Blue Photorefraction And Dual-Wavelength Nonvolatile Holograohic Storage Of In:Fe:Cu:LiNbO₃ Crystals

The optical holographic storage is the promising storage technique because of its high storage density, high transfer speed, fast parallel access, and so on. Lithium niobate (LiNbO₃) crystal has become one important holographic storage materials contributed by its well-known photorefractive performance, such as high diffraction efficiency, high storage density and long storage lifetime. However, LiNbO₃ crystal exhibits the relatively low response speed, strong light-induced scattering and volatility, which limits the holographic storage. So, improving and optimizing the photorefractive performance of LiNbO₃ crystals attract more attention and become an important research subject in recent years. In this thesis, the triple-doped In:Fe:Cu:LiNbO₃ crystals were grown and investigated for the first time. Based on the defect structure, we investigated the formation mechanism of the dual-wavelength nonvolatile grating. The factors influenced the holographic photorefractive properties as well as the optimized conditions are investigated and discussed in detail. Finally, the dual-wavelength nonvolatile storage system was built up, and the multiplexing storage was further investigated by using angular multiplexing technique.The defect structure and occupied sites of In:Fe:Cu:LiNbO₃ crystals were investigated detailedly by using infrared OH^- absorption spectra, UV-Visible absorption spectra, ICP-AES and the differential thermal analysis. Based on the spectra results, the chemical formulas of the crystals are obtained. The results show that In³⁺ ions replace the Nb antisites and Fe ions occupy the normal Li site when In³⁺ ions doping concentration below its threshold concentration. When In³⁺ ions concentration is above its threshold, all Nb_Li⁴⁺ ions are replaced and In³⁺ ions occupy Nb sites. When the [Li]/[Nb] reaches a certain value, Fe ions on the Li sites will be repelled to Nb sites. In our samples, Cu occupies the normal Li sites.The dual-wavelength nonvolatile recording model for In:Fe:Cu:LiNbO₃ crystal is established based on the Kukhtarev two-center model. The stable solution for spatial charge field as well as the analytic solutions for nonvolatile recording sensitivity and dynamic range is obtained. Using the obtained solutions, the kinetics behavior of dual-wavelength nonvolatile photorefractive characteristics is numeric simulated. The results show that the enhancement of dual-wavelength nonvolatile properties in In:Fe:Cu:LiNbO₃ crystals is contributed by the merits of the direct recording, being in phase between the deep and shallow trap centers and strong absorption at 488nm wavelength.The exposure energy flux threshold effect for light-induced scattering in photorefractive materials is proposed, which can be used to evaluate the light-induced scattering of recording materials effectively. Using the proposed threshold effect, the influences of In³⁺ ions doping concentration, reduction/oxidation treatment as well as [Li]/[Nb] ratios on the light-induced scattering exposure energy flux threshold are studied systematically. The fanning-noise-free reconstructed holograms are also demonstrated experimentally according to the incident light exposure energy flux threshold in triply-doped In:Fe:Cu:LiNbO₃ crystals.The photorefractive properties of Fe:Cu:LiNbO₃ and In-doped Fe:Cu:LiNbO₃ crystals were investigated experimentally at 488nm wavelength by using two-wave coupling experiment and strong blue photorefraction are obtained. Moreover, the blue photorefraction is enhanced significantly with the increasing concentration of In³⁺ ions doping. It has been demonstrated experimentally that the so-called optical damage-resistant dopants such as In³⁺ ions are no longer damage-resistant at 488nm wavelength but they enhance the blue photorefractive characteristics. By optimizing the reduction/oxidation state, [Li]/[Nb] ratios and recording angle, stronger blue photorefraction can be obtained.The dual-wavelength nonvolatile photorefractive performance is investigated. Comparing to the conventional two-color nonvolatile recording, the dual-wavelength nonvolatile photorefractive properties are improved greatly. Based on the dual-wavelength characteristics, the dual-wavelength nonvolatile holographic storage system is built up and high density holographic storage is achieved.