采用紫外光提高双掺杂铌酸锂晶体中全息记录的灵敏度和光栅强度

提出了一种在双掺杂铌酸锂晶体中用调制的双紫外光进行非挥发全息记录的方法。与通常的用紫外光敏化的非挥发全息记录相比,这种方法可以大幅度地提高光栅强度和记录灵敏度。联立双中心物质方程和双光束耦合波方程,数值分析了光栅强度和衍射效率随时间的变化并讨论了掺杂浓度和记录光强对紫外光非挥发全息记录机制下光折变效应的影响。研究发现,紫外光记录得到的深浅中心的光栅具有相同的相位,总的光栅（深浅中心光栅的叠加）强度为两光栅强度之和,固定过程中深中心的光栅得到增强;增大深浅中心掺杂的浓度可以提高光栅强度,增大记录紫外光的光强可以增加光栅的强度和记录灵敏度。理论模拟可以证实并预测实验结果。     

Comparison of two-color holography among different doped LiNbO3 crystals at low intensities

The effects of material and experimental parameters on the nonvolatile two-color holographic recording space charge field and sensitivity for different doped LiNbO₃:Fe crystals have been studied theoretically based on a two-center model. When the direct electron transfer between the deep-trap centers and the shallow-trap centers was considered, the near-stoichiometric LiNbO₃:Fe is confirmed theoretically to be of bigger space charge field and higher recording sensitivity than the LiNbO₃:Fe:Mn and LiNbO₃:Cu:Ce in the low intensity region. A further improvement of the recording sensitivity can be achieved by doping concentration, thermal reduction treatment of Fe, appropriate gating and recording wavelengths with large photo-excitation cross sections.     

Enhancement of nonvolatile blue photorefractive properties in LiNbO<Subscript>3</Subscript>:In:Fe:Cu crystals

The nonvolatile photorefractive characteristics of LiNbO₃:Fe:Cu and In-doped LiNbO₃:Fe:Cu crystals are investigated. The stronger nonvolatile blue photorefraction observed can be ascribed to its remarkable characteristic of being in phase between the two gratings recorded in shallow and deep trap centers, which is one or two orders of magnitude higher than those obtained in conventional two-color recordings under the same recording conditions. Furthermore, it is interesting that, compared with LiNbO₃:Fe:Cu, the recording properties, such as the saturation refractive index change, nonvolatile sensitivity and response time at 488 nm wavelength are enhanced in LiNbO₃:In:Fe:Cu crystals under the same recording conditions. The so-called damage-resistant dopants such as In³⁺ ions in red photorefraction are not damage resistant at 488 nm wavelength but they enhance the blue photorefraction. PACS  42.40.Ht; 42.40.Lx; 42.70.Ln     

Nonvolatile photorefractive holographic recording in Sc:Ce:Cu:LiNbO3

In this paper experimental studies of nonvolatile photorefractive holographic recording in Ce:Cu:LiNbO₃ crystals doped with Sc(0,1,2,3 mol%) were carried out. The Sc:Ce:Cu:LiNbO₃ crystals were grown by the Czochralski method and oxidized in Nb₂O₅ powders. The nonvolatile holographic recording in Sc:Ce:Cu:LiNbO₃ crystals was realized by the two-photon fixed method. We found that the recording time of Sc:Ce:Cu:LiNbO₃ crystal became shorter with the increase of Sc doping concentration, especially doping with Sc(3 mol%), which exceeds the so-called threshold, and there was little loss of nonvolatile diffraction efficiencies between Sc(3 mol%):Ce:Cu:LiNbO₃ and Ce:Cu:LiNbO₃ crystals.     

Nonvolatile hologram storage properties of tri-doped Zn:Ce:Cu:LiNbO3 crystals

Congruent Zn(7 mol%):Ce:Cu:LiNbO₃ single crystal was grown by the Czochralski method in air. The occupation mechanism of the Zn²⁺ was discussed by an infrared transmittance spectrum. The nonvolatile holographic recording in Zn(7 mol%):Ce:Cu:LiNbO₃ single crystal was measured by two-photon fixed method. Zn(7 mol%):Ce:Cu:LiNbO₃ single crystals present the faster recording time and higher light-induced scattering resistance ability comparing with Ce:Cu:LiNbO₃ single crystals.     

Structure and nonvolatile holographic recording of Mg:Ce:Cu:LiNbO3 crystals

A series of Mg:Ce:Cu:LiNbO₃ crystals has been grown by Czochralski method. Their infrared transmittance spectra and ultraviolet-visible absorption spectra were measured and discussed to investigate their defect structure. The nonvolatile holographic recording of Mg:Ce:Cu:LiNbO₃ crystals was characterized by the two-photon fixed method. We found that the recording time of Mg:Ce:Cu:LiNbO₃ crystals became shorter and nonvolatile diffraction efficiency decreases with the increase of Mg doping concentration, especially doping with Mg approaches and exceeds the so-called threshold. And the nonvolatility vanishes when the concentration of MgO exceeds 4 mol%. The intrinsic and extrinsic defects were discussed to explain the nonvolatile holographic properties in the Mg:Ce:Cu:LiNbO₃ crystals.     

Investigation of nonvolatile holographic storage in LiNbO3:Mn:Ce:Fe crystal

We investigate the persistent holographic recording in triply doped LiNbO₃:Mn:Ce:Fe crystals at different oxidation/reduction states. The experimental results show that there is an optimum oxidation/reduction state, which results in the best dynamic range M/#. Compared with doubly doped LiNbO₃:Ce:Fe, we found that the nonvolatile diffraction efficiency and the best dynamic range M/# obtained in triply doped samples are larger than that obtained in doubly doped samples. The reason for the increase of the crystal about the nonvolatile diffraction efficiency and the dynamic range M/# was also explained.     

Information capacity and resolution in three-dimensional imaging

In this paper we report experimental studies of nonvolatilephotorefractive holographic recording in doping In(3mol%), which exceed the so-called threshold, in Ce:Cu:LiNbO₃ crystals. The In:Ce:Cu:LiNbO₃ crystal was grown by the Czochralski method and oxidized. The OH⁻ absorption band of In:Ce:Cu:LiNbO₃ crystal is 3507cm⁻¹. The nonvolatile holographic recording in In:Ce:Cu:LiNbO₃crystal is realized by two-photon fixed method. From the experiment, we found that the recording time of In:Ce:Cu:LiNbO₃ crystal is lower than that of Ce:Cu:LiNbO₃ crystal and there are a little lose of nonvolatile diffraction efficiencies between In:Ce:Cu:LiNbO₃ and Ce:Cu:LiNbO₃ crystals. In this paper, the mechanism of OH⁻ absorption band shifting and two-photon fixed was discussed, too.     

Grating spacing dependence of nonvolatile holographic recording with arbitrary charge transport lengths

Grating spacing dependence of nonvolatile holographic recording in doubly doped lithium niobate crystals is theoretically investigated allowing arbitrary charge transport lengths. It is shown that the nonvolatile refractive index modulation initially increases with increasing grating spacing, then a saturation behavior arises because of the dominant bulk photovoltaic effect. Although different charge transport length results in different nonvolatile refractive index modulation, the grating spacing dependence of nonvolatile holographic recording obeys almost the same rules for arbitrary charge transport lengths. The experimental results obtained by recording nonvolatile holograms in LiNbO₃:Cu:Ce crystals with different grating spacing are consistent with the theoretical analyses.     

双掺杂LiNbO3微观光学参量对衍射效率的影响

通过联立两中心带输运物质方程和双光束耦合波方程,建立了双掺杂LiNbO3晶体采用双色光(紫外敏化光和HeiNe记录光)实现非挥发性全息存储的动力学模型。理论上分析了深、浅两杂质中心的微观光学参量(包括敏化光的深中心和浅中心光激发系数、记录光的浅中心光激发系数以及深中心和浅中心的电子复合系数)对记录饱和衍射效率和固定衍射效率的影响。数值计算表明,在双掺杂LiNbO3晶体的非挥发性全息存储中,为了得到高的固定衍射效率,应该选择复合系数较大、记录光激发系数较高、敏化光激发系数较低的浅杂质中心以及敏化光激发系数较高、复合系数最佳的深杂质中心。     

用于双色全息存储（Fe，Cu）：LiNbO3晶体的参量优化计算

在确定Cu在LiNbO3晶体中对应于365nm和633nm的激发系数、复合系数、光伏系数的基础上，采用龙格-库塔（Runge-Kutta）数值方法理论研究了双掺杂（Fe，Cu）：LiNbO3晶体的深浅能级的掺杂组分比、氧化-还原状态对双色全息存储的记录灵敏度和动态范围的影响，并探讨了同时取得尽可能大的灵敏度和动态范围的晶体条件。结果表明，为了同时得到较大的记录灵敏度和动态范围，在实际应用中选用浅能级掺杂浓度为5．0×10^25m^-3，深能级掺杂浓度为3×10^24～3×10^25m^-3之间的弱氧化晶体是合适的。     

Influence of post-growth treatment on the optical properties of In:Ce:Cu:LiNbO3 crystals

Congruent In (3 mol%):Ce:Cu:LiNbO₃ crystals have been grown by the Czochralski method in air. Some crystal samples were reduced in Li₂CO₃ power, and others were oxidized in Nb₂O₅ power. The structure of crystals was studied by an infrared transmittance spectrum. The resistance ability to optical damage and the photorefractive properties were measured by light-induced scattering experiments and two-beam coupling, respectively. It has been found that the reduction treatment increased the photoconductivity , which resulted in decreased erasure time and diffraction efficiency, but higher light-induced scattering resistance ability. The oxidation treatment caused the inverse affect. Finally, the nonvolatile holographic recording in In:Ce:Cu:LiNbO3 crystals is realized.     

第一性原理下铜锰共掺铌酸锂晶体的电子结构和吸收光谱

采用基于密度泛函理论的第一性原理研究了Cu、Mn单掺及共掺LiNbO3晶体的电子结构和光学性质.结果显示,Cu、Mn掺杂LiNbO_3晶体禁带中的杂质能级分别由Cu 3d轨道、Mn 3d轨道贡献;各掺杂体系的带隙均较纯LiNbO_3晶体变窄.共掺晶体中Cu离子形成了较单掺时更浅的能级中心,并在2.87eV处有较强的吸收峰;Mn离子在1.73eV附近的吸收较单掺时减弱且中心略有偏移,在2.24eV处的非光折变峰与Mn~(3+)相关,这对吸收峰的变化被认为与Cu、Mn间电子转移相联系.相对Cu、Fe共掺LiNbO_3晶体,Cu、Mn共掺LiNbO_3晶体可以通过适当提高Cu离子浓度,来改善存储参量中的动态范围和记录灵敏度.由于同一深能级掺杂离子伴以不同浅能级掺离子将呈现出不同的吸收特征并影响存储性能,在共掺离子的配搭选择时对各待选配搭的模拟计算非常必要.     

Oscillation of nonvolatile holographic recording in LiNbO3:Fe:Cu crystals

The oscillation of diffraction efficiency is observed in the nonvolatile holographic recording of lithium niobate crystals doped with iron and copper. The physics of oscillation in doubly doped lithium niobate crystals is studied by using Runge–Kutta methods, and the oscillation can be attributed to the redistribution of electrons in the deeper and shallower traps of the crystals in the initial phase of holographic recording. The effects of Fe concentration and intensity ratio of red beams to UV beam (I_R/I_UV) on the oscillation are investigated theoretically. The results show that with lower Fe concentration, the amplitude of oscillation is larger and with lower intensity ratio I_R/I_UV, the duration of the oscillation is longer.     

双掺杂铌酸锂晶体中体全息记录的振荡特性研究

分析了双中心记录初始阶段振荡现象的产生机制,研究了晶体掺杂浓度和记录光与UV光的束比对记录初期阶段振荡特性的影响.结果表明,振荡现象可以归因于记录介质双能级中心深浅能级电子数平衡分布的改变.在浅能级掺杂浓度较小的晶体中记录时,振荡强度较大;记录光与UV门光束的光束比越大,振荡持续时间越短.     

Formation dynamics of nonvolatile crossed-beam photorefractive gratings in doubly doped LiNbO3 crystals: Theoretical investigation

The formation dynamics of crossed-beam photorefractive gratings formed by the method of two-center holographic recording in doubly doped LiNbO₃ crystals is investigated in this paper based on the theoretical model combining the two-center band transport model with the two-dimensional coupled-wave theory. The numerical simulations are presented for two-center holographic recording crossed-beam photorefractive gratings in LiNbO₃:Fe:Mn crystals. The temporal and spatial evolutions of the refractive index modulation and the diffraction efficiency are shown. The spatial variation of the wave intensity is also presented.     

光色效应(Fe,Mn)\:LiNbO3晶体非挥发性全息存储中自衍射效应的实验研究

在双掺杂 (Fe ,Mn)…LiNbO3 晶体光色效应非挥发性全息存储实验中 ,观测到光束耦合作用导致的自增强和自衰减效应。根据这种自衍射效应 ,归纳了四种记录和光固定实验组合方案。实验结果表明 ,在记录和光固定过程中利用自增强效应得到的最高衍射效率为利用自衰减效应得到的衍射效率的两倍左右 ,说明在实际应用中必须考虑和利用自增强效应。     

双中心全息记录的矢量分析与记录方向优化

对描述双掺杂晶体非挥发性全息记录动力学过程的Kukhtarev方程进行了矢量分析,分析中考虑了体光生伏特效应和外加电场的作用。在小信号近似的基础上给出了双中心全息记录中记录与固定阶段空间电荷场的矢量解析解。在综合考虑空间电荷场的各向异性以及晶体有效电光系数的各向异性后,给出了双中心全息记录的优化记录方向。结果表明,对(Fe,Mn)∶LiNbO3晶体633nm寻常光记录,优化记录方向主要由有效电光系数决定,光栅波矢与光轴夹角为22°,方位角为30°;对(Fe,Mn)∶LiNbO3晶体633nm非寻常光记录,优化记录方向主要由固定空间电荷场决定,光栅波矢与光轴夹角为44°,方位角为90°。     

双掺杂铌酸锂晶体的第一性原理研究

本文利用第一性原理研究了 晶体、 晶体和 晶体的电子结构和光学性质。通过对电子结构的分析,发现杂质能级的深浅与掺杂元素原子序数有关。原子序数越大,杂质能级越深。通过对 晶体、 晶体和 晶体的吸收谱进行分析,并与单掺杂 晶体的吸收谱进行比较,发现双掺杂在可见光区域吸收率明显提高,约为单掺杂的3倍。 晶体分别在380nm和590nm处形成吸收峰, 晶体可见光区域吸收率比 以及 晶体高,分别在450nm附近和660nm附近出现吸收峰, 晶体在430nm到600nm之间区域都有较高的吸收率,并在770nm的长波范围有一小的吸收峰。计算结果与双色全息存储（双光子全息存储）所用记录光的波长相符。