基于涂覆石墨烯的三根电介质纳米线的THz波导的模式特性分析

研究了一种基于涂覆石墨烯的三根电介质纳米线的THz波导,采用多极方法对这种波导所支持的5种低阶模的有效折射率的实部和传播长度进行了解析分析.结果表明,通过改变工作频率、中间纳米线半径、纳米线之间的间距以及石墨烯的费米能,可以有效地调节波导的模式特性.当工作频率从30 THz增加到40 THz时,这些模式的有效折射率的实部增大,传播长度减小,并且在变化的过程中会出现交叉现象.当中间纳米线的半径从25 nm增加到75 nm时,除了模式3和模式4基本不受影响,其他模式有效折射率的实部增大,传播长度变化各不相同.当纳米线之间的间距从10 nm增加到50 nm时,除了模式3和模式4基本不受影响,其他模式有效折射率的实部减小,传播长度增大,并且在变化的过程中会出现交叉现象.当石墨烯的费米能从0.4 eV增加到1.2 eV时,有效折射率的实部减小,传播长度增大.计算表明,多极法得到的结果与有限元方法得到的结果完全一致.本研究可以为基于涂覆石墨烯的电介质纳米线的THz波导的设计、制作和应用提供理论基础.     

涂覆石墨烯的非对称并行电介质纳米线波导的模式特性分析

采用多级展开方法,对涂覆石墨烯的非对称并行电介质纳米线波导的模式特性进行了分析.首先对这种波导中的表面等离子模式进行分类,然后对七种低阶模式的有效折射率和传播长度随工作频率、几何结构参数和石墨烯费米能的依赖关系进行详细的分析.结果表明,通过改变工作频率、几何结构参数和石墨烯的费米能,可以在较大范围内调节模式的特性.与有限元法进行的对比表明,基于多级方法的半解析结果与有限元法的数值结果非常符合.研究结果可为涂覆石墨烯的非对称并行电介质纳米线的设计和制作提供一定的理论基础.     

Study on cylindrical cavity coupling with rectangular waveguides by large apertures

The structure of cylindrical cavity coupling with rectangular waveguides through large apertures on the cavity’s sidewall is studied in detail. The fields in the cavity is properly expanded by derived mode functions. By making reasonable approximation of the boundary conditions, the process of derivation and the formation of the characteristic equation is greatly simplified. The characteristic equation and model coefficients are obtained. The reliability of the derived characteristic equation is tested. Numerical results of characteristic frequency and quality factor changing with dimensions of coupling holes are given.     

Physics‐based modeling and microwave characterization of graphene co‐planar waveguides

In this study, large area mono‐, bi‐, and multi‐layer graphenes (MLG) were synthesized using low pressure chemical vapor deposition (LPCVD) process and co‐planar waveguides (CPW) with graphenes being fabricated using conventional photolithography. The intrinsic room temperature radio frequency (RF) characteristics were measured via direct contact to the graphene surface with a probe–tip in the range of 0.1 GHz to 40 GHz without using metal electrodes. The device characteristics were also evaluated by physics‐based RLC equivalent circuit simulation. The proposed equivalent circuit model reproduced all of the measured characteristics within 3% of RMSE and we numerically extracted the set of component values which minimize the error between the measured data and the simulation results over the measured regime. Our findings demonstrate that MLG is an effective candidate in RF interconnect device applications. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Mode theory of three-layer cylindrical waveguides and its application to aurum(Au)/polystyrene(PS)-coated terahertz hollow waveguides

In this paper, we present a generic mode theory for the three-layer cylindrical waveguides consisting of three arbitrary material mediums. Based on derived eigenvalue equations of the TE, TM, and hybrid modes, the mode conditions of metal/dielectric-coated terahertz hollow waveguides are extracted. In addition, we quantitatively describe both the effective refractive indexes and the loss characteristics of the aurum/polystyrene-coated terahertz hollow waveguides operating at the TE₀₁, TM₀₁, and HE₁₁ modes. It is indicated that the loss coefficient of the TE₀₁ mode can be much lower than that of the TM₀₁, and HE₁₁ modes, and, especially, can tend to that of the terahertz wave in air. So the TE₀₁ mode is very significant for the hollow waveguides in term of low loss propagating of the terahertz wave. We expect these results to enable a variety of new long-distance THz applications in sensing, detecting and communicating.     

Analyzing integrated-optical inhomogeneous planar waveguides by Galerkin's method: A detailed comparison of two different basis functions

The trigonometric and Hermite-Gaussian basis functions for determining the modal characteristics of inhomogeneous optical waveguides by means of the Galerkin's method are presented and analyzed. The results obtained with each set of basis functions for mode spectra and field distributions are compared with other exact and approximate methods. The merits and problems arising with each set of basis functions are discussed.     

涂覆石墨烯的椭圆形电介质纳米线光波导的模式特性分析

设计了一种涂覆石墨烯的椭圆形电介质纳米线光波导.采用分离变量法,在椭圆柱坐标系中,借助Mathieu函数,得到了色散方程.通过数值求解色散方程,可以得到模式的有效折射率和场分布,从而得到模式的传播长度.研究了工作波长、结构参数以及石墨烯的费米能对模式特性的影响,并给出了前五个模式的品质因数.计算表明,当波长从4.3μm增加到8.8μm,这5个模式的有效折射率的实部减小,基模和一阶模的传播长度增大,二阶模的传播长度先增大后减小.当改变纳米线结构参数半长轴和半短轴时,对基模和一阶模的模式特性影响较小,对二阶模的模式特性影响较大.当石墨烯的费米能从0.45 eV增加到0.72 eV时,有效折射率的实部减小,传播长度可以达到2μm左右.分离变量法得到的结果与有限元方法得到的结果完全一致.本文工作可以为基于涂覆石墨烯的电介质纳米线的光波导的设计、制作和应用提供理论基础.     

Quantum steerability of two qubits mediated by one-dimensional plasmonic waveguides

We study the dynamics of the quantum steering between two separated qubits trapped in a one-dimensional plasmonic waveguide. By numerical methods, we calculate the quantum steerability and other quantum correlations, i.e., entanglement, discord, and coherence, for both cases with and without laser driving fields. It is found that steerability may exhibit a sudden disappearance and sudden reappearance phenomenon. Specifically, there exist time windows with no steerability but finite entanglement. The effects of plasmon wavenumber and the distance between the two qubits on steerability are also examined. Furthermore, we show that quantum steerability is tunable by adjusting the laser driving fields.     

任意截面介质波导导模的叠代矩量法分析

本文提出用以计算任意截面介质波导色散关系的叠代矩量法,并给出用叠代矩量法计算椭圆介质波导、三角形波导、平切圆波导、蛋形截面波导等的传播特性的实例.计算结果与有限元法、点匹配法等精确数值计算结果符合很好.本方法简便易行,计算工作量较小.     

Analyzing integrated-optical inhomogeneous planar waveguides by Galerkin's method: A detailed comparison of two different basis functions

Abstract

The trigonometric and Hermite-Gaussian basis functions for determining the modal characteristics of inhomogeneous optical waveguides by means of the Galerkin's method are presented and analyzed. The results obtained with each set of basis functions for mode spectra and field distributions are compared with other exact and approximate methods. The merits and problems arising with each set of basis functions are discussed.     

Enhanced photocatalytic performance of TiO2 nanowires by substituting noble metal particles with reduced graphene oxide

Noble metal particles have been embedded in semiconductors to improve photocatalysis efficiently, but the high cost made this approach difficult to apply widely in industry. Herein titanium dioxide/reduced graphene oxide (TiO₂/rGO) nanowires in a core-shell structure were prepared. The physicochemical properties and photocatalytic performance of the specimen were characterized in comparison with TiO₂ and TiO₂/Pt nanowires. The rGO layer and Pt nanoparticles increased chemical states of the components, reduced bandgap energy of the nanowires, enhanced visible light absorption, improved conductance and capacitance significantly. The methylene blue as catalyzed by TiO₂/Pt and TiO₂/rGO nanowires was degraded to 7.9% and 8.4% in an hour, but retained 25.7% by the TiO₂ nanowires. The properties and function of TiO₂/rGO nanowires were close to those of TiO₂/Pt nanowires, while the rGO price was much lower than that of Pt, which was of great significance for the photocatalytic application of TiO₂ heterojunction materials in industry.     

Power‐ and polarization dependence of two photon luminescence of single CdSe nanowires with tightly focused cylindrical vector beams of ultrashort laser pulses

We investigate two‐photon luminescence (TPL) of single CdSe nanowires (NWs) excited by tightly focused cylindrical vector beams of 150 fs pulses, achieving an optical resolution better than λ/4. Comparing the TPL images recorded by scanning the nanowires through the focal fields created either by a radially or an azimuthally polarized vector beam we observe a distinct anisotropic excitation efficiency which depends on the orientation of the electric field component with respect to the nanowire. The excitation anisotropy of the linear photoluminescence (PL) and TPL can directly be derived from the respective image patterns. Our results show that the highest TPL signal from a single NW is detected when the electric excitation field is parallel to the long axis of the NW, i.e. about one order of magnitude stronger than that excited along the short axis. We attribute the TPL to an emission mechanism based on the recombination of free carriers, which exhibits a fourth order excitation power dependence in low power regime and a second order power dependence in high power regime.

     

Consistency and variations in optical characteristics of four-layer polymeric waveguides

In the present work, four-layer polymeric waveguides have been fabricated and characterized by interchanging the film layer. Using polyvinyl alcohol (PVA) and styrene acrylonitrile (SAN) polymer it has been shown that polymers do not loose their own property in multilayer structure and better efficiency can be achieved in the form of low propagation losses and mode filtration. Due to high contrast of PVA and SAN films, a broad spectrum of refractive index in four-layer structure has been achieved.     

梯形截面硅脊形波导的模式特性及其等离子体色散效应

本文运用有效折射率法和WKB法对具有梯形状截面的硅脊形波导的模式特性作了分析,导出了TE模和TM模的模方程以及截止方程,并运用一阶微扰法对因波导顶部注入载流子而引起的模式调制的大小作了数值估计。其结果可为全硅光波导器件的研制提供指导。     

Electrical characteristics and optoelectronic properties of metal--semiconductor--metal structure with zinc oxide nanowires across Au electrodes

This paper reports on a method of assembling semiconducting ZnO nanowires onto a pair of Au electrodes to construct a metal--semiconductor metal （MSM） structure by dieleetrophoresis and studying on its electrical characteristics by using current-voltage （Ⅰ - Ⅴ） measurements. An electronic model with two back to back Sehottky diodes in series with a semiconductor of nanowires was established to study the electrical transport of the MSM structures. By fitting the measured Ⅰ - Ⅴ characteristics using the proposed model, the parameters of the Schottky contacts and the resistance of nanowires could be acquired. The photoelectric properties of the MSM structures were also investigated by analysing the measurements of the electrical transports under various light intensities. The deduced results demonstrate that ZnO nanowires and their Schottky contacts with Au electrodes both contribute to photosensitivity and the MSM structures with ZnO nanowires are potentially applicable for photonic devices.     

Secondary electron emission characteristics of graphene films with copper substrate

For modern and future circular accelerators, especially high-intensity proton synchrotrons or colliders, the electron cloud effect is a key issue. So, in order to reduce the electron cloud effect, exploring very low secondary electron yield (SEY) material or coating used in vacuum tubes becomes necessary. In this article, we studied the SEY characteristics of graphene films with different thicknesses which were deposited on copper substrates using chemical vapor deposition. The SEY tests were done at temperatures of 25℃ and vacuum pressure of (2-6)×10^-9 torr. The properties of the deposited graphene films were investigated by X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. The SEY curves show that the number of graphene layers has a great effect on the SEY of graphene films. The maximum SEY of graphene films decreases with the increase of the number of layers. The maximum SEY of 6-8 layers of graphene film is 1.25. These results have a great significance for next-generation particle accelerators.     

光脉冲在微环耦合谐振光波导中传输线性特性的数值仿真

利用传输矩阵法得到了微环耦合谐振光波导色散关系的一般表达式,并由色散关系出发讨论了光脉冲在微环耦合谐振光波导中传输时的线性特性,包括带宽、群速度、色散和线性相位变化,这些特性对微环耦合谐振光波导在光通信和光传感领域的应用有重要意义. 关键词： 微环光波导 光脉冲线性特性 传输矩阵法 数值仿真     

基于模式匹配法的平面波导光场分布分析

采用模式匹配法,对一个三段平面光波导结构的光场分布进行了计算机模拟,数值模拟结果正确地反映了一个宽波导与一个窄波导衔接后,在两者交接处的光波场泄露.该方法可用于分析结构更加复杂的光波导器件.     

Fractal dimension study of polaron effects in cylindrical GaAs/AlxGa1−xAs core–shell nanowires

Polaron effects in cylindrical GaAs/Al_xGa_1−xAs core–shell nanowires are studied by applying the fractal dimension method. In this paper, the polaron properties of GaAs/AlxGa1-xAs core–shell nanowires with different core radii and aluminum concentrations are discussed. The polaron binding energy, polaron mass shift, and fractal dimension parameter are numerically determined as functions of shell width. The calculation results reveal that the binding energy and mass shift of the polaron first increase and then decrease as the shell width increases. A maximum value appears at a certain shell width for different aluminum concentrations and a given core radius. By using the fractal dimension method, polaron problems in cylindrical GaAs/Al_xGa_1-xAs core–shell nanowires are solved in a simple manner that avoids complex and lengthy calculations.