Bridgman method with optimized process and parameter conditions to grow LN crystals,and uranium ions which possess multiple valence states were selected as the dopant.The main research results were as follows:1.By optimizing the growth process conditions and parameters,a series of 1 inch LN crystals doped with different concentrations of uranium dioxide have been successfully grown by the modified vertical Bridgman method.The surface of the crystals was brighter and the color was brown.The XRD results showed that uranium ions occupy the normal lattice position in the crystal.However,due to the large radius of uranium ions,a certain distortion occurred in the lattice and induced the diffraction peaks slightly shift to a small angle.The high-resolution X-ray double rocking curve results showed that the full width at half maximum of the crystal was in the range of 9～13".These peaks were sharp and possessed good symmetry,which meant that the LN:U crystal has good crystalline quality.In addition,the ICP test results showed that the effective segregation coefficient of LN:U was about 1.1which was beneficial to obtain higher radial uniformity.2.The valence states of uranium ions in the crystals were characterized by XPS.The results showed that U ions with three different valence states of+4,+5 and+6 existed in LN,and part of U⁴⁺ions were oxidized to+5 and+6 valence due to the high temperature during crystal growth.Compared with pure congruent LN,the ultraviolet absorption edge of LN:U crystal had a significant red shift in the range of 184～218 nm.Meanwhile,it red shifted greater as more uranium ions were doped.At the same time,there was a strong broadband absorption at 300-600 nm and a strong absorption peak at 740 nm,indicating that carriers at different energy levels could be excited by light of different wavelengths,which was beneficial to realize multi-wavelength all-color holographic in the LN:U crystal.3.The photorefractive characteristics of LN:U crystals was studied by the holographic method.The results showed that the crystals could achieve holographic writing at 488nm,532nm and 671nm.As the writing wavelength decreases,the saturation diffraction efficiency was higher and the response speed was faster.Especially when the doping concentration of U was 2 mol%,the saturation diffraction efficiency of the crystal at 488nm was as high as60.9%,the response time was shortened to 3.9s.Compared with the LN:Fe crystal commonly used in holographic field,its photorefractive response speed was improved more than 2 orders of magnitude.At the same time,by increasing the intensity of the incident light,the response time was further shortened to 1.87s and the sensitivity value was as high as 2.52 cm/J,which was an order of magnitude higher than LN:Mo crystals.In addition,using the dual-light coupling energy transfer method,the dominant carrier type of the LN:U crystal was determined to be electron under the excitation of visible light,and the migration mechanism was diffusion.