One-dimensional optical bright screening photovoltaic photorefractive solitons, soliton pairs and incoherent interaction between them in BaTiO3 crystal

By the use of theory of optical solitons, nonlinear equations governed by screening photorefractive photovoltaic solitons are introduced. By means of numerical methods, intensity profile, refractive index change of the media and also stability under propagation are investigated. Self-bending of solitons under first-order diffusion is also studied. Besides, incoherent coupled bright-bright soliton pairs and incoherent interaction are studied.     

光折变晶体LiNbO_3:Fe中的特殊散射现象

用柱面透镜把经针孔滤波器扩束的激光束沿某一个方向聚焦成细丝状(或长条状),细丝状光束垂直入射到LiNbO_3:Fe晶体上,在远场位置观察散射光.我们发现光散射的方向与晶体的c轴方向不一致,不仅沿着晶体的c轴方向有散射光出现,沿光束的条形方向也出现很强的散射光.     

Holographic recording in Fe:Ce:Ti doped LiNbO3 crystal

High diffraction efficiency measurements in a triply doped Fe:Ce:Ti-LiNbO₃ crystal are reported. The crystal shows an improved recording sensitivity, weak light-induced scattering and extremely slow readout erasure which makes it highly desirable for non-volatile data storage applications. The suitability of the crystal was demonstrated by high quality holographic recording and retrieval of several astronomical images using shift and rotational multiplexing scheme.     

Pyroelectric photovoltaic spatial solitons in unbiased photorefractive crystals

A new type of spatial solitons i.e. pyroelectric photovoltaic spatial solitons based on the combination of pyroelectric and photovoltaic effect is predicted theoretically. It shows that bright, dark and grey spatial solitons can exist in unbiased photovoltaic photorefractive crystals with appreciable pyroelectric effect. Especially, the bright soliton can form in self-defocusing photovoltaic crystals if it gives larger self-focusing pyroelectric effect.     

Higher-order space-charge field effects on the self-deflection of photovoltaic dark spatial solitons

We investigate theoretically the effects of higher-order space-charge field on the self-deflection of photovoltaic dark spatial solitons by numerical method under steady-state conditions. The expression for an induced space-charge electric field, including higher-order space-charge field terms is obtained. Numerical results indicate that photovoltaic dark solitons possess self-deflection process during propagation, and the solitons always bend in the direction of the c-axis of the crystal. The self-deflection of dark solitons can experience considerable increase especially in the regime of high photovoltaic field strengths. Relevant examples are provided.     

亮暗屏蔽光伏孤子在LiNbO3晶体中的大自偏转

用数值方法证实了亮、暗屏蔽光伏孤子在有外加电场的LiNbO₃晶体中都可以发生大自偏转,并验证了这种自偏转现象不但与晶体中受主浓度N_A有关而且还与外加电场E₀有关. 在E₀相同的条件下,N_A越小这种自偏转现象越明显,在N_A相同的条件下,E₀越大自偏转现象越明显. 还发现亮、暗屏蔽光伏孤子的自偏转现象不同：亮屏蔽光伏孤子整体都发生明显偏转,暗屏蔽光伏孤子的偏转只是发生在一侧,其极值位置和另一侧几乎不发生偏转. 关键词： 自偏转 空间孤子 光折变效应     

Fe:LiNbO3和(Fe,Tb):LiNbO3晶体中入射光强

用一种新的测量方法在不同入射光强下同时观测了Fe:LiNbO3和(Fe,Tb):LiNbO3晶体中光致折射率变化Δns、吸收系数α和光电导σph与入射光强I的依赖关系, 并从理论上对观测结果给予了初步解释。     

Self-deflection of steady-state bright spatial solitons in biased centrosymmetric photorefractive crystals

The effects of diffusion on self-deflection of steady-state bright spatial solitons in biased centrosymmetric photorefractive crystals are investigated systematically by both numerical and perturbation methods. The results show that the soliton propagates along a parabolic trajectory and the central spatial frequency component shifts linearly with the propagation distance. Both the deflection effects vary cubically with applied external bias field.     

Crystal diffraction grating in LiNbO3:Fe derived from interferometric volume holography

A 90° two-beam coupling configuration was used to store an image of an extended object in a 0.032 %wt iron-doped LiNbO₃ crystal. A volume hologram was thus generated, via the photorefractive effect, within the microstructure of the crystal. The time evolution of partial spatial erasure of the hologram under illumination was studied by capturing its reconstructed image on a digital camera and performing a two-dimensional inverse Fourier transform. This produced the refractive index grating created through interference of object and reference beams. The nature, intensity distribution and dimensions of the diffraction grating were derived in order to throw light upon the movement and mechanisms of charges, which create the index grating.     

Coherent interactions of multi bright spatial solitons in biased photorefractive crystals

The coherent interaction scenarios of two solitons, three solitons, and four solitons are presented. For two-soliton interactions, energy transfer and fusion between solitons are dependent on the relative phase of the interaction solitons, and for multi-soliton interactions, energy transfer will occur in all three relative phase conditions. The magnitude and direction of energy transfer can be controlled respectively by adjusting the interval and the relative phase of solitons.     

Study on UV-light-induced absorption in LiNbO3:Fe, Co crystal

##正##In this paper,the temporary UV-light-induced absorption in LiNbO_3:Fe,Co crystal is investigated for different UV intensities.It is found that the sensitizing process in LiNbO_3:Fe,Co can be considered as a combination of two sub-processes with different time constants.Based on the lattice structure and the photochromic mechanism of the material,the two sub-processes are suggested to arise from electron transfer from O~(2-) to Fe~(3+) through two paths with different distances.The two-colour recording experiment in LiNbO_3:Fe,Co is also preformed by using UV and green light as gating and writing beams respectively.High sensitivity is obtained in the recording,which is considered as the consequence of direct charge transfer from O~(2-) to Fe~(3+).     

LiNbO3晶体中屏蔽光伏孤子自偏转的 时空演化与可控因素

基于带输运模型理论建立了 LiNbO₃ 晶体屏蔽光伏孤子的时空演化动力学方程, 用有限差分方法求解发现, LiNbO₃ 晶体中明、暗屏蔽光伏孤子存在大的自偏转, 并且光孤子形状变得具有不对称性, 偏转方向的曲线斜率绝对值变大, 偏转反方向的曲线斜率绝对值变小. 分析研究表明影响其自偏转度和形变的因素包括受主浓度 N_A, 暗辐射强度 I_d 和外加电场 E₀ . 其他条件不变的情况下N_A 越大, 明孤子的自偏转度与形变越小, 暗孤子的自偏转度与形变反而越大; 对于 I_d , 它对明暗孤子的影响是相同的, I_d 越小, 晶体里诱导出的空间电荷场越容易达到饱和, 当信号光中心光强与暗辐射强度之比为 10^-1时无饱和现象产生; 随着 E₀ 数值的增大, 明孤子的自偏转度和形变减小, 而暗孤子的自偏转度和形变反而增大.     

Higher-order space-charge field effects on the self-deflection of two-photon bright photovoltaic spatial solitons

We investigate numerically the effects of higher-order space-charge field on the self-deflection of bright photovoltaic spatial solitons in two-photon photovoltaic photorefractive crystals under steady-state conditions. The expression for an induced space-charge electric field, including higher-order space-charge field terms is obtained. The dynamical evolution equation is built in which the effects that arise from these higher-order terms are considered. Numerical results indicate that bright photovoltaic solitons can bend towards both the same direction as the crystal's c-axis and the opposite direction, respectively. Specifically, self-deflection cannot occur for bright photovoltaic solitons if the strength of the photovoltaic field and the intensity of the input beam are appropriately selected. Relevant examples are provided.     

Optical fabrication of three-dimensional photonic lattices in LiNbO3:Fe crystals with a single amplitude mask

We fabricate three-dimensional nonlinear photonic lattices in iron-doped lithium niobate photorefractive crystal for the first time by a single amplitude mask. The experimental setup of this method is very simple and flexible. The period of the lattices can be dominated easily. We analyze the three-dimensional photonic lattices by plane wave guiding and far field diffraction pattern imaging. The induced photonic lattices can exist stably for a long time in the photorefractive crystal.     

Study of near-infrared nonvolatile two-center holographic recording in LiNbO3:Fe:Rh crystal

The near-infrared nonvolatile holographic recording has been realized in a doubly doped LiNbO₃:Fe:Rh crystal by the traditional two-center holographic recording scheme, for the first time. The recording performance of this crystal has been investigated by recording with 633 nm red light, 752 nm red light and 799 nm near-infrared light and sensitizing with 405 nm purple light. The experimental results show that, co-doped with Fe and Rh, the near-infrared absorption and the photovoltaic coefficient of shallow trap Fe are enhanced in this LiNbO₃:Fe:Rh crystal, compared with other doubly doped LiNbO₃ crystals such as LiNbO₃:Fe:Mn. It is also found that the sensitizing light intensity affects the near-infrared recording sensitivity in a different way than two-center holographic recording with shorter wavelength, and the origin of experimental results is analyzed.     

Ultraviolet photorefractive effect in Mg-doped near-stoichiometric LiNbO3

The ultraviolet photorefractive effect of Mg-doped near-stoichiometric LiNbO₃ crystals prepared by vapor transport equilibration (VTE) technique was studied at 351 nm. It was found in the near-stoichiometric LiNbO₃ crystals that the ultraviolet photorefractive effect could be enhanced greatly with the increase of Mg concentration. Based on the activation energy of dark decay of the photorefractive grating, possible centers responsible for the ultraviolet photorefractive effect were also discussed.     

One-dimensional optical dark screening photovoltaic-photorefractive solitons, soliton pairs and incoherent interaction between them in BaTiO3 crystal

We studied the theory of the optical solitons and introduced the nonlinear equations governed by the screening photovoltaic-photorefractive dark solitons. By the use of the numerical methods, the intensity profile, refractive index change of the media and also stability under propagation are investigated. Also, we investigated incoherent coupled dark-dark soliton pairs and incoherent interaction between them.     

LiNbO3晶体中光伏亮孤子实验研究

从理论上分析了具有不同Glass常数的背景光在晶体折射率空间分布相对变化中的作用,得到了折射率改变为负的晶体中形成亮、暗光伏孤子的条件. 以此为基础,用532nm的e光作信号光、488nm的o光作背景光并不断变化背景光和信号光的功率,首次在实验上观察到了LiNbO₃∶Fe晶体中亮光伏孤子的形成. 关键词： 亮光伏孤子 自散焦 自聚焦 光生伏打效应     

UV-photorefractive effect in LiNbO3:Ce:Mn single crystal

In this paper, the ultraviolet (UV) photorefraction effect of lithium niobate doubly doped with Ce and Mn has been investigated. It is found the diffraction efficiency shows oscillating behavior under UV-light-recording. A model based on electrons and holes as the dominant carriers is proposed to study the oscillatory behavior of the diffraction efficiency. On the basis of the material equations and the coupled-wave equations, we found that the oscillatory behavior is due to the oscillation of the relative spatial phase shift Φ. And the electron-hole competition may cause the oscillation of the relative spatial phase shift. This oscillation behavior is different from that caused by very high-strength space electric field. UV recording can suppress the beam-fanning perfectly, and the most important is the high sensitivity.     

Effect of wavelength of sensitizing light on holographic storage properties in LiNbO3:Fe:Ni crystal

By sensitizing with 514 nm green light, 488 nm blue light and 390 nm ultraviolet light, respectively, recording with 633 nm red light, effect of wavelength of sensitizing light on holographic storage properties in LiNbO₃:Fe:Ni crystal is investigated in detail. It is shown that by shortening the wavelength of sensitizing light gradually, nonvolatile holographic recording properties of oxidized LiNbO₃:Fe:Ni crystal is optimized gradually, 390 nm ultraviolet light is the best as the sensitizing light. Considering the absorption of sensitizing light, to obtain the best performance in two-center holographic recording we must choose a sensitizing wavelength that is long enough to prevent unwanted absorptions (band-to-band, etc.) and short enough to result in efficient sensitization from the deep traps. So in practice a trade-off is always needed. Explanation is presented theoretically.