Propagation and interaction of ultra-short pulses in quadratic crystals with controlled dispersion

Propagation of ultra-short optical pulses in an environment with quadratic nonlinearity and modulated dispersion is studied. It is shown that periodic modulation of the velocity-mismatch sign and of the third-order dispersion coefficient allows one to suppress pulse scattering and decompression, which noticeably increases their interaction efficiency. Original Russian Text ? A.A. Ivanov, B.B. Petrov, C.C. Sidorov, 2009, published in Vestnik Moskovskogo Universiteta. Fizika, 2009, No. 1, pp. 90–92.     

Multi-axial strain-stiffening elastic potentials with energy bounds: explicit approach based on uniaxial data

According to the well-known models for rubberlike elasticity with strainstiffening effects, the unbounded strain energy is generated with the unlimitedly growing stress when the stretch approaches certain limits. Toward a solution to this issue, an explicit approach is proposed to derive the multi-axial elastic potentials directly from the uniaxial potentials. Then, a new multi-axial potential is presented to characterize the strain-stiffening effect by prescribing suitable forms of uniaxial potentials so that the strain energy is always bounded as the stress grows to infinity. Numerical examples show good agreement with a number of test data.     

Determination of the microscale stress–strain curve and strain gradient effect from the micro-bend of ultra-thin beams

A simple method is established to determine the microscale uniaxial stress–strain curve from the load and deflection data for a doubly clamped beam. The method is based on the fact that, for beam deflection much larger than the beam thickness, the axial stretching dominates the deformation in the doubly clamped beam and the doubly clamped beam behaves like a simple plastic hinge. The microscale uniaxial stress–strain curve, together with the cantilever beam experiments, is used to determine the strain gradient effect in Au thin beams. The effect of finite rotation is also discussed.     

On the directional approach in constitutive modelling: A general thermomechanical framework and exact solutions for Mooney–Rivlin type elasticity in each direction

In order to represent process-induced anisotropies in continuum mechanics or to transfer one-dimensional material models to three spatial dimensions the directional approach is a helpful technique. Since the essential equations are defined in the orientation space it is also denoted as microsphere approach. In the current article, the relation for the directional stress tensor of the second Piola–Kirchhoff type is motivated using the volumetric/isochoric split of the deformation gradient and the Clausius–Duhem inequality. Owing to inherent nonlinearities, numerical discretisation techniques are usually applied to calculate the total stress by averaging the directional stress tensors over the unit sphere. In order to investigate the accuracy of such simulations, the availability of exact solutions in closed form is essential. To this end, the tension/compression behaviour which belongs to a certain direction in the orientation space is modelled by an elasticity relation of the Mooney Rivlin type. The exact solutions are calculated, visualized and discussed for uniaxial tension and compression as well as for equibiaxial tension.     

THEORETICAL ANALYSIS ON THE NUCLEATION OF MICRO-DAMAGE IN CEMENT MORTAR UNDER COMPRESSIVE LOADING AND SULFATE ATTACK

Effect of uniaxial compression on the nucleation of micro-damage in cement mortar under sulfate attack is investigated.Shape and size of micro-voids in cement mortar is detected using Micro Computed Tomography techniques.The formation of delayed ettringite crystal is analyzed using scanning electron microscope and energy disperse spectrum methods.Deformation of micro-voids and the distribution of stress at the surface of a micro-void are calculated.It is found that the nucleation of micro-cracks is caused by the tensile stress at the voids’ surface,and such damage nucleation will be speeded up by the remote uniaxial compressive load.     

The strain-hardening behaviour of linear and long-chain-branched polyolefin melts in extensional flows

By generalizing the Doi-Edwards model to the Molecular Stress Function theory of Wagner and Schaeffer, the extensional viscosities of polyolefin melts in uniaxial, equibiaxial and planar constant strain-rate experiments starting from the isotropic state can be described quantitatively. While the strain hardening of four linear polymer melts (two high-density polyethylenes, a polystyrene and a polypropylene) can be accounted for by a tube diameter that decreases affinely with the average stretch, the two long-chain-branched polymer melts considered (a low-density polyethylene and a long-chain branched polypropylene) show enhanced strain hardening in extensional flows due to the presence of long-chain branches. This can be quantified by a molecular stress function, the square of which is quadratic in the average stretch and which follows from the junction fluctuation theory of Flory. The ultimate magnitude of the strain-hardening effect is governed by a maximum value of the molecular stress, which is specific to the polymer melt considered and which is the only free non-linear parameter of the theory. Received: 1 June 1999/Accepted: 24 November 1999     

UPML吸收边界条件的无截断数值模拟

Berenger完全匹配层(PML)吸收边界条件完成了对非物理反射波的吸收,初步实现了有限区域对无限开放空间的数值模拟,然而在计算区域的边界上需要对场分量进行分裂,增加了Maxwell方程组中独立方程的个数,使场分量迭代复杂,增大了数值计算量,单轴各向异性完全匹配层(UPML)边界条件则不需要对场分量进行分裂,迭代公式简单,便于程序实现,且具有更好的宽频带吸收特性.首先推导了TMz波的UPML方程组,给出了UPML的介电参数分布方式,详细介绍了该算法的程序实现步骤,并以数值算例进行验证,分别采用常规PML和UPML边界条件进行数值计算,通过对比二者的计算结果,表明UPML边界条件具有更好的吸收效果,且能更有效地吸收低角入射波和低频凋落波,UPML边界条件更逼真地模拟了无限开放空间,更精确地实现了电磁波的无截断数值模拟.     

γ射线总剂量辐照对单轴应变Si纳米n型金属氧化物半导体场效应晶体管栅隧穿电流的影响

基于γ射线辐照条件下单轴应变Si纳米n型金属氧化物半导体场效应晶体管(NMOSFET)载流子的微观输运机制,揭示了单轴应变Si纳米NMOSFET器件电学特性随总剂量辐照的变化规律,同时基于量子机制建立了小尺寸单轴应变Si NMOSFET在γ射线辐照条件下的栅隧穿电流模型,应用Matlab对该模型进行了数值模拟仿真,探究了总剂量、器件几何结构参数、材料物理参数等对栅隧穿电流的影响.此外,通过实验进行对比,该模型仿真结果和总剂量辐照实验测试结果基本符合,从而验证了模型的可行性.本文所建模型为研究纳米级单轴应变Si NMOSFET应变集成器件可靠性及电路的应用提供了有价值的理论指导与实践基础.     

Reflection and transmission of Laguerre Gaussian beam from uniaxial anisotropic multilayered media

Based on angular spectrum expansion and 4 × 4 matrix theory, the reflection and transmission characteristics of a Laguerre Gaussian(LG) beam from uniaxial anisotropic multilayered media are studied. The reflected and transmitted beam fields of an LG beam are derived. In the case where the principal coordinates of the uniaxial anisotropic media coincide with the global coordinates, the reflected and transmitted beam intensities from a uniaxial anisotropic slab and three-layered media are numerically simulated. It is shown that the reflected intensity components of the incident beam, especially the TM polarized incident beam, are smaller than the transmitted intensity components. The distortion of the reflected intensity component is more evident than that of the transmitted intensity component. The distortion of intensity distribution is greatly affected by the dielectric tensor and the thickness of anisotropic media. We finally extend the application of the method to general anisotropic multilayered media.     

傍轴光束在单轴左手介质中传输的矢量性质

研究了沿光轴入射的傍轴光束在单轴左手介质中传输的矢量性质.通过平面角谱的方法得到了横电(TE)波和横磁(TM)波在单轴左手介质中传输的严格解，并得到了傍轴近似下的矢量传输模型.利用这一模型，分析得到垂直于光轴方向不同偏振态的入射光束在单轴左手介质中同时激发横向场和低阶纵向场.如果单轴左手介质的介电常数张量在沿垂直于光轴方向上的元素不相等，同时磁导率张量在沿垂直于光轴方向上的两元素也不相等(即z方向为光轴方向，_x≠_y,μ_x≠μ_y),那么入射光束激发的横向场的分布与入射光束的初始场分布和偏振态有关，而纵向场的分布与入射光束的偏振态无关，只决定于入射光束的初始场分布. 关键词： 傍轴传输 单轴左手介质 矢量效应