Two-period model for calculation of level populations in subbands of multi-period quantum-cascade superlattice structures

For two periods of quantum-cascade laser structures, we propose a system of closed balance equations that make it possible to calculate the occupancy of the energy levels, the quasi-Fermi levels, and also the injection current density taking into account different charge carrier scattering mechanisms. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 74, No. 6, pp. 801–804, November–December, 2007.     

Application of He's homotopy perturbation method to boundary layer flow and convection heat transfer over a flat plate

In this Letter, the problem of forced convection over a horizontal flat plate is presented and the homotopy perturbation method (HPM) is employed to compute an approximation to the solution of the system of nonlinear differential equations governing on the problem. It has been attempted to show the capabilities and wide-range applications of the homotopy perturbation method in comparison with the previous ones in solving heat transfer problems. The obtained solutions, in comparison with the exact solutions admit a remarkable accuracy. A clear conclusion can be drawn from the numerical results that the HPM provides highly accurate numerical solutions for nonlinear differential equations.     

Contagion effects in a chartist-fundamentalist model with time delays

In this paper two models of speculative markets are developed to study the effects of feedback mechanisms in financial markets. In the first model, a crash market model couples a linear chartist-fundamentalist model with time delays with a log-periodic market index I(t) through direct coupling. Numerical solutions to the model show that asset prices exhibit significant persistence as a result of the coupling to the log-periodic market index. An extension to include endogenous wealth dynamics shows that the chartists benefit from the persistent dynamics induced by the coupling. The second model is a two-asset model represented by a 2-dimensional delay-differential equation. Asset one price exhibits limit cycle dynamics while in the second market asset prices follow stable damped oscillations. The markets are coupled through a diffusive coupling term. Solutions to the coupled model show that the dynamics of asset two changes fundamentally with the price now exhibiting a limit cycle. The stable converging dynamics is replaced with limit cycle oscillations around the fundamental.     

Dynamic instabilities in scalar neural field equations with space-dependent delays

In this paper we consider a class of scalar integral equations with a form of space-dependent delay. These nonlocal models arise naturally when modelling neural tissue with active axons and passive dendrites. Such systems are known to support a dynamic (oscillatory) Turing instability of the homogeneous steady state. In this paper we develop a weakly nonlinear analysis of the travelling and standing waves that form beyond the point of instability. The appropriate amplitude equations are found to be the coupled mean-field Ginzburg-Landau equations describing a Turing-Hopf bifurcation with modulation group velocity of O(1). Importantly we are able to obtain the coefficients of terms in the amplitude equations in terms of integral transforms of the spatio-temporal kernels defining the neural field equation of interest. Indeed our results cover not only models with axonal or dendritic delays but also those which are described by a more general distribution of delayed spatio-temporal interactions. We illustrate the predictive power of this form of analysis with comparison against direct numerical simulations, paying particular attention to the competition between standing and travelling waves and the onset of Benjamin-Feir instabilities.     

Unconditionally Stable Complex Envelope Wave Equation PML Algorithm for Band Limited FDTD Simulations

Unconditionally stable complex envelope (CE) perfectly matched layer (PML) absorbing boundary conditions (ABCs) are presented for truncating the scalar wave-equation finite difference time domain (WE-FDTD) grids. The formulations are based on incorporating the alternating direction implicit (ADI) scheme into the CE FDTD implementations of the scalar wave-equation derived in the PML region at the domain boundaries. Numerical example carried out in two dimensional domain shows that the proposed formulations are more accurate than the classical ADI scalar wave equation PML formulations when it is used for modelling band limited electromagnetic applications.     

基于表面等等离波子的新型Y分支波导

新型高效的纳米光波导器件的研制纳米集成光学的核心技术之一。Y分支波导作为最基本的分光和光路连接元件是纳米光学器件设计与制备的基础。运用时域有限差分(FDTD)法,模拟计算了基于表面等等离波子(SPP)的纳米Y分支波导的传输特性。结果表明,该新型Y分支波导在光通信波段可以实现大角度的分光功能,且在180°分支情况下,传输效率仍高达92.8%以上。另外,该波导还具有导波性能良好、对分叉处间隙缺陷大小不敏感及制作容差较大和器件尺寸在纳米量级等特点。对该新型光波导器件的研究为未来纳米集成光学器件的研制和应用有一定的指导意义。     

近场头相关传输函数的测量与分析

设计了测量距离可调整的头相关传输函数的实验测量方法，并采用人工头进行近场头相关传输函数的测量，建立了高空间分辨率的近场头相关传输函数数据库，为进一步开展双耳听觉的研究和虚拟听觉的应用提供了数据基础。根据实验数据，初步分析了远、近场情况下，距离、仰角、方位角等参量对头相关传输函数的影响规律。     

三能级光纤激光器速率方程组的阈值解析解

 在考虑了泵浦损耗及激光损耗的情况下，解析求解了阈值泵浦下的三能级光纤激光器速率方程组，获得了泵浦功率阈值的显函数解析表达式，泵浦功率沿光纤变化的隐函数表达式，以及用功率变化表示的各个能级的粒子数密度随位置变化的显函数表达式。并利用这些表达式对光纤激光器的阈值特性进行了研究，研究表明：泵浦功率阈值随泵浦损耗系数增大而增大；激光上能级粒子数密度随归一化位置呈下降趋势，而下能级粒子数密度的变化与此相反；泵浦功率阈值越大(光纤长度越长),净增益系数随归一位置下降就越快。所获得的结果适用于单包层和双包层光纤激光器。     

双间隙输出腔开放腔的高频特性分析

 建立了S波段相对论速调管放大器双间隙输出腔开放腔的3维模型。采用时域有限差分法，通过监测激励电流源的响应计算了该双间隙输出腔的谐振频率、有载Q值、场分布以及特性阻抗，并分析了腔体结构尺寸对谐振频率、有载Q值和特性阻抗的影响。研究表明：腔体半径对开放腔的谐振频率影响很大，耦合孔尺寸对腔体谐振频率的影响较小；随着耦合孔张角增加，有载Q值逐渐减小；随着腔体半径增大、间隙的减小，腔体特性阻抗降低。研究结果可为S波段强流相对论速调管放大器双间隙输出腔的设计提供理论依据。     

(2 + 1)D exotic Newton-Hooke symmetry, duality and projective phase

A particle system with a (2 + 1)D exotic Newton-Hooke symmetry is constructed by the method of nonlinear realization. It has three essentially different phases depending on the values of the two central charges. The subcritical and supercritical phases (describing 2D isotropic ordinary and exotic oscillators) are separated by the critical phase (one-mode oscillator), and are related by a duality transformation. In the flat limit, the system transforms into a free Galilean exotic particle on the noncommutative plane. The wave equations carrying projective representations of the exotic Newton-Hooke symmetry are constructed.