The Growth And Properties Study Of Hf & Fe Codoped LiNbO₃ Crystals

As a kind of holographic storage material, Fe:LiNbO₃ has the advantage of high diffraction efficiency and long preserving time. But its response time is also long and photodamage resistance ability is poor. The sorage performace can be improved by doping HfO₂.The series of Hf:Fe:LiNbO₃ crystals were grown by the Czochralski method and then the crystals were polarized and processed. The suitable temperature field, growth rate of the crystals and rotation rate were decided by a mount of experiment and theory analysis.Using X-ray diffraction, the phase analysis was proceeded for Hf:Fe:LiNbO₃ crystals. It was shown that Hf:Fe:LiNbO₃ crystals keep the same lattice structure with pure LiNbO₃ crystal. But their lattice constants change with the different doped concentration of Hf and different Li/Nb ratios. The infrared absorption spectra and ultra-visible absorption spectra were measured. It was analyzed that the absorption side of Hf:Fe:LiNbO₃ with various concentrations of Hf and different ratios of Li/Nb shift in comparison with that of pure LiNbO₃. We find that the threshold doping concentration of Hf is about 4mol%. The study shows that in the Hf:Fe:LiNbO₃ crystals Hf4+ first occupies the anti-site Nb and after anti-site Nb is substituted completely Hf4+ will replace Li in normal site. The result was identical with X-ray diffraction analysis.The photodamage resistance ability of Hf:Fe:LiNbO₃ crystals was measured by the directly observing facula distortion method. It was shown that the photodamage resistance ability of Hf:Fe:LiNbO₃ crystals is two orders of magnitude higher than that of Fe doped LiNbO₃ crystal, and it is increasing with the accretion of Li/Nb ratio.The exponent gain coefficient was measured with the change of temperature ranging from 20℃to 120℃. It is found that the exponent gain coefficient increased suddenly at about 55℃, 70℃and 110℃.Thermal-fixing experiment has been done to the photograph of the crystals. We used Hf:Fe:LiNbO₃ crystal as storage material, and measured the decline time (storage lifetime) of thermal-fixing. The decline time is 115 years which is 20 times than that of Fe:LiNbO₃ crystal under 20℃. The result shows that thermal fixing can prolong the storage lifetime of crystal.