向列相液晶中强非局域空间光孤子传输的理论研究

对向列相液晶中非局域空间孤子的传输进行了理论研究.基于非线性液晶孤子传输方程，采用Gauss形式的试探解，不仅得到了空间孤子的解析解,而且还在临界功率附近得到了呼吸子的解析解.通过数值模拟证明我们的结果比Conti和Assanto等人的结果更合理.同时，对液晶中的非局域孤子模型和Snyder等提出的强非局域孤子模型进行了全面的比较. 关键词： 向列相液晶 空间孤子 非局域非线性 呼吸子     

光的折射定律(Snell定律)演示实验装置

基于光子动量在不同媒质分界面上沿分界面方向守恒原理,制作了光的折射定律演示装置.使用该装置可演示入射角连续变化,折射角也自动遵循折射定律相应变化.本文介绍了该演示装置的原理、构造及使用方法.     

多分辨力分析在自动聚焦算法抗噪声性能中的应用

自动聚焦是数码设备、计算机视觉中的一项关键技术。自动聚焦过程中,聚焦的准确性和抗噪声性能至关重要。以高频分量作为度量的聚集评价函数具有灵敏性高、聚焦准确的优点,适用于实时系统,但是对噪声十分敏感,受噪声污染时可能导致聚焦失败。因此,提出了一种具有噪声稳健性的高频分量自动聚焦评价函数。该函数通过小波多分辨力分析提取高频分量,利用了信号的每个子带的小波系数存在一定相关性,而噪声不存在这样的相关性的特点,设定高频子带阈值,认为低于阈值的系数是噪声的贡献,大致分离图像信号与噪声信号,从而将其滤除。经过大量的实验,证明提出的方法具有单峰性好、灵敏度高等优点,特别是在抗噪声性能方面有很大提高。     

向列相液晶中强非局域空间光孤子的实验观察

通过实验对向列相液晶中的非局域空间光孤子的传输进行了研究。理论上基于非线性的液晶孤子传输方程,采用高斯形式的试探解,得到了孤子传输的解析解,以及对液晶中的非局域孤子和Snyder等提出的强非局域孤子模型的结果进行了比较。实验上,观察了空间光孤子在向列相液晶中的传输,找到了不同束宽下空间光孤子的临界功率。比如束宽为2μm时临界功率为2.0mW。观察了向列相液晶中空间光孤子的弛豫过程,并注意到液晶分子的响应时间长达几秒钟,液晶中的孤子形成速度较慢。     

均匀聚焦磁场中束晕-混沌的孤子控制

采用粒子.束核模型,对均匀聚焦磁场中满足K-V分布的离子束进行模拟研究.观察到束晕.混沌现象,基于束晕.混沌的非线性控制策略,提出控制束晕.混沌的孤子函数控制器,并给出具体的实施方案.模拟研究表明,运用这种方法可以消除束晕及其再生现象,达到对束晕.混沌的有效控制.     

Influence of surface interactions on the dynamics of the glass former ortho-terphenyl confined in nanoporous silica

We present results on investigations of the dynamics of the glass forming ortho-terphenyl (oTP) confined in nanoporous silica. Calorimetry experiments showed that the glass transition temperature of the confined liquid, T_gconf, has a non-trivial pore size dependence and is strongly affected by surface interactions. Fluid-wall interactions introduce gradients of structural relaxation times in the pores. The molecules at the surface of the pores are slowed down compared to those at the center of the pores. We focus here on a pore diameter range (7 σ< d < 12 σ, where σ is the molecular diameter), where a large variety of dynamical behavior were observed. Depending on surface properties of the confined media, T _gconf may be smaller or larger than the bulk one. In a quite attractive matrix with a pore size of around 7 nm, the structural relaxation times gradient is important enough to allow the observation of two glass transitions for the same liquid. Effects of fluid wall interactions on the short time dynamics at high temperature were also investigated by quasielastic neutron scattering. The self and collective motions exhibit well above the bulk melting point the same dependence on fluid-wall interactions as at T_g.     

Analysis of plume deflection in the silicon laser ablation process

Changes in target surface morphology and ablation plume direction have been experimentally observed during the initial stages of the silicon laser ablation process. A relationship between both phenomena can be observed upon analysing the temperature field induced by the laser beam in a rough surface material. Theoretical studies on the deflection of the ablation plume are presented. These analyses are based on the hypothesis that particles that reach evaporation temperature will exit normally to the target surface with a velocity that is proportional to the surface temperature and the amount of the ablated material. Numerical solutions and experimental results of laser ablation process of silicon targets are found to agree with theoretical studies. PACS 42.25.Lc; 79.20.Dc; 02.70.Dc     

GISAXS study of carbon nanotubes grown by CVD

We report a quantitative Grazing Incidence Small Angle X‐ray Scattering (GISAXS) study of a dense film of mutually oriented carbon nanotubes (CNTs) grown by a catalytically‐activated DC HF CCVD process after dispersion of metallic catalytic (Co) islands on SiO₂/Si(100) substrates. The GISAXS pattern analysis is expanded to non‐correlated surface science systems and is based on CNTs density, characteristic lengths, atomic Co dispersion throughout the CNTs and roughnesses of uncorrelated particles. The results are closely compared to SEM and TEM observations. The GISAXS patterns, even dominated by envelope features of disordered objects, provide significant complementary quantitative data about CNTs films. The results underline that cobalt continuously fills the nanotube in the course of the growth and that the CNTs experience a large tendency toward mutual alignment. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Aligned carbon nanotubes catalytically grown on iron-based nanoparticles obtained by laser-induced CVD

Iron-based nanoparticles are prepared by a laser-induced chemical vapor deposition (CVD) process. They are characterized as body-centered Fe and Fe₂O₃ (maghemite/magnetite) particles with sizes ≤5 and 10 nm, respectively. The Fe particles are embedded in a protective carbon matrix. Both kind of particles are dispersed by spin-coating on SiO₂/Si(1 0 0) flat substrates. They are used as catalyst to grow carbon nanotubes by a plasma- and filaments-assisted catalytic CVD process (PE-HF-CCVD). Vertically oriented and thin carbon nanotubes (CNTs) were grown with few differences between the two samples, except the diameter in relation to the initial size of the iron particles, and the density. The electron field emission of these samples exhibit quite interesting behavior with a low turn-on voltage at around 1 V/μm.     

Nearest-neighbour spacing distributions of the β-Hermite ensemble of random matrices

The evolution with β of the distributions of the spacing ‘s’ between nearest-neighbour levels of unfolded spectra of random matrices from the β-Hermite ensemble (β-HE) is investigated by Monte Carlo simulations. The random matrices from the β-HE are real symmetric and tridiagonal where β, which can take any positive value, is the reciprocal of the temperature in the classical electrostatic interpretation of eigenvalues. The distribution of eigenvalues coincide with those of the three classical Gaussian ensembles for β=1, 2, 4. The use of the β-HE ensemble results in an incomparable speed up and efficiency of numerical simulations of all spectral characteristics of large random matrices. Generalized gamma distributions are shown to be excellent approximations of the nearest-neighbor spacing (NNS) distributions for any β while being still simple. They account both for the level repulsion in ∼s^β when s→0 and for the whole shape of the NNS distributions in the range of ‘s’ which is accessible to experiment or to most numerical simulations. The exact NNS distribution of the GOE (β=1) is in particular significantly better described by a generalized gamma distribution than it is by the Wigner surmise while the best generalized gamma approximation coincides essentially with the Wigner surmise for β>∼2. They describe too the evolution of the level repulsion between that of a Poisson distribution and that of a GOE distribution when β increases from 0 to 1. The distribution of ln (s), related to the electrostatic interaction energy between neighbouring charges, is accordingly well approximated by a generalized Gumbel distribution for any β?0. The distributions of the minimum NN spacing between eigenvalues of matrices from the β-HE, obtained both from as-calculated eigenvalues and from unfolded eigenvalues are Brody distributions which are classically used to characterize the spectral fluctuations of various physical systems.