Anomalous dispersion relations by symmetry breaking in axially uniform waveguides

We show that modes of axially uniform waveguides of arbitrary cross section can be made to have anomalous dispersion relations resulting from strong repulsion between two modes. When the axial wave vector k is 0, the two modes have different TE/TM symmetry and thus can be brought arbitrarily close to an accidental frequency degeneracy. For nonzero k, the symmetry is broken causing the modes to repel. When the modes are sufficiently close together this repulsion leads to unusual features such as extremely flattened dispersion relations, backward waves, zero group velocity for nonzero k, atypical divergence of the density of states, and nonzero group velocity at k=0.     

Membrane-based acoustic metamaterial with near-zero refractive index

We investigate a one-dimensional acoustic metamaterial with a refractive index of near zero(RINZ) using an array of very thin elastic membranes located along a narrow waveguide pipe. The characteristics of the effective density, refractive index, and phase velocity of the metamaterial indicate that, at the resonant frequency fm, the metamaterial has zero mass density and a phase transmission that is nearly uniform. We present a mechanism for dramatic acoustic energy squeezing and anomalous acoustic transmission by connecting the metamaterial to a normal waveguide with a larger cross-section. It is shown that at a specific frequency f_1, transmission enhancement and energy squeezing are achieved despite the strong geometrical mismatch between the metamaterial and the normal waveguide. Moreover, to confirm the energy transfer properties, the acoustic pressure distribution, acoustic wave reflection coefficient, and energy transmission coefficient are also calculated. These results prove that the RINZ metamaterial provides a new design method for acoustic energy squeezing,super coupling, wave front transformation, and acoustic wave filtering.     

Theory of waveguide light scattering in an integrated optical waveguide in the presence of noise

We develop a vector theory of waveguide monochromatic light scattering in an integrated optical waveguide with arbitrary three-dimensional irregularities in the presence of noise. The electrodynamical problem of laser radiation scattering in an irregular waveguide is solved by the method of coupled modes with the help of the theory of perturbations. The approximate solution of the inhomogeneous three-dimensional wave equation is derived by the method of modes and the method of Greens functions. The obtained Greens function is analyzed for the cases of propagating and evanescent modes of an irregular asymmetric optical waveguide. Analytical formulas for the near- and far-zone radiation fields are presented. A preliminary analysis of these formulas is given.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 48, No. 1, pp. 63–75, January 2005.     

Two-fluid equations for phonons without momentum

In the customary microscopic derivation of the two-fluid equations of liquid He II explicit use is made of the assumption that an elementary excitation of wave vector k carries a momentum ?k. In this paper it is shown that phonons in a liquid can be defined as carrying momentum ?k (Eulerian phonons) or zero momentum (Lagrangian phonons). A careful analysis—in particular of the concept “velocity of the bearer fluid”—shows that the two-fluid equations turn out to be the same in either case.     

Transmission of quasi-monochromatic light wave through dispersive medium and measurement of refractive index

Relations used in the measurements of the refractive index in a non-absorbing dispersive medium are discussed, taking into account the quasi-monochromaticity of the light wave. It is shown that when interference methods using compensation are employed, it is always necessary to allow for corrections for dispersion. Relations describing the diffraction of a ray at the boundary of vacuum and a non-absorbing dispersive medium can be obtained only from the continuity of the Poynting vector at the interface of these media. The stationary phase method may lead to erroneous conclusions. The group velocity vector, which for diffraction describes correctly not only the absolute magnitude but also the direction of this velocity in a dispersive non-absorbing medium, is of the form $$v_g = \frac{{k(\omega _0 )}}{{|k|}}|\upsilon _g |$$ whereω ₀ is the frequency belonging to the maximum amplitude of a given quasi-monochromatic wave.     

Parametrical Instability of Surface Type X-modes Immersed into a Nonmonochromatic Pumping Electric Field

Parametrical excitation of surface type X-modes (STXM) at the second harmonic of electron cyclotron frequency by nonmonochromatic external alternating electric field is under consideration. STXM are the eigenmodes of a planar magnetoactive plasma waveguide structure consisting of a metal wall with dielectric coating and uniform plasma filling. An external steady magnetic field is applied along the plasma interface, so it is perpendicular to the group velocity of the considered extraordinarily polarized waves. Influence of the plasma waveguide parameters on the parametrical instability of the STXM is studied. External alternating electric field is assumed to consist of two fields with different amplitudes and frequencies. A theoretical investigation is carried out using kinetic equation for plasma particles under the conditions of weak plasma spatial dispersion and small amplitudes of external electric fields. The obtained results can be useful for research in branch of edge plasma physics.     

Shear Alfvén wave perpendicular propagation from the kinetic to the inertial regime

We report on observations of shear Alfvén waves radiated from a source of small transverse size, and the subsequent radial confinement of wave magnetic field energy within a cylindrical plasma. The radius of confinement lies between the kinetic regime of the bulk plasma and the inertial regime at the plasma edge; this radius is found to be a function of wave frequency. Numerical calculations using kinetic theory predict a zero in the perpendicular group velocity at a radius which varies in accord with the observations. An analytic expression for the perpendicular group velocity (valid for small perpendicular wave numbers) is given in the vicinity of the zero crossing.     

Traveling Wave Solutions of the Incompressible Ideal Hall Magnetohydrodynamics

The solutions of incompressible ideal Hall magnetohydrodynamics are obtained by using the traveling wave method.It is shown that the velocity and magnetic field parallel to the wave vector can be arbitrary constants.The velocity and magnetic field perpendicular to the wave vector are both helical waves.Moreover,the amplitude of the velocity perpendicular to the wave vector is related to the wave number and the circular frequency.In addition,further studies indicate that,no matter whether the uniform ambient magnetic field exists or not,the forms of the travelling wave solutions do not change.     

Theory of cyclotron superradiance from a moving electron bunch under group synchronism conditions

A theory is presented of cyclotron superradiance from an electron bunch rotating in a uniform magnetic field and drifting at a velocity close to the group velocity of a wave propagating in a waveguide. It is shown that, in a comoving frame of reference, the bunch emits radiation at a frequency close to the cutoff frequency of the waveguide. Superradiance implies the azimuthal self-bunching of electrons, which is accompanied by coherent emission of the stored rotational energy in a short electromagnetic pulse. Linear and nonlinear stages of the process are analyzed. The growth rate of the superradiance instability is determined. It is shown that the maximum growth rate is attained under group synchronism conditions. The peak power and the characteristic duration of the cyclotron superradiance pulse are determined by numerical simulation. The characteristic features of the superradiance pulses are described in the comoving and laboratory frames. The results of theoretical analysis are compared with experimental data.     

Mode selection of Lamb waves for the evaluation of solid plates with liquid loading

This paper studies the mode selection of Lamb waves for evaluating solid plates with liquid loading. For this purpose, the Lamb wave selected should have the features such as zero normal displacement components at the plate surface in contact with liquid, small dispersion, and maximum group velocity. It is found that when the phase velocity of Lamb wave is equal to the longitudinal wave velocity of the plate material, its normal displacement at the plate surface is always zero. Through the numerical analyses, the specific S₂ Lamb wave that has zero normal displacement component at the plate surface, small dispersion and maximum group velocity compared with the other Lamb waves has been found. With respect to the specific S₂ Lamb wave, some experimental examinations have been carried out. It is found that the liquid loading on the plate surface has less influence on the specific S₂ Lamb wave signal but it can effectively eliminate the other signals. Moreover, the specific S₂ Lamb wave selected exhibits the capability of detecting multiple defects in the solid plate with the liquid loading. It can be concluded that the specific S₂ Lamb wave selected is suitable for the evaluation of solid plates with liquid loading.     

Propagation of quasi-two dimensional exciton polaritons in a quantum well waveguide

The propagation of exciton polaritons in an optical waveguide with a quantum well is studied. Spatial dispersion of the excitons causes the wave vector of the exciton polaritons to split between waveguide and exciton modes at resonance. The magnitude of this splitting is determined by the radiative decay parameter of excitons with corresponding polarization in the quantum well. The group velocity of the waveguide exciton polaritons in the resonance region can be three or four orders of magnitude lower than the speed of light in vacuum. Fiz. Tverd. Tela (St. Petersburg) 40, 362–365 (February 1998)     

二维正方光子晶体波导中的慢光传输

采用平面波展开法分析了正方晶格光子晶体线缺陷波导中的慢光传输.数值模拟结果表明,介质柱光子晶体线缺陷波导中,缺陷模的群速度与填充比、缺陷处介质柱半径及与波导相邻介质柱半径有关,其中缺陷处介质柱半径对缺陷模的群速度影响最大|随着填充比和缺陷处介质柱半径的增大及与波导相邻介质柱半径的减小,缺陷模的群速度逐渐减小且最小减至0.015c.进而对比研究空气柱光子晶体线缺陷波导,发现缺陷模的群速度随着缺陷处空气柱半径的减小而减小,最小减至0.008c.     

Low group velocity in a photonic crystal coupled-cavity waveguide

A two-dimensional photonic crystal coupled-cavity waveguide is designed and optimized, the transmission spectrum is calculated by using the finite-difference time-domain method, and the group velocity of c/1856 is obtained. To our knowledge, this value of group velocity is the lowest group velocity in a photonic crystal waveguide calculated from its transmission spectrum so far. The result is confirmed by the photonic band structure calculated by using the plane wave expansion method, and it is found that the photonic crystal waveguide modes in a photonic band structure are in accordance with those in the transmission spectrum by using the finite-difference time-domain method. The mechanism of slow light in the coupled-cavity waveguide of photonic crystal is analysed.     

Wideband and low dispersion slow light in slotted photonic crystal waveguide

We present a procedure to generate wideband and low dispersion slow light in slotted photonic crystal waveguide. By shifting the first and second rows of air holes of slotted photonic crystal waveguide, the bandwidth of slow light can be increased, with small group velocity dispersion. Using 2D plane wave expansion method, we numerically demonstrate slow light with the nearly constant group indices of 23, 42, and 54 over 17.6 nm, 6.7 nm and 3.3 nm bandwidth, respectively. The maximal normalized delay-bandwidth product is 0.26. From the fabrication's point of review, shifting the position of holes is easier to be controlled technically than changing the diameters of air holes. In addition, our simulations suggest this design is tolerant to deviation for positions of the first two rows of air holes. Therefore, the proposed approach decreases the dependence on the fabrication accuracy.     

Second-order solutions for random interfacial waves in N-layer density-stratified fluid with steady uniform currents

In the present paper, the random interfacial waves in N-layer density-stratified fluids moving at different steady uniform speeds are researched by using an expansion technique, and the second-order asymptotic solutions of the random displacements of the density interfaces and the associated velocity potentials in N-layer fluid are presented based on the small amplitude wave theory. The obtained results indicate that the wave-wave second-order nonlinear interactions of the wave components and the second-order nonlinear interactions between the waves and currents are described. As expected, the solutions include those derived by Chen （2006） as a special case where the steady uniform currents of the N-layer fluids are taken as zero, and the solutions also reduce to those obtained by Song （2005） for second-order solutions for random interfacial waves with steady uniform currents if N = 2.     

Light transmission along a slab waveguide with a core of anisotropic metamaterial

We investigated the dispersion property of a slab waveguide with an anisotropic metamaterial core whose permittivity tensor is partially negative. The subwavelength guidance characteristics are presented based on the boundary conditions. The results show that, at some specific frequencies, many high-order modes can exist in present waveguide even with the thickness of the guiding core 10 times smaller than the working wavelength. It is also found that different orientations of the optical axis of the anisotropic core will lead to different dispersion of the guided modes. If the orientation of the optical axis is properly chosen, the guided modes show a transition from backward wave to a forward wave as the frequency increases. During this transition, the group velocity of some guided modes can approach zero. Since the anisotropic metamaterial we discuss here can be fabricated in GHz, near- and mid-infra-red frequencies, our result may find some applications in wave trapper, integrated optical and nanophotonic devices.     

基于超指向性多极子矢量阵的水下低频声源方位估计方法研究

针对水下低频声源的方位估计问题,基于基元紧密排列的二维矢量阵,建立了一种超指向性波束形成方法.根据矢量基元差分运算构建各阶多极子模型,获得了几乎与频率无关的模态函数,并经加权计算可在低频条件下实现超指向性波束,以解决阵列孔径对波束性能的限制.同时,结合输出信噪比最大准则所得波束,分析了超指向性波束形成算法的稳定性与波导的影响程度,探索模态阶数与阵列参数的选取原则.通过阵列性能的仿真计算与实际阵列的测量数据表明,该算法可在小尺寸阵列孔径下获得良好的阵列波束,兼具了水下线型超指向性阵和环形超指向性阵的优点,可有效实现水下低频声源的水平方位估计;且波束性能可通过调节模态阶数与基元间距以达到最佳,并受水下声波导多途与频散效应影响有限.     

负折射率介质层中光波的相位和传输特性研究

针对在理解负折射率材料中光波的传播特性时相位的特殊性质 ,详细分析推证了负折射率介质中折射率、波矢k的大小和方向、空间坐标系以及波相相位互之间的关系。分析结果表明负折射率介质层中的波矢k的反常取向是由于其标量值为负 ;在统一的坐标系下 ,由该标量值参与数值计算和分析 ,k则由标量值和单位矢量k0 (k0 >0 )共同决定 ,而不必再考虑左、右手系的区别。在此基础上依据电磁场理论推导了光波在负折射率介质层中的传输矩阵 ,用以分析含有负折射率介质层的复杂薄膜系统的光学性质。初步计算结果显示含有负折射率介质层的膜系对TE和TM波的光谱特性都有显著改变     

对称平面单轴晶体金属波导的模式场(1)

由于平面各向异性介质金属波导的重要应用,本文讨论了光在对称平面单轴晶体金属波导(波导层是单轴晶体,两个波导界面均为金属)内的传输特性.对于晶体光轴位于波导界面法方向与传输方向构成的平面内的特殊波导结构,利用麦克斯韦方程组并结合单轴晶体的性质,精确解出了波导模式场.结果表明:1)此种特殊结构波导内存在横电波(TE)和横磁波(TM);2)TM波的坡印廷矢量与波矢方向不在一条直线上;3)波导层为负单轴晶体时波导主模是TE波主模,而波导层为正单轴晶体时波导主模是TM波主模.