Broadband noise reduction by circular multi-cavity mufflers operating in multimodal propagation conditions

In this study, sound propagation through a circular duct with non-locally lining is investigated both numerically and experimentally. The liner concept is based on perforated screens backed by air cavities. Dimensions of the cavity are chosen to be of the order or bigger than the wavelength so acoustic waves within the liner can propagate parallel to the duct surface. This gives rise to complex scattering mechanisms among duct modes which renders the muffler more effective over a broader frequency range. This work emanates from the Cleansky European HEXENOR project which aim is to identify the best multi-cavity muffler configuration for reduction of exhaust noise from helicopter turboshaft engines. Here, design parameters are the cavity dimensions in both longitudinal and azimuthal directions. The best cavity configuration must in addition fit weight specifications which implies that the number of walls separating each cavity should be chosen as small as possible. To achieve these objectives, the scattering matrix of the lined duct section is obtained experimentally for two specific muffler configurations operating in multimodal propagation conditions. The good agreement with numerical predictions serves to validate the perforate plate impedance model used in our calculation. Finally, given an incident acoustic pressure which is representative of typical combustion noise spectrum, the best cavity configuration achieving the maximum overall acoustic Transmission Loss is selected numerically. The study also illustrates how the acoustic performances are dependent on the nature of the incident field.     

冷原子系综中偏振关联光子对的产生和测量

我们在~(87)Rb冷原子系综中进行了关联光子对的产生和测量。通过自发拉曼散射过程,产生了一个Stokes光子和一个原子自旋波激发。利用电磁感应透明效应将存储在原子系综中的自旋波转化为一个anti-Stokes光子。在两相互正交的偏振基下,测量了Stokes光子和anti-Stokes光子之间的交叉关联函数。实验结果表明,交叉关联函数g~(2)达到~75,表现了强的非经典关联。基于现场可编程门阵列(FPGA)的自编程的多通道符合器被用于光子脉冲信号的采集和逻辑处理,大大提高了偏振关联光子对的产生和测量速度,为以后量子中继技术中的高效信息处理提供了重要基础。     

Global well-posedness and scattering for the energy-critical, defocusing Hartree equation for radial data

We consider the defocusing, -critical Hartree equation for the radial data in all dimensions (n5). We show the global well-posedness and scattering results in the energy space. The new ingredient in this paper is that we first take advantage of the term in the localized Morawetz identity to rule out the possibility of energy concentration, instead of the classical Morawetz estimate dependent of the nonlinearity.     

生物组织中的光分布研究

本文利用蒙特卡罗方法模拟了单层生物组织中光的传输与分布情况,给出了生物组织中的光子数分布随着g因子、层厚d、吸收系数以及散射系数的变化而变化的关系,分析了其变化的原因     

Magnetic impurities in gapless Fermi systems: perturbation theory

We consider a symmetric Anderson impurity model with a soft-gap hybridization vanishing at the Fermi level, with r>0. Three facets of the problem are examined. First the non-interacting limit, which despite its simplicity contains much physics relevant to the U>0case: it exhibits both strong coupling (SC) states (for r<1) and local moment states (for r>1), with characteristic signatures in both spectral properties and thermodynamic functions. Second, we establish general conditions upon the interaction self-energy for the occurence of a SC state for U>0. This leads to a pinning theorem, whereby the modified spectral function is pinned at the Fermi level for any U where a SC state obtains; it generalizes to arbitrary r the pinning condition upon familiar in the normal r=0 Anderson model. Finally, we consider explicitly spectral functions at the simplest level: second order perturbation theory in U, which we conclude is applicable for and r>1 but not for . Characteristic spectral features observed in numerical renormalization group calculations are thereby recovered, for both SC and LM phases; and for the SC state the modified spectral functions are found to contain a generalized Abrikosov-Suhl resonance exhibiting a characteristic low-energy Kondo scale with increasing interaction strength. Received 26 August 1999     

聚乙烯-碘化铯中子阴极的最佳化

本文的中子阴级由聚乙烯、导电基底(Al)和碘化铯二次电子发射体组成.文中建立了一个此中子阴级的新的物理模型,并采用Monte Carlo方法模拟了中子同阴极作用产生低能二次电子的物理过程.模拟得到的出射二次电子能谱和产额与实验结果和理论分析符合得较好.我们还得到了最大探测效率时各中子阴极组分的最佳厚度.     

Restoring entanglement in atomic collisions: A gedanken experiment

A new type of collision experiments is discussed, where observations of two successive collisions of the same pair of particles would be possible. When such technology is available, a surprising restoring of entanglement, normally considered broken in usual collision experiments, could be observed. As an illustration the collision partners He⁺ and He⁺⁺ in a collision regime where the resonant charge transfer is dominating are considered. In the analysis it is shown that in such experiments, two spatially widely separated ion paths, corresponding in fact to two different charge states, would contribute coherently to the final amplitudes, describing which of the ions emerges as singly charged, i.e. which carries the single electron involved. The double collision experiments are not trivial, since their overall cross-sections are extremely small. Development of relevant experimental techniques will decide if the proposed phenomena remain in the field of gedanken experiments or enter the world of real experimental physics. Received 2 December 1999 and Received in final form 12 May 2000     

Nonlinear Optical Properties of Semiconductor-Doped Sol-Gel Waveguides

Thin silica-titania films doped with CdS and PbS nanocrystals have been prepared by the sol-gel route. Their nonlinear properties have been studied using the techniques of degenerated four-wave mixing and m-lines with picosecond (ps) and nanosecond laser pulses. Depending on wavelength, doping level, and laser pulse duration, high negative nonlinearity was found for CdS-doped (n₂ = -2 x 10^-8 cm² per kW) and PbS-doped films (-10^-10 to -2 x 10^-7 cm² per kW). The response time of the nonlinearity was below 35 ps. Saturation of the nonlinearity was observed. Straight, monomode channel waveguides have been fabricated on these films. The influence of MIE-scattering due to the nanoparticles is investigated.     

石英砂-聚乙烯二元混合体系的太赫兹光谱特性研究

太赫兹时域光谱技术目前逐渐应用于对岩石的研究中。在制备样品时通常需要将岩石磨碎后与粘合剂混合压片,岩石的含量、粒径等都会对测试结果造成影响。将自然界中常见的石英砂粉末（不同粒径）与聚乙烯（PE）微粒以不同的比例混合,通过压片的手段将其制成适用于太赫兹系统测试的样品,用以探究样品中石英砂的含量以及其粒径对实验结果的影响。首先研究石英砂含量对实验结果造成的影响。在保持样品中石英砂的粒径不变的情况下,发现不同石英砂含量样品的时域光谱图中,时间延迟以及峰值都呈现出非单调的变化趋势。为了探究出现这种现象的原因,对样品的折射率以及吸收系数做了进一步的分析。结果表明样品的折射率会随着样品中石英砂含量的增加而逐渐增大,通过适用于本实验的有效介质理论能够解释这一现象。样品对于太赫兹波段的吸收系数随着石英砂含量的增加呈现出先增大后减小的现象,并且在石英砂质量分数为60%时达到最大值。为了解释这一现象产生的原因,利用扫描电子显微镜对样品的微观形貌进行观察,发现随着石英砂含量的增加,在压制样品时PE颗粒破碎程度加剧,导致PE的粒径变小。根据米氏散射以及瑞利散射的原理,石英砂粒径不变而PE粒径减小,随之降低的散射强度与石英砂的吸收效应发生竞争,从而导致了吸收系数先增大后减小的现象。进一步研究了石英砂粒径对实验结果的影响,对不同粒径的石英砂样品进行测试,发现折射率不随石英砂粒径的变化而改变,但其吸收系数随着石英砂粒径的减小而逐渐减小。根据米氏散射,样品吸收系数的变化是由于散射强度随石英砂粒径的减小而逐渐减小。研究表明,样品中石英砂的含量和粒径都会对实验结果产生影响。石英砂粒径相同时,样品的吸收系数随石英砂含量的增加呈现先增大后减小的趋势,样品的折射率随石英砂含量的增加而逐渐增大;石英砂含量相同时,样品的吸收系数随石英砂粒径的减小而逐渐减小,而样品的折射率基本不变。这一结论对矿物样品的制备以及对混合物的实验结果分析有一定程度的指导意义。     

Diffraction of a mode close to its cut-off by a transversal screen in a planar waveguide

The problem of diffraction of a waveguide mode by a thin Neumann screen is considered. The incident mode is assumed to have frequency close to the cut-off. The problem is reduced to a propagation problem on a branched surface and then is considered in the parabolic approximation. Using the embedding formula approach, the reflection and transmission coefficients are expressed through the directivities of the edge Green’s function of the propagation problem. The asymptotics of the directivities of the edge Green’s functions are constructed for the case of small gaps between the screen and the walls of the waveguide. As the result, the reflection and transmission coefficients are found. The validity of known asymptotics of these coefficients is studied.