Localized states of acoustic waves in three-dimensional periodic composites with point defects

Acoustic point defect states in the three-dimensional simple-cubic arrays of water spheres embedded in a mercury host are studied. Two kinds of defects are introduced, one is a sphere defect created by changing its radius, and another is a cubic defect obtained by replacing one of the spheres with a cube. The results show that a defect band appears in the band gap of the perfect crystals. The calculations show that the defect modes are localized around the defect. The influence of the filling fraction and the geometry of the defect on the defect modes are investigated in detail.Received: 5 May 2003, Published online: 11 August 2003PACS: 43.20.+g General linear acoustics - 62.60.+v Acoustical properties of liquids - 61.72.Ji Point defects (vacancies, interstitials, color centers, etc.) and defect clusters     

Localized defect modes in finite magnetic two-dimensional photonic crystals

We present a detailed study of localized defect modes in finite two-dimensional magnetic photonic crystal consisting of cylindrical ferrite material in air background by using the scattering matrix method. It is shown that due to having positive and negative value of the effective magnetic permeability in interested photonic band gap, the defect modes present different behavior. Our results show that the modes appeared in positive permeability region have features similar to the modes supported by the dielectric photonic crystal while those created in the negative permeability region are strongly localized at the interface of defect ferrite rod with air background. Moreover, results indicate that defect modes occurred in the negative region are sharper than those appeared in the positive one which means the quality factor of former case is extremely high.     

二维磁振子晶体中点缺陷模的耦合性质研究

在超原胞近似下, 利用平面波展开法数值计算了多点缺陷的二维磁振子晶体带结构及其部分缺 陷模的磁化强度场分布. 研究结果表明, 点缺陷模间的能量发生了耦合, 使自旋波沿着缺陷方向传播. 利用该性质可将含多点缺陷体的二维磁 振子晶体作为自旋波导波器件的制作材料.     

Localized optical modes induced by point and extended defects in ionic crystals

It is shown that the vector character of the relative displacement of atoms in ionic crystals causes a nonexponential (power) law decay of localized optical mode amplitudes at large distances from point or linear defects.     

Coherence of acoustic modes propagating through shallow water internal waves

The 1995 Shallow Water Acoustics in a Random Medium (SWARM) experiment [Apel et al., IEEE J. Ocean. Eng. 22, 445-464 (1997)] was conducted off the New Jersey coast. The experiment featured two well-populated vertical receiving arrays, which permitted the measured acoustic field to be decomposed into its normal modes. The decomposition was repeated for successive transmissions allowing the amplitude of each mode to be tracked. The modal amplitudes were observed to decorrelate with time scales on the order of 100 s [Headrick et al., J. Acoust. Soc. Am. 107(1), 201-220 (2000)]. In the present work, a theoretical model is proposed to explain the observed decorrelation. Packets of intense internal waves are modeled as coherent structures moving along the acoustic propagation path without changing shape. The packets cause mode coupling and their motion results in a changing acoustic interference pattern. The model is consistent with the rapid decorrelation observed in SWARM. The model also predicts the observed partial recorrelation of the field at longer time scales. The model is first tested in simple continuous-wave simulations using canonical representations for the internal waves. More detailed time-domain simulations are presented mimicking the situation in SWARM. Modeling results are compared to experimental data.     

Localized vibrational modes of a persistent defect in ion-implanted SiC

The low-temperature luminescence spectrum of an intrinsic defect in ion-implanted cubic SiC reveals a number of high-energy localized modes. One has an energy of 164·7 meV, equivalent to the highest lattice frequency in diamond, and far above the 120·5 meV lattice limit of SiC. A carbon di-interstitial is a plausible model for the defect, which appears after a 1300°C anneal, and persists after a 1700°C anneal.     

Magnetoplasma modes of a spatially periodic two-dimensional electron gas

We examine the magnetoplasma dispersion of a two-dimensional electron gas with a spatially periodic charge density. The system studied is a periodic array of two-dimensional electron gas strips with constant equilibrium density. The integral equation describing the charge fluctuations on the strips, has been derived and solved numerically. The spatial dependence of the form of either propagating or evanescent waves. The latter are associated with the edge modes, recently discovered in electrons on liquid⁴ He. For a periodic array of two-dimensional strips, the modes in different strips, interact and form bands.     

一维多缺陷光子晶体的缺陷模

设计了一个多缺陷的一维光子晶体,并利用传输理论研究其透射谱.利用泰勒展式的一级近似,得到了缺陷模频率的解析表达式,进而得到紧束缚理论中的耦合因子.这些结论与实验结果或数值模拟相符合,可以很好地描述缺陷模的有关规律,对多通道滤波器的设计具有指导意义. 关键词： 光子晶体 缺陷模 紧束缚方法 耦合因子     

Localized vibrations of point defects in body-centred cubic lattices

The paper gives an exact calculation of the localized frequencies of substitutional defects in a body-centred cubic lattice by the method of Green's functions and compares it with the approximate calculation carried out after [14]. The exact calculation is based on newly computed Green's functions of a b.c.c. lattice [18]. It is shown how by means of group theory the symmetry of the system can be used both in an approximate and in the exact calculation. Some symmetry relations between Green's functions, which limit the number of functions necessary for numerical calculations, are derived.     

Magnetoplasma modes of a two-dimensional electron gas with spatially periodic charge density

The plasmon band structure of a two-dimensional electron gas with an arbitrary spatially periodic equilibrium density is studied.     

Localized vibrational,edges and breathing modes of graphene nanoribbons with topological line defects

Peculiar vibrational modes of graphene nanoribbons (GNRs) with topological line defects were presented. We find that phonon dispersion relations of the topological defective GNRs are more similar to those of perfect armchair-edge GNR than to zigzag-edge GNR in spite of their zigzag edge. All vibrational modes at Γ point are assigned in detail by analyzing their eigenvectors and are presented by video. Three types of characteristic vibrational modes, namely, localized vibrational modes in defect sites, edges, and breathing modes, are observed. Five localized vibrational modes near the defect sites are found to be robust against the width of the topological line-defective GNR. The Raman D’ band just originates from one localized mode, 1622 cm^-1. The vibrational mode is sensitive to symmetry. The edge modes are related with structural symmetry but not with widths. Two edge modes are asymmetrical and only one is symmetrical. The breathing modes are inversely proportional to the width for wide-defect GNRs, and inversely proportional to the square root of the width for narrow-defect GNRs. The breathing mode frequencies of defective GNRs are slightly higher than those of perfect GNRs. These vibrational modes may be useful in the manipulation of thermal conductance and implementation of single phonon storage.     

Modulated periodic stokes waves in deep water

Modulated deep-water 1D Stokes waves are considered experimentally and theoretically. Wave trains are modulated in a controlled fashion and their evolution is recorded. Data from repeated laboratory experiments are reproducible near the wave maker, but diverge away from the wave maker. Numerical integration of a perturbed nonlinear Schrodinger equation and an associated linear spectral problem indicate that under suitable conditions modulated periodic Stokes waves evolve chaotically. Sensitive spectral evolution in the neighborhood of homoclinic manifolds of the unperturbed nonlinear Schrodinger equation is found.     

Tunneling of electromagnetic waves through a barrier with a periodic structure

The propagation of an electromagnetic wave through a barrier with a periodic structure is considered. An interesting manifestation of electromagnetic wave tunneling through a barrier with a periodic structure is revealed. Specifically, it is theoretically and experimentally shown that, when an electromagnetic wave passes through a medium with a periodic structure in the diffraction mode within a photonic band gap, the field in the medium is localized near its boundaries. Theoretical calculations have been performed for a cholesteric liquid crystal layer of finite thickness and for a 1D photonic crystal. The experiment was carried out with perfect Si single crystals in reflection from the $ (2\bar 20)$ (2\bar 20) and $ (4\bar 40)$ (4\bar 40) planes using MoK _α X rays.     

Localized electromagnetic modes and the transmission spectrum of a one-dimensional photonic crystal with lattice defects

The properties of localized electromagnetic modes in a one-dimensional photonic crystal with a structural defect layer were studied. The role of the defect was played by an anisotropic nematic liquid crystal layer. The frequency and the damping factor of defect modes were shown to substantially depend on the defect layer thickness and the orientation of the optical axis of the nematic. The transmission spectra of photonic crystals with one and two lattice defects were studied. Taking into account the special feature of nematic liquid crystals distinguishing them from usual crystals, namely, large permittivity anisotropy, it was shown that the transmission spectrum of the photonic crystal could be controlled by varying the orientation of the optical axis of the nematic, for instance, under the action of an external electric field.     

Localized modes due to a line defect in a diatomic cubic lattice

By a transformation into a monoatomic lattice and utilization of the translation symmetry in direction of a defect three regions of possible localized mode frequencies are generally determined. For some simple combinations of parameters of a defect concrete conditions are specified for the occurrence of frequency bands of localized modes and for a more complicated defect the bands are determined numerically.The author thanks Prof. O. Litzman for his attention paid to this work.     

From point defects in graphene to two-dimensional amorphous carbon

While crystalline two-dimensional materials have become an experimental reality during the past few years, an amorphous 2D material has not been reported before. Here, using electron irradiation we create an sp2-hybridized one-atom-thick flat carbon membrane with a random arrangement of polygons, including four-membered carbon rings. We show how the transformation occurs step by step by nucleation and growth of low-energy multivacancy structures constructed of rotated hexagons and other polygons. Our observations, along with first-principles calculations, provide new insights to the bonding behavior of carbon and dynamics of defects in graphene. The created domains possess a band gap, which may open new possibilities for engineering graphene-based electronic devices.     

点缺陷二维光子晶体波导的出射光集束

光子晶体器件在高密度集成光通信中有广泛的应用,为解决光子晶体波导出射光场的空间控制,采用时域有限差分法分析光子晶体波导结构的缺陷传播特性,提出基于点缺陷优化波导结构,通过在波导出射口两侧加上点缺陷,出射光方向性有显著提高,实现三点光源干涉系统的光集束。模拟结果表明缺陷态越靠近能带结构中央,共振腔的耦合效率越高;相反,缺陷态越靠近能带结构边缘位置,则共振腔耦合效率越低,因此,选取禁带区域四分之一处对应的点缺陷,可以有效实现波导出射的光集束。