Study On The Growth And Photorefractive Characteristics Of Magnesium And Uranium Co-Doped Lithium Niobate Crystal

Lithium niobate crystal（LiNbO₃,LN）is an artificial crystal with superior physical properties and easy growth.It was not only evaluated as a versatile"optical silicon"material by"Nature Materials Update"in 2002,but also proposed by Harvard University in 2017 that human beings are entering the era of"Lithium Niobate Valley".Its excellent photorefractive properties are widely used in holographic display,holographic storage and other fields.However,there are still many difficulties in the commercialization of lithium niobate crystals in the holographic field,such as low saturation diffraction efficiency,long response time,and low optical damage threshold,etc.In view of the problems faced by lithium niobate crystals,this paper selected magnesium ions with optical damage resistance characteristics for co-doping based on the previous research on single-doped lithium niobate crystals,and studied their growth and photorefractive properties.The following research results were obtained:1.A series of magnesium uranium co-doped lithium niobate crystals（LN:U,Mg）with different concentrations of 1 inch in diameter were successfully grown by the improved by the modified vertical Bridgman method.The crystals had good integrity.XRD and EDS results showed that Mg and U ions successfully enter the lattice position,and the rocking curve showed that the crystals had good crystal quality.2.The true threshold concentration and related defects of LN:U,Mg were studied by OH^-absorption spectra,raman spectra and absorption spectra.The OH^-absorption spectra of LN:U,Mg crystals showed that the doping of 7mol%Mg²⁺reached the real doping threshold concentration in LN:U,which was consistent with the calculated results based on the model of lithium vacancy defect.The results of Raman spectra of LN:U,Mg showed that the characteristic peak of antisite niobium at253cm^-1in LN:U,Mg_7.0was significantly lower than that of LN:U,Mg_5.0.The absorption edge of LN:U,Mg had a significant violet shift relative to the single-doped lithium niobate（LN:U）,while LN:U,Mg_7.0had a red shift relative to LN:U,Mg_5.0.The main reason was that the Mg doping concentration exceeded the true threshold and eventually occupied the Nb site,causing greater lattice distortion and leading to the red shift of the absorption edge.3.The photorefractive properties of LN:U,Mg crystals were studied by holographic method.The crystals could be written under the irradiation of 488nm,532nm and 671nm lasers and the saturated diffraction efficiency was over 50%.Among them,the response time of the three crystals at 532nm was shortened to about2s,which was close to that of LN:U and twice as fast as that of LN:Fe,Mg,indicating that LN:U,Mg realized multi-band holographic data storage.The optical damage resistance of Mg-U co-doped lithium niobate was studied.It was found that the optical damage resistance of LN:U,Mg_7.0crystals reached 2.36×10⁵W/cm²,which increased by three orders of magnitude compared with that of LN:U,Mg_5.0.The dominant carrier type of the crystal was electron type and the diffusion mechanism was the main mechanism by the two-wave coupling method.Finally,the photorefractive center of the crystal was discussed by absorption difference spectra.