Forming stable nonlinear lightguides with cross-modulation self-locking of incoherent soliton modes

This paper discusses a new class of spatially localized soliton solutions with bounded energy in the problem of the nonlinear propagation of a light beam along a photorefractive crystal (PRC) with a drift mechanism of nonlinear response. Solitons of this class correspond to stable propagation in the PRC of two or more wave packets (components) that are matched in intensity distribution but incoherent. Their spatial distributions correspond to simultaneous cross-modulation self-locking of its first-and higher-order modes into the common nonlinear waveguide formed by them. It is shown that such multicomponent solitons can be implemented and that they are structurally stable both with respect to collisions and with respect to substantial (more than 10% in intensity) stochastic perturbations of their components as they propagate to distances of the order of several diffraction lengths. The paper discusses the possibility that conjugate polymers, antiferromagnets, and superconductors can contain unusual states corresponding to the stable propagation of two or more mutually incoherent but matched electron wave packets along conjugate chains or along isolated atomic planes. Zh. éksp. Teor. Fiz. 113, 1167–1180 (April 1998)     

Formation of spatial solitons and spatial shock waves in photorefractive crystals

An analytical model, which describes the drift and diffusion mechanisms for the formation of the nonlinear response (local and nonlocal nonlinearities) of photorefractive crystals on the microscopic level, is constructed. New types of stable self-consistent distributions of the light field intensity, i.e., spatial solitons, are found. The trajectories of their motion (self-bending) are calculated, and the possibility of observing a new nonlinear-optical effect in photorefractive crystals, viz., the formation of spatial shock waves, is demonstrated. The modulation instability appearing when plane waves propagate in photorefractive crystals is analyzed, and the characteristic spatial scales of the light field distribution formed as a result of self-interaction (fanning) are determined. The results of the analysis are confirmed by computer simulation data. Zh. éksp. Teor. Fiz. 111, 705–716 (February 1997)     

Nonlinear response of photorefractive lithium tantalate and niobate at acoustic frequencies

An amplification of the intensity of pump oscillations is observed experimentally at frequencies from 100 Hz to 1 kHz during photoinduced light scattering and holographic-type parametric scattering in photorefractive lithium tantalate and niobate. Possible ways are analyzed for explaining the existence of a photorefractive response in these crystals over times of 10⁻²–10⁻³ s, which are five orders of magnitude shorter than the Maxwell time. Zh. éksp. Teor. Fiz. 112, 1490–1498 (October 1997)     

Linear and nonlinear propagation of light beams in two-dimensional photorefractive photonic lattices formed in lithium niobate

Special features of linear and nonlinear propagation of coherent light beams in two-dimensional photorefractive photonic lattices optically induced in the lithium niobate crystal doped with iron are experimentally investigated. The two-dimensional gratings are formed by consecutive recording of two one-dimensional photorefractive gratings in the crystal. Experiments were performed using He-Ne laser irradiation with a wavelength of 633 nm. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 9, pp. 58–62, September, 2006.     

Ultraslow Gaussian pulse propagation induced by a dispersive phase coupling in photorefractive bismuth silicon oxide crystals at room temperature

By use of the highly dispersive phase coupling effect in a photorefractive wave mixing process, we have observed ultraslow propagation of a single Gaussian light pulse with a group velocity ∼0.5 m/s in a photorefractive Bi₁₂SiO₂₀ crystal at room temperature. The ultraslow Gaussian pulse is amplified due to an intensity coupling effect but keeping its Gaussian profile with high fidelity. The group velocity of the Gaussian pulse can be controlled to a large extent. This technique is useful for controllable optical delay lines.     

Incoherent interaction between bright–bright photovoltaic soliton in an unbiased series two-photon photorefractive crystal circuit

We have investigated incoherent interaction between photovoltaic bright–bright soliton pairs in photorefractive crystals under steady-state condition in an unbiased series two-photon photorefractive crystal circuit in one dimension. The numerical scheme according to the Crank-Nicholson and Runge-kutta methods are applied to simulate the propagation of incoherent interaction for different normalized separation distances and different E₀. Results show that in the case of one-dimensional interaction between these photovoltaic solitons, attraction occurs and width of beams decreases with increasing biased field E₀ and two soliton interact in longer distance for smaller E₀. The result can be used for design optical switches that controlled by biased field.     

光折变表面波诱导的薄层波导中的导波模式

利用数值模拟对扩散非线性机制下由光折变表面波诱导的薄层波导中导模的形成和特点进行了研究。采用分布傅里叶法对导模的传输特性进行了模拟。通过求解本征方程,对光折变表面波诱导的薄层波导中存在的导波模式进行了数值求解。通过调节传播常数和波导参数的方法,可以控制导模的阶数和传播波形。随着阶数的增加,导模轮廓的对称性越来越差;随着波导参数的增加,导模的峰值振幅单调递增。导模的能量主要聚集在晶体薄层波导中靠近-c轴的一侧,随着传播常数的增大,导模能量先减小后增大,且导模可以稳定传播。     

Photorefractive effect in sillenite crystals with shallow traps in a sign-alternating electric field

The results of a theoretical analysis of the photorefractive response in crystals with shallow traps to in a sign-alternating, square-wave electric field are presented. The numerical analysis method developed imposes no restrictions on the frequency of the external field and the period of the photorefractive grating. The parameters characterizing deep donor and shallow trap centers are estimated on the basis of investigations of two-beam interaction in a Bi₁₂SiO₂₀:Cd crystal with the application of a sign-alternating, square-wave electric field. Fiz. Tverd. Tela (St. Petersburg) 40, 2037–2043 (November 1998)     

在金属与光折变晶体界面形成的表面波研究

应用质点振荡模型和数值模拟方法研究了在金属与光折变晶体界面形成表面波的条件及其能量变化. 结果表明: 传播常数的正负影响表面波的类型及波能量分布, 当传播常数取负值时在界面处形成非局域表面波, 取正值时在界面处形成振荡表面波和局域表面波, 局域表面波的能量随传播常数的变大而单调递增. 在一给定的物理系统中, 可通过调节决定非线性效应强度的可变参量控制不同阶数局域表面波模及其传播波形.     

外加电场光折变有机聚合物串联回路中独立空间孤子对

对外加电场光折变有机聚合物串联回路中独立空间孤子对进行了理论研究,导出了在光折变有机聚合物串联回路中两光束传播的耦合方程,证明了串联回路中存在暗-暗、明-暗和明-明三种独立空间孤子对.在光束的空间展宽远小于有机聚合物宽度的极限条件下,暗-暗孤子对之间孤子入射光强的改变可相互影响到对方的包络和动态演化.对于明-暗孤子对,暗孤子通过光电流能影响明孤子,但明孤子不能影响暗孤子.明-明孤子对之间没有影响. 关键词： 非线性光学 光折变效应 光折变聚合物 空间孤子对     

Hexagonal optical structures in photorefractive crystals with a feedback mirror

A nonlinear theory is presented for the formation of hexagonal optical structures in a photorefractive medium equipped with a feedback mirror. Oppositely directed beams in photorefractive crystals are unstable against the excitation of sideband waves. It is shown here that as this instability evolves to its nonlinear stage, the three-wave interaction between weak sideband beams does not stabilize it, but rather leads to explosive growth of the amplitudes of beams whose transverse wave vectors form angles that are multiples of π/3. As a result, sideband beams at these angles are found to be correlated. A range of parameters is found in which four-wave interactions saturate the explosive instability, which explains the appearance of stable hexagons in the experiment. Outside this region, nonlinearities of higher order saturate the explosive instability, and the process of hexagon generation must be studied numerically. Matrix elements are obtained for the three-and four-wave interactions as functions of the distance to the feedback mirror, and an equation for the time evolution of the sideband wave amplitudes is derived that describes the hexagon generation. A comparison is made with experimental results for the photorefractive crystals KNbO₃ and BaTiO₃. Zh. éksp. Teor. Fiz. 113, 1122–1146 (March 1998)     

有偏压中心对称光折变晶体中的屏蔽孤子

给出了中心对称光折变晶体中屏蔽孤子的高阶空间电荷场.当高阶项可以忽略时,这个电荷场就变为早前在中心对称光折变晶体中研究的屏蔽孤子的空间电荷场.研究了中心对称光折变晶体中屏蔽孤子的高阶非线性波动方程.在适当的条件下,这个非线性波动方程能够展示明暗空间光孤子.这类晶体不同于非中心对称晶体,其非线性折射率的改变来源于二次电光效应,而不是一般的线性电光效应.应用光束传播的方法,对这些孤子的稳定性进行了讨论.表明在小的微扰下这类孤子是稳定的,不会发生分裂.     

光折变介质中空间光孤子自弯曲现象研究

光折变非线性介质在外加直流电压时会引起介质内部电荷移动,而移动后的电荷又会导致空间电荷的扩散效应,从而产生非局域非线性现象。从理论上研究了具有流动和扩散非局域非线性的光折变晶体中所支持的空间光学孤子的传播行为。应用等效粒子近似方法分析了这类介质中(1 1)维空间光学孤子动力学行为,得出孤子运动"加速度"显式解。孤子的有效"加速度"决定于孤子参量和光折变非局域参量。所得的解析结果可在一定参量范围内直接用来计算孤子的传播轨迹。对孤子在光折变非局域非线性作用下的传播动力学行为做了仿真模拟,数值模拟结果与理论分析结论符合得很好。     

Influence of an external electric field on self-focusing of light beams in cubic photorefractive crystals

Self-focusing of an optical paraxial beam in a photorefractive cubic crystal exposed to a strong external electric field E ₀ of arbitrary orientation has been considered with regard to the Pockels effect. The best localization of radiation is shown to be attained when the vector E ₀ is oriented along diagonals of the cubic cell. Numerical modeling revealed that the beam width increased significantly for non-optimal electric field orientations.     

The role of the longitudinal component of the electric field vector in two-wave mixing in photorefractive BaTiO3

It is shown that for waves incident with a polarisation in the plane of the grating the component of the electric field in the direction of propagation must be included in the calculations. Coupled differential equations for both the transverse and longitudinal components are derived for wave interaction in photorefractive BaTiO₃. Analytic solutions for the electric field of the signal wave are obtained under the assumption that the pump wave is undepleted. For the parameters considered the longitudinal component is shown to be negligible.     

Photoinduced gratings in a Sn<Subscript>2</Subscript>P<Subscript>2</Subscript>S<Subscript>6</Subscript> ferroelectric crystal with the period depending on the optical pump power

The light scattering in the form of ring structures in Sn₂P₂S₆ crystals at the propagation of intense laser pump radiation along the b crystallographic axis is detected. Radiation passing through a crystal is completely scattered into a cone whose angle increases with the pump power and decreases reversibly with a decrease in this power. The observed effect can be attributed to a spontaneous increase in the amplitude of photoinduced bulk diffraction gratings and to the scattering of light on them in the directions where the phase increments of the photorefractive and diffraction natures compensate each other. A similar type of scattering was observed previously in other photorefractive crystals, but the appearance of gratings with the period easily varied by varying the pump power is demonstrated for the first time.     

金属-光折变材料复合全息结构对表面等离激元的波前调控

表面等离激元(surface plasmon polaritons, SPPs)控制具有重要意义.表面电磁波全息法是在金属表面设计能有效控制SPP传输的凹槽阵列结构.本文提出一种新的SPP传输的控制方法,利用金属-光折变材料复合全息结构控制SPP传播.在金属表面覆盖一层光折变材料,两束SPP波在光折变材料内干涉生成全息结构,利用此全息结构能够控制SPP的传播.通过时域有限差分法模拟验证,结果显示,通过金属-光折变材料复合全息结构可以有效地控制SPP波束的传输,实现SPP平面波束的单点聚焦、两点聚焦,以及生成零阶和一阶高斯SPP波束.经过优化发现,光折变材料的最佳厚度为3.3μm,最佳折射率调制度为0.06.现有SPP控制器件主要是通过离子束刻蚀,而金属-光折变材料复合全息结构不需要刻蚀,从而扩展了SPP控制的器件的制作方法,为SPPs的全光控制提供了新的思路,使SPP全光控制成为可能,进一步实现了SPP全光开关等功能.     

Wave optics analysis by phase-shifting real-time holographic interferometry

The purpose of this work is to study the potentialities in the phase-shifting real-time holographic interferometry using photorefractive crystals as the recording medium for wave-optics analysis in optical elements and non-linear optical materials. This technique was used for obtaining quantitative measurements from the phase distributions of the wave front of lens and lens systems along the propagation direction with in situ visualization, monitoring and analysis in real time.