非线性光学 非线性光学材料

O437 2005010225 两组份复合材料光学非线性性质的临界行为=Critical behavior of optical nonlinear properties in two-component composites[刊．中]／谢秉川(江苏大学理学院物理系,江苏,镇江(212013)),沈廷根∥光学学报,-2004,24 (10).-1358-1362 对两组份非线性复合材料的光学非线性性质的临界行为进行了研究。考虑第一组份为非线性材料,其电流、电压间服从I=g1V+x1Vβ关系;而第二组份为线性材料, 电流、电压间满足I=g2V,其中g1,x1是第一组份的线性     

光学双稳性临界点的相变行为

本文详细地讨论了光学双稳性(OB)的临界现象。理论分析主要以忽略量子涨落的Fokker-Plank方程的静态解为基础。结果发现,OB的临界现象可以纳入Landau二级相变理论的框架,并且有关临界指数之间的关系也服从标度律。 关键词：     

长程电子关联对聚合物中激子极化率的影响

本文研究了在外电场下高分子的线性极化率和三阶非线性极化率,发现电子-电子相互作用使激子线性极化率χ(1)xx和三阶非线性极化率χ(3)xxxx都降低,而长程电子关联效应的作用更进一步降低χ(1)xx和χ(3)xxxx.准确地计算高分子聚合物的极化率可以更好的理解共轭聚合物的长程电子关联对有机分子线性和非线性响应的影响,确定其结构与非线性光学性质的关系.     

非平衡态相变的两例——光学参量振荡和激光

本文就光学参量振荡和激光两个物理问题,讨论了非平衡态相变过程的动力特点。相变点是由稳定结点状态突变到鞍点时的转变点。说明了涨落和耗散在相变中所起的作用,得到第二类相变的临界指数(β=1/2,γ=1)。 关键词：     

绝缘颗粒液体主体基质复合介质的非线性光学性质

利用Maxwell-Garnett近似，并结合谱表示方法，理论研究了具有一般非线性的绝缘颗粒，无规则浸入液体主体基质复合体系的非线性光学性质-在弱非线性条件下，数值研究了体系的有效线性介电函数和光学非线性随体积分数的变化，并发现体系的线性介电函数比光学非线性更依赖于绝缘颗粒组分的体积分数-在一般非线性条件下，研究体系的有效介电函数随入射光强度的依赖关系，还研究了s方向和p方向极化的总反射率的行为- 关键词： 复合介质 谱表示 非线性光学性质     

具L-近邻键的一维渗流模型临界指数的精确解

应用实空间重正群方法于一维具L-近邻键的点-渗流及键-渗流模型,得到类-温度标度率与类-场标度率,再利用普适标度律得到全部临界指数的精确结果。对于点-渗流模型有α_p=2-L,β_p=0,γ_p=L,δ_p=∞,η_p=1及v_p=L。这与生成函数方法结果一致;对于键-渗流模型有α_p=2-(L(L+1))/2,β_p=0,γ_p=(L(L+1))/2,δ_p=∞,η_p=1及v_p=(L(L+1))/2,其中的“热”临界指数与转移矩阵方法结果一致,磁临界指数是新的结果。由点-渗流及键-渗流模型求出Suzuki的弱普适律的重正化临界指数为φ≡(2-α)/v=1,β≡β/v=0,γ≡γ/v=1,δ≡δ=∞及η≡η=1。即重正化的临界指数不仅与L无关,而且也不依赖于是点抑键的渗流模型,即普适律对Suzuki重正化临界指数仍得以保持。 关键词：     

关于透明电介质晶体的非线性折射率

近年来,光学由线性现象扩展至非线性现象,高功率激光的发展,使很多非线性光学效应容易出现,激起人们对研究非线性光学和材料的非线性光学性质的广泛兴趣。目前关于非线性光学的理论日趋完善,但我们不能满足于一般地描述材料的诸非线性张量,应该深入地了解其机制以及诸非线性系数和材料的原子、分子结构以及其他物理化学性质的关联,这是为广泛地寻找许多新的光学材料的物理判据。 透明电介质晶体的宏观极化矢量P可以表示为 P=X(1)E+X(2) E2 + X(3)E3十…,(1)其中 X为各次极化张量,E 为电场强度。不存在对称中心的晶体皆具有二次(三阶)极…     

柱形量子点中的三阶极化率

利用量子力学的密度矩阵理论,在有效质量近似下，采用有限深势阱模型，导出了柱形量子点的三阶非线性极化率的解析表达式.通过数值计算，分析了GaAs/AlβGa1－βAs柱形量子点的三阶非线性极化率与量子点尺寸、掺杂浓度、入射光的频率和偏振方向等参量的关系.结果表明，三阶极化率峰值位置与这些参量密切相关，并且对于确定频率和偏振方向的入射光以及材料的掺杂浓度，存在与之相匹配的量子点尺寸，使三阶极化率达到极大，比体材料的相应值高出2个数量级以上.     

梯度圆柱颗粒复合介质的有效非线性响应

应用非线性微分有效偶极近似方法,导出了关于非线性梯度圆柱颗粒的等效线性介电常数和三阶非线性极化率的微分式,研究了稀释极限下,圆柱形梯度颗粒无规置入线性介质中组成的复合介质的有效线性介电常数和三阶非线性极化率.作为应用,数值计算了具有幂指数梯度分布的介电常数的颗粒复合介质的有效非线性响应,及梯度金属电介质颗粒复合介质的表面plasmon共振效应.     

β-Si3N4电子结构和光学性质的第一性原理研究

采用基于密度泛函的平面波赝势方法(PWP)和广义梯度近似(GGA),计算了β相氮化硅(β-Si3N4)的电子结构和光学性质,得到的晶格常数、能带结构等均与实验结果较好符合.进一步还研究了β-Si3N4的光吸收系数以及禁带宽度随外压力的变化规律,为β-Si3N4材料在高压条件下的应用提供了理论参考.     

Statical critical properties of gadolinium models

Microscopic models of real ferromagnetic gadolinium are proposed, and their critical properties are studied by the Monte Carlo method. The critical exponents α (heat capacity), γ (susceptibility), and β (magnetization) are calculated. The α, β, and γ exponents are determined by the approximation of the data on the basis of traditional power functions and in the framework of the finite-size scaling theory. It is revealed that the critical behavior of gadolinium is affected by the dipole-dipole interactions. It is shown that the Monte Carlo method is a powerful tool for investigations into the critical properties of complex models in which two types of weak relativistic interactions are jointly taken into account against the background of each of these interactions.     

Renormalized tricritical behaviour for wetting transitions

In this paper we study tricritical wetting behaviour in three dimensions. In particular we concentrate on systems with short-ranged forces and apply linear functional renormalization group techniques to elucidate the effect of fluctuations upon tricriticality. In comparison with studies of critical wetting we identify an additional fluctuation regime which is relevant for values of the capillary parameter between 2/9 and 1/2. We demonstrate that this regime essentially provides a crossover from mean-field like behaviour, in which tricritical exponents are always distinct from their critical counterparts, from intermediate- and strong-fluctuation behaviour where the critical exponents for tricritical and critical wetting are found to always coincide. We conclude by discussing briefly the possible relevance of these results for experimental studies of wetting. Received 4 January 2001 and Received in final form 11 May 2001     

An improved decoupling approximation method for nonlinear granular composites

An improved decoupling approximation is proposed to estimate the spatial average of ∥E∥^β, where ∥E∥ denotes the magnitude of the local electric field E and β (β ≥ 3) is the nonlinear exponent. As an example, we recalculate the nonlinear susceptibilities of weakly nonlinear composites and find that our present results are very close to the simulation data.     

Random-bond Ising chain in a transverse magnetic field: A finite-size scaling analysis

We investigate the zero-temperature quantum phase transition of the randombond Ising chain in a transverse magnetic field. Its critical properties are identical to those of the McCoy-Wu model, which is a classical Ising model in two dimensions with layered disorder. The latter is studied via Monte Carlo simulations and transfer matrix calculations and the critical exponents are determined with a finite-size scaling analysis. The magnetization and susceptibility obey conventional rather than activated scaling. We observe that the order parameter and correlation function probability distribution show a nontrivial scaling near the critical point, which implies a hierarchy of critical exponents associated with the critical behavior of the generalized correlation lengths.     

Nonlinear photonic crystals: IV. Nonlinear Schrödinger equation regime

We study here the nonlinear Schrödinger (NLS) equation as the first term in a sequence of approximations for an electromagnetic (EM) wave propagating according to the nonlinear Maxwell (NLMs) equations. The dielectric medium is assumed to be periodic, with a cubic nonlinearity, and with its linear background possessing inversion symmetric dispersion relations. The medium is excited by a current J producing an EM wave. The wave nonlinear evolution is analysed based on the modal decomposition and an expansion of the exact solution to the NLM into an asymptotic series with respect to three small parameters α, β and ?. These parameters are introduced through the excitation current J to scale, respectively (i) its amplitude and consequently the magnitude of the nonlinearity; (ii) the range of wavevectors involved in its modal composition, with β^?1 scaling its spatial extension; (iii) its frequency bandwidth, with ?^?1 scaling its time extension. We develop a consistent theory of approximations of increasing accuracy for the NLM with its first term governed by the NLS. We show that such NLS regime is the medium response to an almost monochromatic excitation current J. The developed approach not only provides rigorous estimates of the approximation accuracy of the NLM with the NLS in terms of powers of α, β and ?, but it also produces a new extended NLS (ENLS) providing better approximations. Remarkably, quantitative estimates show that properly tailored ENLS can significantly improve the approximation accuracy of the NLM compared with the classical NLS equation.     

Large-scale structure of fluctuating order parameter field

An effective free energy for a fluctuating system is investigated using an exact (local) renormalization group (RG) equation. This equation accounts for the fluctuation interaction in a reduced manner (at Fisher exponent =0) and leads to a physical solution branch which gives realistic estimations for the free energy and nice critical exponents. It is shown that in spite of the monotonic character of the effective free energy in the critical region, all vertices should be taken into account in the effective Ginzburg-Landau-Wilson functional. The large-scale structure of the fluctuating field at a second-order phase transition is studied utilizing the calculated free energy and localized nonlinear excitations are found with profiles rather like those previously obtained in a model approach.     

Renormalization point invariance,twisted fans,and critical exponents at finite ϵ in the reggeon calculus.

A procedure for calculating critical exponents directly at finite ? is proposed. It relies on the invariance of the critical exponents at the critical coupling g^c of the full theory with respect to finite changes in the renormalization point. This is expressed as the coincidence of curves at the point β = 0 in the plane of β versus a critical exponent parametrically described by the renormalized coupling for various values of the renormalization point (the “twisted fan”). If more than one critical exponent is present the fan is a set of curves in a multidimensional space with the twist at β = 0 and the exact values of the critical exponents. In perturbative approximations, an approximate invariance may result whether or not a zero of β exists to that order. We show that in the one and two loop approximations to the Reggeon calculus this approximate invariance does occur. The values of the critical exponents at the approximate twists show remarkable stability properties. We obtain σ_tot ≈ (lns)^?γ where ?γ ≈ 0.11 and 0.17 for one and two loops respectively.     

Effective Nonlinear Response of the Nonlinear Random Mixed Composites

A simple self-consistent mean-field theory is used to study the nonlinear composites consisting of linear and strongly nonlinear materials or strongly nonlinear materials with different nonlinear exponents. The effective nonlinear response obtained is compared with simulation data. Good agreement is found. Meanwhile, we study the effect of granular shapes (depolarization factor g) in different external applied fields on the nonlinear response of nonlinear random composites. It is found that the factor g affects the effective nonlinear response greatly in certain conditions.     

Monte Carlo simulation of very large kinetic Ising models

We study the relaxation of Ising models in three and four dimensions aboveT _c , using multi-spin coding for lattices up to 360³ and 40⁴. The nonlinear relaxation time diverges as (T–T _c )^{–1.05±0.04} in three dimensions, where corrections to scaling are taken into account. In four dimensions the effective exponent is about 0.72; logarithmic correction factors make the analysis difficult here. The linear relaxation time for the asymptotic exponential decay is found to be larger, with effective exponents 1.31 (d=2) and 0.97 (d=4). The difference in the linear and nonlinear relaxation exponents is compatible in three dimensions with the orderparameter exponent , as predicted by Racz.Work supported by SFB 125 Aachen-Jülich-KölnWork started at Department de Physique des Systemes Desordonnes, Universite de Provence, Centre St-Jerome, F-13397 Marseille Cedex 13, France