Study On The Optical Properties Of Doubly And Triply Doped Lithium Niobate

Lithium niobate is one of the excellent optical materials with the reputation of "optical silicon". A variety of elements can be doped into lithium niobate crystal, which makes the material presenting different physical properties. Doubly and triply doped lithium niobate have many applications in the field of photonics, for example, optical waveguide and highdensity information storage. Previous studies show that the photochromic effect of LiNbO3:Fe:Mn crystals can be used to realize the nonvolatile recording of holograpy. However, a long recording time in LiNbO3:Fe:Mn crystals hinder the further improvement of nonvolatile recording preformance. In order to overcome this shortcoming, two solutions were proposed in this thesis: 1. hafnium element was introduced into the LiNbO3:Fe:Mn crystals for increasing the photoconductivity and speeding up the recording response; 2. the element of Mn in LiNbO3:Fe:Mn was replaced by another transition-mental-element of Co.LiNbO₃:Fe:Mn codoped with different concentrations of Hf were prepared. The influence of heat treament and UV pre-sensitization on the UV-Vis spectrum of these triply doped LN samples are studied. It was found that in the Hf concentration range below 2mol% the crystal absorption edge exhibits the red shift with the increase of Hf concentration. However, above the 2mol% the absorption edges shift to the shorter wavelength with the increasing Hf concentration. This result indicates that the 2mol% concentration corresponds the threshold value where hafnium ions substitude the Nb ions at Li sites completely. Moreover, the optimal hafnium ion concentration and heat treatment conditions for application of nonvolatile storage was proposed. Crystals doped with Hf concentrations of 2mol% and 3mol% demonstrate a strong photochromic effect, which is quite benifical to the nonvolatile storage applications. Oxidation heat treatment has little effects on the UV-Vis absorption spectrum of these sample. However, reduction heat treatment show some helps for enhancing the photochromic effect of LN crystals doped with 4mol% Hf. Futhermore, the reduction heat treatment leads also to the red shift of the absorption edge, which may be explained by the fact that hafnium doping over 4 mol% begin to influence the Nb sites.Secondly, the temperal dynamic processes of the light-induced absorption and transparency of LiNbO3:Fe:Mn:Hf were studied. The response times under different sensitization and bleaching intensities were recorded. The results show that LiNbO3:Fe:Mn:Hf responds faster than LiNbO3:Fe:Mn during photochromic precess. By compareing their response times under different intensities, we also obtain the optimum ratio for bleaching and sensitizing lintensity used in subsequent nonvolatile recording.Performance parameters for nonvolatile storage using LiNbO3:Fe:Mn:Hf with different hafnium doping levels were studied. The experimental resutls proves that fast photochromic response of LiNbO3:Fe:Mn:Hf leads to the fast recording reponse, as compared with the case of LiNbO3:Fe:Mn. 2mol% Hf doping level shows the best nonvolatile storage performance.Finally, the photochromic process of LiNbO3:Fe:Co was studied. The response time of LiNbO3:Fe:Co is also found to be shorter than that of the LiNbO3:Fe:Mn. From the bleaching and sensitizing time constants, we estimate the optimum ratio of recording and sensitizing intensities for nonvolatile recording to be ranging approximately from 35 to 40. In addition, the charge-transfer mechanism of the photochromic process of LiNbO3:Fe:Co was proposed at the first time in the end of text.