Sharpened Aharanov-Bohm oscillations near resonance in a balanced ring with double quantum dots

In a balanced Aharonov-Bohm ring with an identical quantum dot (QD) in each arm, the transmission amplitude near resonance consists of a zero and pole which can be made to nearly or exactly overlap in the complex-energy plane. This presents a sensitively balanced region of parameter space in which arbitrarily sharp transmission resonance spikes manifest. Analytical results based on a tight-binding solution of the Schrödinger Equation for this system demonstrate the extreme flux sensitivity of the balanced ring in the vicinity of the resonance near the quasi-bound state energy of the QD’s. This effect may have practical applications as a sensitive monitor of magnetic flux.     

Control of Fano resonances and phase of a multi-terminal Aharanov-Bohm ring with three embedded quantum dots

We study a four-terminal AB-ring with three embedded quantum dots in one arm. A tight-binding model is employed to analytically obtain the transmission through the system. Results show that the magnitude and sharpness of the resonance phase-jumps diminish as a function of the degree of coupling to the extra terminals. The Fano resonance zeros become complex with a non-zero coupling matrix element to the extra terminals. The zeros can be returned to the real energy axis by increasing the flux for values of the coupling less than a critical value. A simple analytical model of the Fano resonances shows naturally how the phase transition across the resonance peak will soften and diminish in the case where the transmission zero energy is no longer real, but complex.     

Theory of GaN Quantum Dots for Optical Applications

The optical properties of III-N wurtzite heterostructures are dominated by the built-in polarization potential. We first review the dependence of III-N bulk valence band structure on strain and the key factors determining the polarization vector in polar and nonpolar quantum wells, including electromechanical effects. We then present a surface integral technique to determine the built-in potential in quantum dots (QDs) of arbitrary shape. We show for polar QDs how the polarization potential spatially separates electrons and holes vertically but confines them laterally, causing the radiative recombination rate to decrease rapidly with increasing dot height and a strong blueshift with increasing carrier density. Finally, we show that although the polarization potential can be much reduced in nonpolar GaN/AlN QDs, it is likely to remain significant in nonpolar InN/GaN QD structures.      

基于跑道型微环谐振腔的Fano共振器件设计及传感特性研究

与传统洛伦兹线形相比,具有不对称线形的Fano共振光谱有更高的光谱分辨率,尤其适合传感应用。通过在硅基总线波导与跑道微环谐振腔耦合区域引入两个空气孔使得连续光谱产生突变相移,并与跑道微环谐振腔耦合形成Fano共振,且在较宽谱段范围内的每个共振峰都表现为非对称线形。通过仿真优化耦合间距以及空气孔与耦合区域中心的偏移量,获得最大光谱分辨率为312.05 dB/nm,消光比为53.09 dB的硅基Fano共振器件。在模仿被液体的折射率变化范围为1.33～1.332的条件下,仿真得到折射率传感灵敏度为125 nm/RIU。仿真结果表明本论文提出的器件结构简单,尺寸紧凑且制造误差小,为硅光子器件应用到高灵度敏度的集成生化传感提供新思路。     

特高压复合绝缘子电场计算及基于神经网络遗传算法的均压环结构优化设计

为了满足超/特高压大容量输电技术的发展需求,研究复合绝缘子的电场分布,寻找合理的电场优化措施已成为影响输电线路安全运行的关键因素。利用有限元法(FEM)建立了工程实际应用情况下特高压(UHV)交流输电线路复合绝缘子的单串、双串和V串电场计算模型,在考虑横担、分裂导线和均压环及其金具连接情况下,对复合绝缘子的电位和沿面电场分布进行了计算。针对特高压复合绝缘子电位分布极不均匀,且最大沿面电场强度大大超过电晕起始场强的情况,分析了不同的均压环配合方式对复合绝缘子沿面电场分布的影响。针对单串绝缘子下的计算结果,引入神经网络遗传算法(BP-GA),对均压环的结构参数进行了优化设计,利用BP神经网络的高度非线性映射能力直接计算遗传算法的适应度函数值,解决了穷举法计算时间冗长的问题。计算结果表明:加装均压环能够大大降低最大沿面电场强度,明显改善复合绝缘子沿面电位和电场的分布;通过遗传算法优化后的均压环结构参数,能使均压环表面场强和绝缘子沿面场强均小于电晕起始场强,为特高压绝缘子均压环的选取提供了一种可靠、实用的设计方法,可为我国特高压输电线路外绝缘优化设计提供新的参考。     

Exciton relaxation and dephasing in quantum-dot amplifiers from room to cryogenic temperature

We present an extensive experimental study of the exciton relaxation and dephasing in InGaAs quantum dots (QDs) in the temperature range from 10 K to 295 K. The QDs are embedded in the active region of an electrically pumped semiconductor optical amplifier. Ultrafast four-wave mixing and differential transmission spectroscopy on the dot ground-state transition are performed with a sensitive heterodyne detection technique. The importance of the population relaxation dynamics to the dephasing is determined as a function of injection current and temperature. Above 150 K dephasing processes much faster than the population relaxation are present, due to both carrier-phonon scattering and Coulomb interaction with the injected carriers. Only at low temperatures (<30 K) does population relaxation of multiexcitons in the gain regime fully determine the dephasing.     

Manipulating of Resonances in Conductance of an Electron Waveguide with Anti-Dots

A new scenario of quantum interference in open systems when a decaying (quasi-bound) state can interfere with the continuum is investigated. The resonant perturbation theory and numerical methods are applied to treat a scattering problem in an electron waveguide. Specifically, we use the Kapura and Peierls (KP) approach to extract resonances. In the waveguide with two and three anti-dots, we observe an interesting phenomenon: the interference between one subband of the quasi-bound states and background second subband, and the interference of resonant group states of different subbands. Our results show that the zero of Fano resonance, arising from the interference between the quasi-bound states and the continuum, is generally placed in the complex plane.     

Electronic and excitonic processes in multilayer organic light emitting devices incorporating PbSe quantum dots

In this work we investigate the operation mechanism of hybrid organic/inorganic quantum dot light emitting devices (QD-LEDs). We employ a numerical method previously established to describe current-voltage characteristics, spatial distributions of charge, electric field, and recombination rate in organic light emitting devices (OLEDs). The numerical solution of the continuity and Poisson equations have been extended to treat internal organic/Quantum Dot (QD) interfaces, recombination processes in the polymer matrix and in the QDs, and the charge acquired by the QDs. The contact boundary condition is taken to be Schottky contact boundary condition. Also, we consider the exciton formation and diffusion processes. The simulation results trend and experimental data are in good agreement.     

Numerical Calculation of Electronic Structure for Three-Dimensional Nanoscale Semiconductor Quantum Dots and Rings

In this paper the electronic structure of nanoscale ellipsoid-torus-shaped semiconductor quantum dot and quantum ring is investigated of utilizing a unified model. This three-dimensional model considers the effective one-band Hamiltonian, the position- and energy-dependent effective mass approximation and Landé factor, the finite hard wall confinement potential, and the Ben Daniel-Duke boundary conditions. It is solved numerically without any fitting parameters by using a computationally cost effective nonlinear iterative method. It is found that the penetration of magnetic fields into non-simply connected topology of structures leads to substantial difference in the transition energy between InAs/GaAs quantum dot and quantum ring. The quantum ring exhibits non-periodical electron-hole transition energies when the magnetic field increases. Contrary to the one-dimensional periodical argument on the ring's energy spectra, our examination into the nanoscale semiconductor quantum ring agrees with the experimental result. The energy band gap of quantum dot is an increasing function of the magnetic field. For quantum rings the energy band gap oscillates non-periodically and the oscillation period is strongly controlled by the inner radius of structures. The magnetization of quantum ring not only jumps non-periodically but also saturates eventually when the magnetic field increases.     

Effects of spontaneous and piezoelectric polarization fields on the electronic and optical properties in GaN/AlN quantum dots: multimillion‐atom sp3d5s* tight‐binding simulations

GaN quantum dots (QDs) grown on AlN substrates are strong candidates for UV and near‐infrared applications. The wurtzite crystal symmetry in these dots induces internal fields arising from the following: (i) crystal atomicity;(ii) strained active region; (iii) piezoelectricity; and (iv) spontaneous polarization (pyroelectricity). Accurate modeling of electronic and optical properties of these QDs must capture the interplay of these atomistic and long‐range fields and the size quantization on an equal footing. In this work, single‐particle electronic structure and interband optical transition rates of a GaN/AlN QD grown along the c‐axis are studied using a coupled molecular mechanics‐atomistic 20‐band sp³d⁵s^* tight‐binding (VFF‐TB) framework. To calculate piezoelectricity, a recently reported model that takes into account both the linear and the nonlinear dependence of polarization on the strain tensors has been employed. The simulated GaN/AlN dot is realistically sized (containing ~3 million atoms) and of hexagonal disk shape having height and base length of 4.1 and 17 nm, respectively. It is found that, in contrast to the well‐studied InN/GaN systems, the pyroelectric potential in GaN/AlN dot is larger than the piezoelectric counterpart, and the effects of piezoelectric and pyroelectric fields add up. The internal fields result in a large redshift in the electronic states near the Brillouin zone center (known as quantum confined Stark effect), pronounced non‐degeneracy in the excited states, strongly suppressed optical transition, and anisotropic emission spectra. Copyright © 2014 John Wiley & Sons, Ltd.     

量子点发光材料在多色照明上的应用

半导体量子点作为一种最新型的荧光材料,与传统的荧光材料相比具有多种优势.由于量子局域效应,单一种类的量子点材料就能够通过改变其尺寸来产生不同颜色的单色光.本文简要描述了量子点材料的发光特性及其在照明光源器件中的应用前景.文中给出了CdSe量子点材料的制备方法和其发光特性,探讨了如何将不同尺寸的CdSe量子点复合在一起来获得多色照明器件.     

±800kV与±500kV换流站共用接地极时入地电流对极址附近电位分布的影响

±800 kV楚穗直流换流站与±500 kV兴安直流换流站所共用的鱼龙岭接地极其中一回线路检修,另一回线路单极大地回路运行时,极地附近10 km范围内的电位分布决定了检修线路的检修措施;利用CDEGS软件计算了接地极附近电位分布,并与该极址投运前系统调试期间的测量结果进行了对比。结果表明：单回线路检修,另一回线路单极大地回路运行时,接地极内外环上方呈地表电位峰值,且接地极内环上方地电位高于外环,接地极外环地电位衰减迅速;跨步电压也在极环处达到峰值,但小于设计值,接触电压按标准采取措施后可满足限值要求;实测结果与计算结果基本一致;因土壤电阻率较低使得跨步电压偏高的部分区域,检修作业时应采取相应的防护措施。     

Optical properties of quantum dots versus quantum antidots: Effects of hydrostatic pressure and temperature

The effects of hydrostatic pressure and temperature on the linear, nonlinear and total absorption coefficients (ACs) of a hydrogenic impurity in the center of spherical quantum dot (QD) and quantum antidot (QAD) have been investigated. The comparative approach is used for presenting the results of both models. Our numerical results indicate that for QD nano-systems, by increasing the pressure, the resonance peak positions (RPPs) of ACs shift towards higher energies, while for QAD nano-systems, RPPs of ACs approximately remain unchanged. Furthermore, the larger pressure leads to the smaller height of resonance peak in both models. Also, our results show that the temperature increasing imposes the opposite effect on RPPs than the pressure increasing to the both models.     

Voltage distribution analysis and grading ring design for porcelain insulators on 750‐kV compact transmission lines

On a compact transmission line, there are three phases arranged in one tower window. They form an inverted triangle with a small air gap, having no shielding between each other. This characteristic results in mutual effects among the three phases, which cause higher voltages and electric field intensities on the porcelain insulators and fittings than the ordinary lines. In this paper, a three‐dimensional finite element method (FEM) is presented to calculate the potential and electric field distributions of the porcelain insulators on a 750‐kV compact double‐circuit transmission line. Considering the mutual effects among the three phases, the structural parameters of the grading ring are optimized to reduce the voltages and electric field intensities on the insulators. The results show that the significant mutual effects among the three phases need to be taken into account in the design and testing of the insulators and fittings. In addition, structural parameters of the optimized grading ring are proposed on the basis of the above‐mentioned analysis. The comparison between the numerical calculations and the test results shows the accuracy of the FEM. Thus, in the approval test, it might be more reasonable and efficient to determine the correction factors through analyzing the calculations of the actual line and test arrangement. © 2012 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.     

Microscopic Description of Nanostructures Grown on (N11) Surfaces

We report on a theoretical study of GaAs/InGaAs based nanostructures grown along the [N11] direction. The elastic deformations of the structures were calculated by means of the continuum elasticity theory, taking into account commensurability constraints at the interfaces. The strained atomic positions were derived, as well as the strain induced piezoelectric polarizations and electric fields. These data were used as an input for the calculation of the fundamental electronic transitions of our systems within the empirical tight-binding approach. These results are compared with the envelope function methods. We applied our approach to a (211) oriented InAs quantum dot embedded in a GaAs matrix, and to a (311) oriented InGaAs quantum wire, embedded in AlGaAs barriers. In both cases, we obtained a non-symmetric elastic deformation due to the lower symmetry of (N11)-oriented structures. Moreover, the atomic displacements and the strain induced piezoelectric potential induce a separation of the hole and electron wave functions, which are shifted from the dot center.     

Progress in GaN-based quantum dots for optoelectronics applications

Our recent progress in GaN-based quantum dots (QDs) for optoelectronics application is discussed. First, we discussed an impact of the use of GaN-based QDs on semiconductor lasers, showing theoretically that reduction of threshold current by using the QDs in GaN-based lasers is much more effective compared to those in GaAs-based or InP-based lasers. Then discussed are our growth technology including self-assembling growth of InGaN QDs on sapphire substrates by atmospheric-pressure metalorganic chemical vapor deposition. Using the self-assembling growth technique, we have succeeded in obtaining lasing action in an edge-emitting laser structure with the InGaN QDs embedded in the active layer under optical excitation with the emission wavelength of 410 nm. Toward UV light wavelength emission, we have recently established self-assembled GaN QDs of high quality and high density under very low V-III ratio. We clearly observed two photoluminescence peaks from both the QDs and the wetting layer at room temperature, which clearly shows the nanostructures are formed with the Stranski-Krastanow growth mode.     

GaN基多量子阱蓝色发光二极管的实验与理论分析

本研究采用量子点模型对蓝色InGaN/GaN多量子阱发光二极管电致发光光谱进行考察,并和实验测量结果进行了比对。结果发现,利用量子点模型计算出的自发辐射发光峰位与实验得出的电致发光的峰位很好地吻合,表明了在多量子阱发光二极管中由于InN和GaN相分离而形成的富In类量子点结构,主导着InGaN基发光二极管发光波长,体现了InGaN基发光二极管量子点发光的本质。同时,基于量子点模型的理论,本文讨论了以合适组分的四元A lInGaN材料取代传统的GaN材料作为量子阱垒层对发光二极管发光特性的影响。     

偏远地区风电场并网系统串联谐振分析

针对偏远风电场并网时易被忽略的并网系统串联谐振问题,提出结合回路元件灵敏度的串联谐振模态分析法进行研究。虽然并联谐振在电力系统中所占比例较大,但串联谐振的危害亦不可忽略。偏远风电场因其送出线较长,并网结构薄弱,输电线路分布电容所带来的影响不容忽视。建立线路分布参数模型,并分析其阻抗特性,基于已建立的风电场并网模型,采用串联谐振模态分析法分析系统串联谐振现象,准确判断串联谐振频率的同时,给出串联回路电气元件的模态灵敏度值。最后,以某风电场为实例建立仿真模型,仿真结果验证了所采用分析方法的可行性。通过元件灵敏度值的计算,进一步阐述线路长度的改变对风电场并网系统串联谐振频率和幅值的影响规律,为风电场实际规划建设提供理论指导。     

New Efficient Three-Party Quantum Key Distribution Protocols

Most of the existing quantum key distribution protocols (QKDPs) assume that every communicating party is equipped with quantum devices (QDs), e.g., the qubit generating machine, or the quantum memory, or the qubit measuring machine. However, in the practical situation, these QDs are expensive, and a center may be the only party that affords to own these devices. Though Phoenix first realize the idea, their scheme only has 13% of qubit efficiency. This paper proposes two three-party QKDPs. The first QKDP with an honest center allows communicants to share a session key by only performing unitary operations. Moreover, considering the trustworthiness of the center, this paper further proposes the second QKDP without the assumption of a trusted center. Besides, the proposed three-party QKDPs provide better qubit efficiency than the other QKDPs due to the use of the quantum memory and the use of one-way hash function for the eavesdropping check, and the security of proposed QKDPs is shown in the “sequence-of-games approach.”      

Contactless magnetic gear for robot control application

This paper describes the application of a magnetic gear to a robot by fulfilling the essential requirements for a robot control, which are velocity control, position control, and force control. A magnetic gear is a transmission device that realizes contactless torque transmission by applying a magnetic force. When using a magnetic gear, cogging torque and spring characteristics need to be considered. In this paper, we introduce an approximate model of cogging torque. This model is used for velocity control to attenuate the disturbance due to cogging torque. In the case of position control, the oscillations due to the spring effect of the magnetic attractive force become a problem. To reduce the adverse effect due to these oscillations, resonance ratio control is applied. We also propose to use a magnetic gear for realizing force sensorless bilateral control of teleoperation. Because of the frictionless transmission of a magnetic gear, the force sensorless estimation of a reaction force can be realized using a reaction force observer. © 2013 Wiley Periodicals, Inc. Electr Eng Jpn, 184(4): 32–41, 2013; Published online in Wiley Online Library ( wileyonlinelibrary.com ). DOI 10.1002/eej.22414