三维时域有限差分法计算金纳米粒子尺寸与SERS活性的关联

通过合成一系列不同粒径(16～160 nm)的金纳米粒子.观察到120～135 nm的金纳米粒子在632.8nm波长激发下具有最高的SERS活性,这与前人报道的电磁场理论及实验的结果不同.利用三维时域有限差分法对金纳米粒子的SERS活性与其尺寸以及入射光波长的关系进行模拟计算.在632.8 nm激发线下,金纳米粒子二聚体体系在粒径为110 nm左右具有最佳增强效应,其光电场耦合最强的热点处的增强因子高达109考虑到体系的平均SERS增强因子通常会比最大值低约2个数量级,计算得到的107的增强因子与实验测量值相符.同时对目前实验上尚难以合成的大尺寸的金纳米粒子进行模拟,结果表明受多极矩和大尺寸效应的影响在粒径220 nm时又出现SERS增强另一峰值.在325 nm的紫外激发线下,计算得到的增强因子仅为102.     

Comparison of 1D, 2D and 3D quench onset simulations

Measured and simulated minimum quench energies (MQE) for short samples are doubtful when estimating disturbance tolerance of an impregnated superconducting magnet. If measurements are performed at liquid cryogen, the cooling provided by the cryogen causes MQE to be high. At a conduction-cooled system, the transverse heat conduction is neglected causing low MQE. Even though these conditions can be artificially brought closer to the ones in an impregnated magnet, it is difficult to estimate by short-sample measurement the MQE of a superconducting coil. These similar difficulties are present at simulations. On the other hand, 1D and 2D measurements and simulations can be used, e.g., to compare wires or consider effect of insulation layer thickness, but according to the results only in a rough quantitative way. In this paper, we compare MQE and normal zone propagation velocity simulations of an MgB₂ wire with 1D, 2D and 3D finite element method models to focus on the different results given by the models employing different dimensions.     

使用广义粗网有限差分方法加速中子输运特征线方法

提出广义粗网有限差分方法(GCMFD),可以使用三维任意几何形状的粗网格来加速中子输运特征线方法(MOC),同时给出确定广义粗网有限差分方法中宽度因子的方法.将广义粗网有限差分方法应用到三维特征线方法程序TCM中,若干基准题的验证表明,广义粗网有限差分方法可以使用任意形状粗网格来加速特征线方法,使用自动调整宽度因子的方法后,广义粗网格有限差分方法可以得到较好的加速效果.     

Design of Hamming Code Checker Using Titanium-Diffused Lithium Niobate-Based Waveguide

ABSTRACT

This work presents an electro-optic effect-based design for the widely used Hamming code for checking single bit errors in the received data. The structure is based on Mach-Zehnder Interferometer, designed using Titanium-diffused Lithium Niobate-based waveguides. The proposed device has been successfully tested using the beam propagation method. For a received 7-bit code, the three simultaneously generated check bits are combined to check the error position. Critical performance parameters like extinction ratio (24.39dB), contrast ratio (25.711dB), insertion loss (0.041dB), and amplitude modulation (0.079dB) calculated for the overall device are within acceptable limits for the feasibility of the device.     

Variations of the elastic constants of InSb near the covalent-metallic transition

Abstract

The pulse-echo-overlap method was used to measure the longitudinal and transverse wave velocities in single-crystal (100) and (111) samples of InSb up to 3 GPa at room temperature. The peculiar variations of the elastic constants were observed near the covalent-metallic transition.     

Alternative difference analysis scheme combining R‐space EXAFS fit with global optimization XANES fit for X‐ray transient absorption spectroscopy

Time‐resolved X‐ray absorption spectroscopy (TR‐XAS), based on the laser‐pump/X‐ray‐probe method, is powerful in capturing the change of the geometrical and electronic structure of the absorbing atom upon excitation. TR‐XAS data analysis is generally performed on the laser‐on minus laser‐off difference spectrum. Here, a new analysis scheme is presented for the TR‐XAS difference fitting in both the extended X‐ray absorption fine‐structure (EXAFS) and the X‐ray absorption near‐edge structure (XANES) regions. R‐space EXAFS difference fitting could quickly provide the main quantitative structure change of the first shell. The XANES fitting part introduces a global non‐derivative optimization algorithm and optimizes the local structure change in a flexible way where both the core XAS calculation package and the search method in the fitting shell are changeable. The scheme was applied to the TR‐XAS difference analysis of Fe(phen)₃ spin crossover complex and yielded reliable distance change and excitation population.     

Ultrasonic and FT-IR studies on Bi2O3–Er2O3–PbO glasses

Glasses in the 90Bi₂O₃–(10?x)Er₂O₃?xPbO (x = 3, 5, 7, 9 and 10 mol%) system have been prepared by the melt-quenching technique. Elastic properties and FT-IR spectroscopic studies have been employed to study the role of PbO in the structure of the investigated system. Elastic properties and Debye temperature were recorded using sound wave velocity measurements at 4 MHz at room temperature. The results showed that density increased and molar volume decreased, while both sound velocities increased with an increase in x. Infrared spectra of the glasses revealed that the bismuthate network is affected by an increase in PbO content. The results are interpreted in terms of the conversion of [BiO₆] into [BiO₃] structural units, indicating that Pb ions have been substituted for erbium ions as tetrahedral network formers. The elastic moduli increased with increasing PbO content due to the increased average bond strength and degree of connectivity, as a direct effect of the increase in [BiO₃] structural units.     

A mathematical model of hotspot condensed phase ignition in the presence of a competitive endothermic reaction

We consider the propagation of a combustion front resulting from the gasless combustion of a condensed state fuel. The propagation of the front, essentially a premixed laminar flame, is supported by an exothermic reaction subject to possible heat loss through a competitive endothermic reaction. The dynamics of the endothermic process inducing the heat loss strongly depend on the temperature and the local fuel concentration. Through an analysis based on high activation energy, the steady-state values of the final burnt temperature as well as the burning velocity are obtained, and the control parameters are identified. Using a linear perturbation method, we assess the stability of the propagating front and obtain a condition for oscillatory behaviour. The critical parameter values for the transition from steady to oscillatory burning speeds are identified. The results represent a generalization of those obtained by Matkowsky and Sivashinsky to include the effects of heat loss induced by a competitive endothermic reaction.     

新型光弹调制干涉具研究

针对单个光弹调制器((Photoelastic Modulator,PEM))干涉具受到材料本身热、力学的限制,产生的最大光程差小,而多个PEM串联又难以控制,且多个PEM界面的多次反射将使光能大量损失等缺点,提出一种在单PEM上贴全反射膜经过多次反射有效提高最大光程的方法。理论推导分析了不同反射情况对光程差的影响,并得到任意角任意位置入射经多次反射后产生最大光程差的公式与光谱反演公式,且通过COMSOL、MATLAB、ZEMAX仿真和实验验证其可行性。实验选用的弹光晶体为硒化锌(ZeSe),压电晶体为石英,结果显示实验与理论的相对误差为0.21%,为该方法的工程应用提供了参考。     

Plasmonic planar lens based on stack of metal–insulator–metal waveguides with height tuning

We introduce a technique capable of focusing electromagnetic (EM) waves through plasmonic nanoslits symmetrically arranged along the indented semi-circular surface in silver background. The EM transports through the tuning slits in the form of surface plasmon polaritons (SPPs), and gets the required phase retardations to focus at the focal plane. Due to the subwavelength nature of planar metallic lens, we present the rigorous electromagnetic analysis by using two dimensional (2D) finite difference time domain (FDTD) method. These height-modulated slits with uniform width are demonstrated to have unique advantages in beam manipulation. In combination with previous studies, it is expected that our structure with small number of slits could lead to realization of optimum designs of plasmonic nano-lens.