金属介电函数的正确运用——以金属纳米球诱导的二能级系统的基态能级移动为例

金属对电磁场的响应包括自由和束缚电荷两部分的贡献,大学物理教学中通常分别用Drude模型和Lorentz模型描述,然而,以上模型所得的介电函数只在一段较窄的频率范围内能与实验数据吻合.因此,在实际科研工作中,当物理量与较宽频率范围内的电磁场响应有关时,原则上介电函数应当采用实验测量值而不能采用模型化的结果.本文以金属纳米球诱导的二能级系统的基态能级移动为例,分别展示采用模型化的介电函数和实验介电函数时,能级移动计算结果的差异.     

Continuum Model for Electronic Polarization Based on a Novel Dielectric Response Function

基于连续介质模型推导了一个普适的描述电介质介电弛豫过程的响应函数. 该介电响应函数依赖于电介质的介电谱. 基于该函数推导得到了以前特殊情况下用于描述溶剂弛豫的响应函数一致的表达式.结合三种典型极性溶剂,水、甲醇和乙腈的介电谱,研究了三种溶剂在外加电场线性变化时的电子极化过程. 结果表明,溶剂的电子极化伴随着电子跃迁同步发生,没有时滞.     

用椭偏术研究与接触介质有关的贵金属的介电函数

为了研究接触介质对金属光学性质的影响，实验中使用具有不同折射率的梯形棱镜作为衬底，将金和银蒸发到棱镜底部，用椭偏术分别测量了薄膜在金属-空气、金属-衬底界面的介电函数.结果表明：无论在Drude区，还是在带间跃迁区，金属-衬底界面处薄膜介电函数不仅与金属-空气界面处的测量值不同，而且随衬底折射率改变而改变.在固体接触条件下获得的结果与其他作者在液体接触条件下获得的结果相一致，但尚难被现有机制所解释. 关键词：     

太赫兹与远红外频段下铝质目标电磁特性与计算

在太赫兹与远红外频段,铝处于由导体到介质的过渡,研究该频段铝质目标与电磁波的相互作用机理对于实现太赫兹频段目标精确电磁散射计算具有重要意义.基于实验测量数据,设计有效误差准则模型拟合得到了太赫兹与远红外频段铝的介电系数模型;基于拟合模型通过推导过渡阶段不同损耗机理下铝中传播电磁波的空间相位系数与铝的波阻抗等参数,分析了太赫兹与远红外频段电磁波在铝中的透射与反射特性,给出了铝的反射率关于频率的变化曲线.结果表明铝中电磁波传播参数从微波向太赫兹频段过渡时具有很好的连续性与一致性;基于阻抗边界条件的雷达散射截面计算结果表明太赫兹频段光滑铝质目标可视做理想导体进行计算,太赫兹雷达散射截面测量中可利用光滑铝板或铝球做为定标体.     

关于色散流体的介电方程

指出了在宏观推导运动流体的非局域型材料关系时，所涉及到的局部静止坐标必须随同流体一起转动．为了确定它与实验室坐标之间的关系，描述流体状态的场变量，除平动速度外，还应有三个空间角度．这些角变量满足一组一般的偏微分方程．尽管本文的内容是针对简单的弛豫时间模型而给出的，其物理方法可适用于推导洛伦兹协变的、含任何复杂的色散和非线性项的材料方程 关键词：     

任意色散周期结构的时域有限差分方法分析

利用辅助微分时域有限差分法求解了任意色散周期模型的电磁波传播问题。利用共轭复数对形式对任意色散媒质进行参数拟合,并将任意色散媒质的介电常数表示成公式形式,在FDTD迭代式中引入辅助微分方程,推导出了适用于多层任意色散模型的通用递推公式,分别求解了Debye、Drude与太阳能电池周期结构模型的电磁特性仿真问题。仿真结果表明:数值计算结果与CST商业软件仿真结果基本吻合,证明了所构建方法的有效性与普适性。     

Dielectric Properties of GaN in THz Frequencies

The complex refractive indices and the dielectric function of GaN for frequencies ranging from 0.25 to 1.22THz are obtained using THz time-domain spectroscopy. The real part of the dielectric function first decreases from 0.25 to 0.42THz and then oscillates from 0.42 to 1.22THz, whereas the imaginary part of the dielectric function is oscillating within the whole range of frequency. The simple Drude model is extended to take into account the effect of defects on the dielectric function. The extended model is in agreement with the experimental data.     

电介质的频域特性与测量

从物质的分子结构基本模型出发，简要介绍了介电频域理论，提出了有关新的测量方法。     

混合电解质溶液中考虑介电饱和度的表面电位评估原理

基于非线性泊松-玻尔兹曼方程，推导了混合电解质溶液中考虑介电饱和度的表面电位的解析表达式. 近似解析解和精确数值解计算出的表面电位在很大范围的电荷密度和离子强度条件下均具有很好的一致性. 当表面电荷密度大于0.30 C/m^{2 时，介电饱和度对表面电位的影响变得尤为重要；当表面电荷密度小于0.30 C/m2时，可忽略介电饱和度的影响，即基于经典泊松-玻尔兹曼方程可获得有效的表面电位解析模型. 因此，0.3 C/m2可作为是否考虑介电饱和度的颗粒临界表面电荷密度值. 在低表面电荷密度时，考虑介质饱和度的表面电位解析模型可自然回归到经典泊松-玻尔兹曼理论的结果，得到的表面电位可以正确地预测一价和二价反离子之间的吸附选择性.}     

双色散模型的辛时域有限差分算法

结合有耗的Drude-Lorentz色散模型,提出了处理双色散模型的辛时域有限差分算法.基于矩阵分裂,辛传播算子和辅助差分方程技术,结合严格而巧妙的公式推导,构建了算法框架,并给出了详细的公式推导过程.为了验证本文算法的有效性和精确性,首先计算了一维空间双色散平板的透射系数,并与解析解对比,结果较好地符合,证明了该算法是有效而精确的.然后计算了三维空间中有实际意义的银分裂环,金属银的介电参数由Drude模型拟合.计算了该结构的透射系数,反射系数和吸收系数,得到了银分裂环的谐振频率和吸收频率,为实际实验结果提供了可供参考的计算结果.     

高阶色散介质的改进移位算子时域有限差分方法

提出一种适用于高阶Debye,Drude,Lorentz及其混合模型的改进移位算子的时域有限差分(SO-FDTD)方法。从介质极化率函数出发,将其写成一阶或二阶有理分式求和的形式,并在随时间步推进计算的过程中,通过引入中间变量和设置临时变量,克服了常规SO-FDTD将高阶模型直接转化为有理分式所导致的计算复杂性和内存占用量大的问题。同时,改进SO-FDTD方法的时域推进计算步骤具有通用性,克服了常规递归卷积(RC-FDTD)方法对各种高阶模型具有不同计算公式,因而不能形成通用计算程序的问题。最后,通过空气-高阶色散介质界面的反射系数计算验证了算法的有效性和通用性。     

Two kinds of upper hybrid soliton bistability

The basic model employed to describe nonlinear upper hybrid wave structures is the generalized nonlinear Schrödinger equation including second and fourth order dispersive effects as well as local and nonlocal nonlinearity. For two kinds of such an equation the existence of two stable solitons with the same plasmon number but with different spatial scales and amplitudes is shown as two qualitatively different kinds of upper hybrid soliton bistability. An integral relation for an arbitrary nonlinear upper hybrid wave packet evolution is derived taking into account higher order dispersive effects. Necessary conditions for soliton formation from arbitrary wave packets and the impossibility of wave packet collapse are demonstrated taking into account higher order dispersive effects.     

A generalized 2D FDTD model for photonic crystal fibers with frequency dependent media

A generalized model based on the two-dimensional finite-difference time-domain (2D FDTD) method for photonic crystal fibers (PCF) with frequency dependent media is presented. The Maxwell’s curl equations are formulated using flux density and the magnetic field. Auxiliary differential equations are used with complex-conjugate pole-residue pairs which incorporate the material dispersion of the dispersive media. The model is demonstrated to be a unified approach for arbitrary dispersive materials; therefore, it definitely reduces implementation cost when dealing with different frequency-dependent materials.     

Frequency response of 1D dispersive sea surface by the ADE-FDTD algorithm

The frequency response of a one-dimensional (1D) sea surface was investigated using the finite-difference time-domain (FDTD) algorithm with pulsed wave excitation. Where the sea water is regarded as an isotropic and dispersive medium, it satisfies the single-pole Debye model. The auxiliary differential equation (ADE) technique is incorporated into the FDTD scheme to deal with the dispersive sea surface. To ensure the feasibility of our present method, the frequency response of dispersive sea surface by the ADE-FDTD method is compared with the result by the conventional method of moments (MOM), which requires an individual calculation for every frequency point of interest. Finally, the scattering characteristic of dispersive sea water is investigated for different wind speed, temperature, and salinity. The representative far zone scattered field versus time is also analyzed.     

Dynamism of two-mode’s parameters on the field function for third-order dispersive Fisher: application for fibre optics

Two-mode equations model the spread of two different wave modes simultaneously. A new two-mode dispersive Fisher of third order (TMDF) is proposed and different types of solutions are obtained. We studied the effect of TMDF’s factors (dispersive, nonlinearity and the phase velocity) on the behavior of the obtained field function. We believe that our findings can be useful for investigating further analysis in fibre optics and engineering applications.     

一种处理色散介质问题的通用时域有限差分方法

色散介质的介电系数是频率的函数,使本构关系在时域成为卷积关系.这就给用时域有限差分方法计算色散介质中波的散射和传播带来了困难.现有算法往往要针对不同色散介质模型推导相应的递推公式,算法的通用性较差.本文完善和发展了移位算子-时域有限差分方法,使之成为一种处理色散介质电磁问题的通用方法.首先,证明了常见的三种色散介质模型(德拜模型、洛伦兹模型和德鲁模型)的介电系数均可以写成适于移位算子法计算的有理分式函数形式.然后,用/t代替jω,过渡到时域,再引入时域移位算子z_t代替时间微分算子来处理有理分式函数形式的介电系数,给出离散时域本构关系的表示式,进而导出时域有限差分方法当中电位移矢量和电场强度之间的关系.最后,计算了几种色散介质的电磁散射,数值结果表明了本文方法和程序的通用性和正确有效性. 关键词： 时域有限差分方法 色散介质 移位算子     

Transmissivity and reflectivity of spatially dispersive thin films

We calculate the transmission and reflection coefficients for electromagnetic radiation incident normally on the surfaces of thin, spatially dispersive, absorbing, dielectric films. Results are obtained for four models of spatially dispersive dielectrics and for the case in which spatial dispersion is neglected. For each model the effects of spatial dispersion are to introduce additional fine structure into the transmission and reflection coefficients, regarded either as functions of frequency or of the thickness of the film. In addition, the results reveal an interleaving between the maxima and minima of the spectra for different models, which may provide a basis for an experimental discrimination between different phenomenological models for the nonlocal dielectric constant of a spatially dispersive dielectric medium.     

An estimation of EDXRF spectrometer properties,based on a two‐layer composite Si‐Ge detector

A mathematical model for the two‐layer composite Si‐Ge energy dispersive X‐ray detector is proposed, based on analyses of radiation and electron transport in the detector, and a mathematical model of an energy dispersive X‐ray fluorescent spectrometer with the detector is considered. The Monte Carlo method is applied to calculate probabilities of photon detection in different parts of the detector's response function. The composite detector with the time anti‐coincidence scheme is proposed; its first layer is Si detector, and the second layer is Ge detector. It is shown that this composite detector has some advantages, such as reduced Ge photo escape peaks intensities and efficiency of detection of high energy photons similar to efficiency of Ge detector. Applying the X‐ray detector for the energy dispersive X‐ray fluorescent spectrometer provides for a lower background level. Copyright © 2012 John Wiley & Sons, Ltd.     

Stability of Bragg grating solitons in a cubic-quintic nonlinear medium with dispersive reflectivity

We investigate the existence and stability of Bragg grating solitons in a cubic-quintic medium with dispersive reflectivity. It is found that the model supports two disjoint families of solitons. One family can be viewed as the generalization of the Bragg grating solitons in Kerr nonlinearity with dispersive reflectivity. On the other hand, the quintic nonlinearity is dominant in the other family. Stability regions are identified by means of systematic numerical stability analysis. In the case of the first family, the size of the stability region increases up to moderate values of dispersive reflectivity. However for the second family (i.e. region where quintic nonlinearity dominates), the size of the stability region increases even for strong dispersive reflectivity. For all values of m, there exists a subset of the unstable solitons belonging to the first family for which the instability development leads to deformation and subsequent splitting of the soliton into two moving solitons with different amplitudes and velocities.