Active vibration control for structural-acoustic coupling system of a 3-D vehicle cabin model

This paper presents an active vibration control system for use with structural-acoustic coupling system using piezoelectric actuators and piezoelectric sensors. For modelling a complicated 3-D vehicle cabin model, the structural-acoustic coupling system is analyzed by combining the structural data from modal testing with the acoustic data from the finite element method. Through the structural-acoustic analysis program, the control plate and the control modes are selected, which are most effective for attenuating its noise. A robust LQG controller with two sensor signal filters is designed to remove the experimental problems such as the spillover effect due to uncontrolled modes. The robust LQG controller for the structural-acoustic coupling system can reduce the interior noise of the cavity as well as the structural vibration of the cabin.     

Easily coupled whispering gallery plasmons in dielectric nanospheres embedded in gold films

A new self-aligned robust method for coupling to whispering gallery modes (WGMs) of submicron microspheres utilizes their periodic arrangement without relying on nanopositioned external coupling devices. The microspheres are embedded in a nanostructured gold surface supporting delocalized plasmonic crystal modes that mediate the coupling, and can be tuned by the geometry. Detailed measurements of the angle- and orientation-dependent reflectivity reveal localized plasmonic WGMs whose energies scale with sphere diameter and agree closely with Mie calculations. Coupling between these plasmonic WGMs leads to mode splitting and the formation of plasmonic minibands of a controllable bandwidth.     

双层非线性耦合反应扩散系统中复杂Turing斑图

采用双层耦合的Brusselator模型, 研究了两个子系统非线性耦合时Turing 模对斑图的影响, 发现两子系统Turing 模的波数比和耦合系数的大小对斑图的形成起着重要作用. 模拟结果表明: 斑图类型随波数比值的增加, 从简单斑图发展到复杂斑图; 非线性耦合项系数在0–0.1时, 系统1中短波模在系统2失稳模的影响下不仅可形成简单六边形、四边形和条纹斑图, 两模共振耦合还可以形成蜂窝六边形、超六边形和复杂的黑眼斑图等超点阵图形, 首次在一定范围内调整控制参量观察到由简单正四边形向超六边形斑图的转化过程; 耦合系数在0.1–1时, 系统1中短波模与系统2失稳模未发生共振耦合仅观察到与系统2相同形状的简单六边形、四边形和条纹斑图. 关键词： Brusselator模型 非线性耦合 Turing模     

双芯光子晶体光纤中的模式干涉

应用全矢量超格子叠加模型分析了双芯光子晶体光纤(PCF)的模式特征.双芯PCF的基模和二次模分别由偏振方向不同的两个偶模和两个奇模组成，讨论了这四个模式的模式电场奇偶性.基于对双芯PCF模式电场奇偶性的认识，讨论了光纤中的矢量模式干涉问题.分析表明，不同偏振态模式的干涉不会引起芯间的功率转移，双芯PCF的芯间功率耦合是由相同偏振模式的干涉引起的.在相同偏振模式干涉过程中，光功率在两个芯子中呈余弦振荡，光纤的两个芯子一般存在约π／2的相位差.还讨论了两个不同偏振方向的耦合系数与波长的关系. 关键词： 光子晶体光纤 双芯光纤 模式 干涉 耦合     

Counterdirectional mode coupling in ring resonators with QPM nonlinear crystals and effects on the characteristics of cw optical parametric oscillation

The effects of coupling between clockwise and counterclockwise modes in ring resonators due to back reflections from QPM gratings in nonlinear crystals are investigated. We demonstrate, using simulations, that the non-phase-matched back reflection due to typical imperfections of the QPM grating has a relevant magnitude and can give rise to the coupling. We developed an analytical model and evaluated parameters of a singly resonant OPO in the presence of this coupling, which indicates that the threshold pump power is doubled in the limit of strong coupling. We also evaluated the effect of the coupling with respect to resonant power enhancement for cavity-enhanced nonlinear frequency mixing applications, finding that the enhancement is reduced by up to a factor four. Experimental observations based on PPLN and orientation-patterned GaAs crystals and model predictions are in good agreement. We propose and demonstrate a method for active stabilization of ring resonators with mode coupling that relies on the dissimilarity of the coupled forward and backward modes.     

Collective excitations in fast ion conductor superlattices

Within the framework of the electromagnetic theory, the collective modes in the superlattice system composed of superionic conductors and ionic crystals are studied. The superionic conductor is described by the hydrodynamical model in which the anion cage is immersed in the cation liquid. The behavior of the modes are analysed in terms of the coupling strength between excitations pertaining to different layers. The coupling strength is controlled by varying the slab thicknesses. An interesting behavior in which the diffusion mode transforms to the relaxation mode when the coupling strength is varied from strong to weak is obtained. Also, the effect of the coupling strength on the acoustical and optical modes are shown.     

时间反演对称性破缺系统中的拓扑零能模

Su-Schreiffer-Heeger模型预测了在一维周期晶格的边缘处可能出现零维的拓扑零能模,其能量本征值总是出现在能隙的正中间.本文以半导体微腔阵列中光子和激子在强耦合情况下形成的准粒子为例,通过准粒子的自旋轨道耦合与Zeeman效应,研究了时间反演对称性破缺对拓扑零能模的影响.发现拓扑零能模的能量本征值可以随着自旋轨道耦合强度的变化在整个带隙内移动,自旋相反的模式移动方向相反;在二维微腔阵列中发现了沿着晶格边缘移动的拓扑零能模,提出了一维零能模的概念.由于时间反演对称性的破缺,这种一维拓扑零能模解除了在相反传输方向上的能级的简并,从而在传输过程中出现极强的绕过障碍物的能力.     

Wave propagation in uniaxial graded-index optical waveguides

Propagation of guided-modes in uniaxial optical waveguides is investigated. When the optical axis lies in the plane of the waveguide, a coupling between TE and TM modes takes place. Based on the coupled-mode theory, the propagating constants of the hybrid modes, the coupling coefficient and the interaction length are determined. A simple model is proposed to describe the guided mode behaviour. The results are illustrated for the case of Ti-diffused LiNbO₃ waveguide.     

Acoustic scattering by a sphere with a hemispherically split boundary condition

A general analytical model is developed for the scattering of sound by a sphere with a nonuniform impedance boundary condition that is divided into two uniformly distributed hemispheres. In addition to the overall solution for the time harmonic pressure, the analytical result gives insight into the modal contributions and coupling for different cases of source incidence and boundary impedance. Modal cross coupling is shown to exist between incoming and scattered wave modes of equi-order and nonequal degree when the degrees are opposite in parity (odd-even or even-odd coupling). This cross coupling is strongest between modes of adjacent degree, and decreases as the degrees become dissimilar. The overall magnitude of the cross coupling is dependent on the extent of the impedance mismatch between the two surface hemispheres. Simulation and discussion are given for several specific cases of source incidence and impedance (each hemisphere is given a different constant impedance value). These results are consistent with expectations from the scattering of sound by a sphere with a uniformly distributed surface boundary. The broad scattering characteristics of the hemispherically divided sphere are shown to be analogous to connecting the appropriate sectors from the corresponding uniformly distributed spheres.     

Reduction of Bragg Grating-Induced Coupling to Cladding Modes

We discuss fiber designs that have been suggested for the reduction of Bragg-grating-induced coupling to cladding modes. The discussion is based on a theoretical approach that includes the effect of asymmetry in the UV-induced index grating, made by UV-side writing. Experimental results from gratings in a depressed-cladding fiber are compared with simulations. The model gives good agreement with the measured transmission spectrum and accounts for the pronounced coupling to asymmetrical cladding modes, even when the grating is written with smallest possible blaze. The asymmetry causing this is accounted for by the unavoidable attenuation of the UV light. It is found for the considered fiber designs that a high numerical-aperture fiber increases the spectral separation between the Bragg resonance and the onset of cladding-mode losses. A depressed-cladding fiber reduces the coupling strength to the lower order cladding modes, and the UV-sensitive cladding design reduces the cladding-mode coupling loss. The analysis suggests that the UV-sensitive cladding design is the most effective in reducing the cladding-mode-coupling losses.     

Soliton contribution to correlations in a system of coupled chains

A model representing a two- or a three-dimensional array of classical harmonic chains withnonlinear coupling between them is investigated. Physically real systems to which this model applies are discussed. The model exhibits soliton-like nonlinear modes. The influence of these nonlinear modes on the static and the dynamic correlation functions is calculated by generalizing techniques developed for strictly one-dimensional systems. In the static correlation functions these modes lead to minor quantitative changes only. In certain dynamic correlation functions, however, a central peak is found to occur due to the nonlinear modes. The total weight and the width of this peak are calculated for a real spin system.     

Hybrid waveguide-surface plasmon polariton modes in a guided-mode resonance grating

We analyze the coupling between surface plasmon polaritons in a metal grating and the guided modes of a dielectric waveguide. Our model structure is a gold wire grating on a slab waveguide made of silicon nitride on silica wafer. The excitation of guided-mode resonances, surface plasmon polariton modes and hybrid waveguide-plasmon modes are observed in numerical simulations. Our experiments verify the existence of the predicted modes. These hybrid modes add significant degrees of freedom in designing structures for plasmonic applications.     

Elastic model for simulating ultrasonic inspection of smooth and rough defects

A model is presented for the scattering of ultrasonic waves from smooth and randomly rough defects. The model uses Kirchhoff theory and is elastic, such that mode-conversion between compressional and shear waves is included in the formulation. The model is designed to simulate ultrasonic non-destructive testing situations, by specifying the location and orientation of a defect within an isotropic material, together with transmitter and receiver locations on an inspection surface. Results are presented for the scattering of both monochromatic waves and of pulses. It is shown how small levels of roughness can affect echodynamic curves and how diffracted signals may become lost due to scatter from the rough faces of defects. It is also shown that the usual rules for coupling between waves of all three modes no longer hold when roughness is present. In particular, roughness leads to coupling between horizontally polarized shear (SH) waves and the other two wave modes. The model predictions are also compared with an earlier acoustic model, indicating the importance of mode-conversion effects when considering rough defects embedded within solids.     

单负材料组成一维光子晶体双量子阱结构的共振模

通过传输矩阵方法, 计算模拟了两种单负材料组成一维光子晶体双量子阱结构的透射谱. 研究发现: 由于双量子阱结构双阱之间的相互耦合作用, 共振模发生双重劈裂, 共振峰之间的距离可以通过调节双阱之间的耦合强度控制, 共振模的品质因子可以通过调节外部障碍光子晶体的周期数控制. 并且, 共振模受入射角和光偏振模式的影响都比较小, 适合全方向滤波. 当考虑两种单负材料不同损耗的影响时, 研究结果表明, 电损耗对低频处的共振模影响大, 而磁损耗对高频和低频处的共振模影响都比较大.     

模式耦合理论在圆周对称长周期光纤光栅建模中的应用

长周期光纤光栅是不同于光纤Bragg光栅的一种光纤光栅器件 ,根据模式耦合理论 ,长周期光纤光栅表现为前向传播的纤芯导模和同向的各阶次包层模式之间的耦合。分析研究了长周期光纤光栅轴向的模场变化。忽略轴向的模式耦合以及包层模式之间的相互耦合作用 ,并认为折射率指数的调制只存在于纤芯中 ,建立了简化的长周期光纤光栅数学模型。对圆周对称轴向均匀型长周期光纤光栅谱特性进行了仿真 ,其结果与实验结果基本吻合 ,表明了简化的数学模型的合理性     

Collective excitations in electron-hole bilayers

We report a combined analytic and molecular dynamics analysis of the collective mode spectrum of a bipolar (electron-hole) bilayer in the strong coupling classical limit. A robust, isotropic energy gap is identified in the out-of-phase spectra, generated by the combined effect of correlations and of the excitation of the bound dipoles. In the in-phase spectra we identify longitudinal and transverse acoustic modes wholly maintained by correlations. Strong nonlinear generation of higher harmonics of the fundamental dipole oscillation frequency and the transfer of harmonics between different modes is observed.     

Noise transmission from a curved panel into a cylindrical enclosure: analysis of structural acoustic coupling

Much of the research on sound transmission through the aircraft fuselage into the interior of aircraft has considered coupling of the entire cylinder to the acoustic modes of the enclosure. Yet, much of the work on structural acoustic control of sound radiation has focused on reducing sound radiation from individual panels into an acoustic space. Research by the authors seeks to bridge this gap by considering the transmission of sound from individual panels on the fuselage to the interior of the aircraft. As part of this research, an analytical model of a curved panel, with attached piezoelectric actuators, subjected to a static pressure load was previously developed. In the present work, the analytical model is extended to consider the coupling of a curved panel to the interior acoustics of a rigid-walled cylinder. Insight gained from an accurate analytical model of the dynamics of the noise transmission from the curved panels of the fuselage into the cylindrical enclosure of an aircraft is essential to the development of feedback control systems for the control of stochastic inputs, such as turbulent boundary layer excitation. The criteria for maximal structural acoustic coupling between the modes of the curved panel and the modes of the cylindrical enclosure are studied. For panels with aspect ratios typical of those found in aircraft, results indicate that predominately axial structural modes couple most efficiently to the acoustic modes of the enclosure. The effects of the position of the curved panel on the cylinder are also studied. Structural acoustic coupling is found to not be significantly affected by varying panel position. The impact of the findings of this study on structural acoustic control design is discussed.     

Modifications of acoustic modes and coupling due to a leaning wall in a rectangular cavity

Acoustic modes and the coupling characteristics of a rectangular-like cavity with a slight geometrical distortion introduced through a leaning wall are investigated in this paper. A pressure variation index is proposed to quantify the global changes in acoustic modes caused by the inclination of the wall. Effects on the coupling between acoustic modes and structural modes are investigated using coupling coefficients. Numerical results show a simple relationship between the distortion effect and the acoustic wavelength. The effect is most significant when the distortion approaches the half wavelength. Compared with a rectangular enclosure, the existence of the leaning wall gives rise to a much more effective coupling between the structure and the enclosure.     

Stationary entanglement between macroscopic mechanical oscillators

We show that the optomechanical coupling between an optical cavity mode and two movable cavity mirrors is able to entangle two different macroscopic oscillation modes of the mirrors. This continuous variable entanglement is maintained by the light bouncing between the mirrors and is robust against thermal noise. In fact, it could be experimentally demonstrated using present technology. Received 2 September 2002 / Received in final form 10 October 2002 Published online 7 January 2003     

Realizing Majorana fermion modes in the Kitaev model

We study the possibility to realize a Majorana zero mode that is robust and may be easily manipulated for braiding in quantum computing in the ground state of the Kitaev model in this work. To achieve this we first apply a uniform [111] magnetic field to the gapless Kitaev model and turn the Kitaev model to an effective p+ip topological superconductor of spinons. We then study possible vortex binding in such system to a topologically trivial spot in the ground state. We consider two cases in the system: one is a vacancy and the other is a fully polarized spin. We show that in both cases, the system binds a vortex with the defect and a robust Majorana zero mode in the ground state at a weak uniform [111] magnetic field. The distribution and asymptotic behavior of these Majorana zero modes are studied. The Majorana zero modes in both cases decay exponentially in space, and are robust against local perturbations and other Majorana zero modes far away, which makes them promising candidates for braiding in topological quantum computing.