Propagation of low-intensity Gaussian fields in photorefractive media with real and imaginary intensity-dependent refractive index components

A theoretical analysis of a low power Gaussian field propagating in unbiased self-defocussing photorefractive media that includes both real and imaginary components of the intensity-dependent refractive index is presented. The analysis, which is in excellent agreement with experimental observations, shows that the imaginary component of the intensity-dependent refractive index can have a focussing effect independent of the focussing or defocussing effect of the real component of the intensity-dependent refractive index. These findings suggest that the imaginary component of the intensity-dependent refractive index is the cause of the previously observed apparent self-focussing in unbiased self-defocussing photorefractive media.     

Influence of surroundings on photorefractive effect in lithium niobate crystals

In the paper results of the investigation of the influence of electric properties of the environment surrounding LiNbO₃ crystals on photorefractive effect induced in these crystals by Gaussian Ar⁺ laser beam with various intensities are presented. We show spatial and temporal dependences of changes of the refractive index obtained experimentally in LiNbO₃: Fe and LiNbO₃: Fe:Mn samples surrounded by media with different electric conductivities and different permittivities (water, air, water solution of CaCl₂). The space and time dependences of the refractive index changes induced by the Ar⁺ laser beam are observed by means of the Mach–Zehnder interferometer using light from HeNe laser. The experimentally obtained results are in a good agreement with those following from numerical calculations using the manifold mirroring method. The agreement between calculated and experimental results indicates that the polarization charge at the photorefractive crystal/surrounding medium boundary significantly influences the photorefractive process in the crystal. The experimentally observed slow spontaneous decrease of the refractive index change in a sample placed into a slightly conducting medium (air) after switching off the beam also indicates that the polarization charge in the sample's surroundings affects the photorefraction.     

光折变晶体中光诱导波导的折射率分布

分别讨论了亮孤子和暗孤子在光折变晶体中诱导波导的折射率分布,结果表明光诱导波导的折射率分布除与光强的倒数有关外,还与光强对数的一阶导数有关.后者对折射率的影响引起波导折射率的非对称性分布.     

光生伏打LiNbO3:Fe晶体从自散焦到等效“自聚焦”的动态转换

实验研究了在一定条件下LiNbO₃:Fe晶体中从自散焦到等效“自聚焦”转换的动态行为,提出了出现自聚焦的物理机理为双相位共轭的法布里珀罗干涉腔中的多光束干涉与光折变效应的共同结果.该效应有希望在光折变自散焦介质中形成亮空间孤子 关键词： 自散焦 自聚焦 光折变     

光折变有机聚合物中的双色光空间孤子

对光折变有机聚合物中的双色光空间孤子进行了理论分析,证明双色信号光的两个频率成分相互作用可以在折射率改变为正的光折变有机聚合物中形成暗暗和亮暗空间孤子,在折射率改变为负的光折变有机聚合物中形成亮亮和亮暗空间孤子。与单色光孤子的情况不同,对于双色光孤子情况,通过与一形成暗孤子的频率成分相互作用,另一频率成分在折射率改变为正的有机聚合物中也可形成亮孤子;通过与一形成亮孤子的频率成分相互作用,另一频率成分在折射率改变为负的材料中也可形成暗孤子。     

高斯光束通过非线性折射和吸收介质的光强分布

以衍射理论为基础,推导并模拟了高斯光束通过非线性折射和吸收效应介质后的远场光强分布。结果表明:入射高斯光束引起的介质折射率变化和光束波前曲率影响介质出射面光束的径向调制相位,安装在焦点前的有自聚焦效应的介质和安装在焦点后的有自散焦效应的介质具有闪耀光栅的能力,使远场出现中心为暗斑外侧为亮环的衍射图样。非线性折射效应为主的介质,其非线性吸收效应的变化同时改变远场衍射图样的数目和光强值,但对于非线性吸收效应为主的介质,衍射图样的数目随吸收效应的变化不明显,吸收效应只影响光强值。     

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.     

热自聚焦诱导光折变非对称自散焦

用波长为５３２ｎｍ的高斯光束入射到掺杂的ＫＮＳＢＮ晶体中，对足够大的光强，观察到由热自聚焦效应诱导产生的光折变非对称自散焦．非对称自散焦的空间分布与性质都有别于噪声受激放大．它对自泵浦相位共轭无甚贡献．对开始产生非对称自散焦时晶体中光束的直径作出估计 关键词：     

低功率近红外光的偏振方向对光致折射率变化影响的实验研究

研究了利用低功率近红外光辐照In:Fe:LiNbO3晶体时写入光束的偏振方向对光致折射率变化(Δn)的影响.分别采用正常偏振(o光)和非常偏振(e光)的近红外细激光束,在In:Fe:LiNbO3晶体中进行了光折变实验.研究表明,两种偏振方向引起晶体的Δn实测曲线相似,但变化规律恰好相反,o光引起的折射率变化量约是e光的3倍左右.近红外光写入下两种偏振光束引起晶体的Δn分布规律都不同于可见光,尤其是e光辐照区域中心晶体的折射率升高.因此,通过选择不同偏振方向的近红外光可以在光折变晶体中制作不同折射率分布的非线性光学器件.     

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.     

LN晶体自散焦到自聚焦转换的耦合理论分析

对铌酸锂晶体进行了三阶非线性折射系数测量的Z扫描实验和光折变非线性系数测量的时间扫描实验。为了分析时间分辨扫描曲线的特点，基于非线性介质中光折变非线性与三阶非线性光学效应的耦合作用，引入非线性折射率随时间变化的表达式，分析了空间亮孤子的形成过程，进而揭示了其在一定条件下从自散焦到自聚焦转换的动态行为的物理机制,得到了与实验现象相一致的结论，并说明了自聚焦光折变晶体中可能存在稳定的暗空间孤子。     

光生伏打LiNbO_3:Fe晶体从自散焦到等效“自聚焦”的动态转换

实验研究了在一定条件下LiNbO3:Fe晶体中从自散焦到等效“自聚焦”转换的动态行为 ,提出了出现自聚焦的物理机理为双相位共轭的法布里 珀罗干涉腔中的多光束干涉与光折变效应的共同结果 .该效应有希望在光折变自散焦介质中形成亮空间孤子     

Laser induced memory bits in photorefractive LiNbO3 and LiTaO3

We study experimentally the formation of refractive index voxels (volume elements) in photorefractive LiNbO₃ and LiTaO₃ crystals illuminated with high irradiance femtosecond laser pulses. We used 150 fs pulses at 800 nm wavelength (energy 6–50 nJ) tightly focused inside the crystals in a single shot regime. This resulted in a formation of a micrometer size region of elevated refractive index, which may be used as memory bits in information storage/retrieval application. The maximum refractive index change of 5×10⁻⁴ was recorded in undoped LiNbO₃ at an average light intensity of ∼TW/cm² that is close to the breakdown threshold. A simple setup for photorefractive recording and in situ monitoring of the refractive index changes has been proposed. M. Sūdžius leaves from: the Institute of Materials Science and Applied Research of Vilnius University, Lithuania.     

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.     

Holographic recording by excitation of metastable electronic states in Na2[Fe(CN)5NO]·2H2O: a new photorefractive effect

Elementary holographic phase gratings can be written in single crystals of Na₂[Fe(CN)₅NO]·2H₂O, sodiumnitroprusside, by excitation of metastable electronic states in the blue–green spectral range. For light polarized parallel to the crystallographic a and b axes of the orthorhombic crystal the light-induced modulation of the refractive index reaches Δn≈2×10^-3 at λ=514.5 nm. Although the largest population of the metastable states is reached for light polarized parallel to the crystallographic c axis, a photorefractive response is not observed. In contrast to electro-optic photorefractive materials the photorefractive effect depends mono-exponentially on the exposure and on the modulation of the incident light interference pattern. Beam-coupling experiments demonstrate that written gratings are in phase with the interference pattern in correspondence with the fact that the excitation of the metastable electronic states is local. The width of the rocking curve shows that the holographic gratings are written completely over the volume of the crystal. Variations of the wavelengths within the excitation range as well as of the crystal thickness do not influence the maximum photorefractive response. Investigations on the grating vector of the written gratings show unambiguously that charge migration is not responsible for the photorefractive effect. Received: 18 November 1998 / Revised version: 26 January 1999 / Published online: 12 April 1999     

(2+1)维光折变波导阵列的制作

对四个点光源进行光学傅里叶变换,在自散焦光折变晶体中成功地制作了6μm×6μm间隔的并行(2+1)维阵列波导300余条,并证明了阵列波导实质上是二维体相位栅.实验研究了点光源间距、点光源尺寸及写入时间对阵列波导制作的影响,并分析了在自散焦光生伏打光折变介质中形成阵列波导的原理.     

向列相液晶中弱光引致各向异性衍射图样的研究

研究了弱线偏振光(≈0.16 W/cm²)通过垂直排列C₆₀掺杂的向列相液晶(5CB)薄膜的远场衍射图样.基于取向光折变机理,二波耦合使液晶分子进行二次取向之后,强度为高斯分布的光束通过样品时将形成高斯分布的空间电荷场,偏振光束通过样品时将产生偏振衍射图样.衍射图样的轮廓是同心圆环,在垂直于光的偏振方向有对称缺口.改变入射光的偏振方向可以看到衍射图样也随之改变, 有效非线性折射率系数n₂≈0.3cm²/W 关键词： 向列相液晶 取向光折变效应 自相位调制 衍射图样     

Measurement of nonlinear refractive index of Ag and Au nano-particles using moiré technique

In this paper we have measured second order nonlinear refractive index of Ag and Au nano-particles using the moiré deflectometry with a CW He-Ne laser at wavelength of 632.8 nm. The main advantages of this technique are its extreme experimental simplicity and low sensitivity to external distribution in comparison with other interfrometric methods that need very stable experimental set-up. Because of self-focusing (or self-defocusing) that is due to intensity dependent refractive index, presence of sample in the moiré deflectometry set-up lead to rotation of moiré fringes. By moving the sample in set-up we have measured these rotation angles in different displacements from focus point of lens used in this set-up and then we have calculated the nonlinear refractive index of Ag and Au nano-particles using their values. Also we have obtained the sign of n₂ regard to the direction of rotations. Consequently by using the Mathematica software we have exhibited the suitable fitness of theoretical curve with experimental results.     

Modulation instability with incoherent white light in self-defocusing photorefractive crystal

We report on the first experimental observation of modulation instability and spontaneous pattern formation with incoherent white light emitted from an incandescent lamp in self-defocusing photorefractive LiNbO₃:Fe crystal. Experimental results show that the modulation instability of white light in self-defocusing medium is related to the input intensity, illumination time and the direction of the crystalline c-axis with respect to that of the lamp filament. At the illumination time t = 0 and the steady-state of MI, we give the spatial distribution of the optical Fourier power spectrum experimentally and the corresponding Fourier transformation of the output intensity numerically, and observe the emergence of the high frequency component along the c-axis.     

Nonlinear Optical Interactions in Bacteriorhodopsin Using Z-Scan

Nonlinear refractive index coefficient n₂ of bacteriorhodopsin suspensions in water is measured by the Z-scan technique with a low power continuous wave laser at 647.1 manometer wavelength. Our results indicate that both the magnitude and the sign of n₂ depend strongly on the light intensity. Negative values for n₂ are obtained for on axis laser irradiance at the focus above 3 W/cm². The observed self-defocusing phenomena can be attributed to the index change due to the light induced transition between the photochromic states. The results elucidate the origin of n₂ and offer a plausible explanation for the differences in the reported n₂ measurements.Presented at the International Commission for Optics Topical Meeting, Kyoto, 1994.