Phonon Dispersion in Polyadenylic Acid

A study of the normal modes of vibration and their dispersion in polyadenylic acid based on the Urey–Bradley force field is reported. It gives a better interpretation of FTIR spectra as compared with the valence force field. Characteristic features of dispersion curves such as regions of high density‐of‐states, repulsion, and character mixing of dispersive modes are discussed. Predictive valuzes of heat capacity as a function of temperature are reported.     

Vibrational Dynamics of Poly(l-asparagine)

The normal modes and their dispersions for poly(l-asparagine) (PLN) in the α-helical form have been obtained in a reduced zone scheme with Wilson's GF matrix method as modified by Higgs. The results indicate that the modes below 1350 cm^?1 show appreciable dispersion. Optically active frequencies corresponding to the zone center and zone boundary are identified and discussed. Some of the characteristic features of the dispersion curves are repulsion accompanied by an exchange of character and von Hove singularities. Amide modes, side chain modes, and mixed modes are reported. The heat capacity calculated from the dispersion curves via density-of-states using Debye's relation is compared with the experimental data in the temperature range of 220–390 K.     

双层石墨烯位于1800—2150cm~(-1)频率范围内的和频拉曼模

文章利用拉曼光谱研究了双层石墨烯在1800—2150 cm-1范围内的和频拉曼模.基于双共振拉曼散射理论,利用多波长激光拉曼散射结合声子色散曲线分别从实验上和理论上分析发现,双层石墨烯在此频率范围内主要存在4个拉曼模,它们主要由LO和LA或iTA按不同共振散射方式所组成的4个和频模,而iTO和oTO参与和频的可能性很小.文章澄清了学术界在1800—2150 cm-1频率范围内和频模的解释,有助于进一步深入理解多层石墨烯在此范围内的和频模.     

Properties of plasmon-polariton modes in one-dimensional photonic superlattices made of Rudin-Shapiro bases

Theoretical investigations on the features of the plasmon-polariton frequency spectrum in one-dimensional photonic superlattices with Rudin-Shapiro dielectric layered unit cell are preformed. The plasmon polariton dispersion relations as well as the dependence on the angle of incidence of the Bragg and plasmon-polariton photonic bandgaps are reported. Results show that plasmon-polariton modes are robust even in the presence of rather large absorption. The outcome of the study shows that the reduced plasmon-polariton dispersion relation obtained within a Drude-like dielectric model points at a multifractal character under lossless conditions.     

Observation of trapped light induced by Dwarf Dirac-cone in out-of-plane condition for photonic crystals

Existence of out-of-plane conical dispersion for a triangular photonic crystal lattice is reported. It is observed that conical dispersion is maintained for a number of out-of-plane wave vectors(k_z). We study a case where Dirac like linear dispersion exists but the photonic density of states is not vanishing, called Dwarf Dirac cone(DDC) which does not support localized modes. We demonstrate the trapping of such modes by introducing defects in the crystal. Interestingly, we find by k-point sampling as well as by tuning trapped frequency that such a conical dispersion has an inherent light confining property and it is governed by neither of the known wave confining mechanisms like total internal reflection, band gap guidance. Our study reveals that such a conical dispersion in a non-vanishing photonic density of states induces unexpected intense trapping of light compared with those at other points in the continuum. Such studies provoke fabrication of new devices with exciting properties and new functionalities.     

双介质片平面波导契伦科夫自由电子激光

给出双介质片平面波导契伦科夫自由电子激光波导模式的色散关系,求出器件的单程增益并证明存在一组新的波导模式__奇模。讨论了器件的工作频率和单程增益随介质片厚度、电子束能量和电子束通道宽度的变化关系。还研究了这组奇模对器件的影响。     

Spatial dispersion in metamaterials with negative dielectric permittivity and its effect on surface waves

The effect of spatial dispersion on the electromagnetic properties of a metamaterial consisting of a three-dimensional mesh of crossing metallic wires is reported. The effective dielectric permittivity tensor epsilon(ij)(omega, k) of the wire mesh is calculated in the limit of small wavenumbers. The procedure for extracting the spatial dispersion from the omega versus k dependence for electromagnetic waves propagating in the bulk of the metamaterial is developed. These propagating modes are identified as similar to the longitudinal (plasmon) and transverse (photon) waves in a plasma. Spatial dispersion is found to have the most dramatic effect on the surface waves that exist at the wire mesh-vacuum interface.     

Ultrasound propagation near the critical mixing point of sodium-ammonia solutions

Ultrasonic absorption and dispersion measurements near the critical mixing point of sodium-ammonia solutions are reported. The results are interpreted in terms of a coupling of the ultrasonic wave and mass diffusional modes. The critical behavior of sound propagation shows features not observed in organic binary mixtures.     

The modes of an ultra-cold strongly magnetized charged Bose gas

The modes of a strongly magnetized charged Bose gas are presented for ultra-low temperatures. For longitudinal oscillations propagating parallel to the magnetic field the dispersion relation is found to be dominated by the one-dimensional field-free plasmon dispersion relation as found by Alexandrov, Beere and Kabanov recently in reference [1], while for propagation perpendicular to the magnetic field they are found to be influenced by the cyclotron motion of the particles. Dispersion relations for these modes known as Bernstein modes are given near the cyclotron frequency and its first two harmonics. The dispersion relations for transverse modes in the system are then presented for the cases of photon propagation perpendicular and parallel to the direction of the magnetic field. Received: 3 July 1997 / Revised: 12 August 1997 / Accepted: 4 November 1997     

Characteristics of Photonic Bandgap Fibres with Hollow Core＇s Inner Surface Coated by a Layer Material

Hollow core＇s inner surface coating in a photonic bandgap fibre （PBCF） is investigated by means of finite element method. The coat material and thickness-dependence dispersion curve and group velocity dispersion are numerically studied. The coating with materials of low index or small thickness will rise up the dispersion curve but will not induce surface modes. However, coating with materials of high index or big coat thickness will induce surface modes and avoided-crossings. By varying coat material＇s refractive index and thickness, the appearances of surface modes and avoided-crossings can be changed. It is found that the avoided-crossing can enormously enlarge the negative dispersion which can find applications in dispersion compensation. We numerically achieve a negative dispersion as large as -21416.15ps/nm/km. The results give a physical insight into the propagation properties of PBGFs with the hollow core coated by a layer of material and are of crucial significance in the applications of PBGF coating.     

Guided Waves in a Multi-Layered Cylindrical Elastic Solid Medium

We investigate the guided waves in a multi-layered cylindrical elastic solid medium. The dispersion function of guided waves is usually complex and the dispersion curves of all modes are not conveniently obtained. Here we present an effective method to obtain the dispersion curves of all modes. First, the dispersion function of the guided waves is transformed into a real function. The dispersion curves are then calculated for all the modes of the guided waves by the bisection method. The modes with the orders n = 0, 1, and 2 are analysed in two- and three-layer media. The existence condition of Stoneley wave is discussed. The modes of the guided waves are also investigated in a two-layer medium, in which the velocity of shear wave in the outer layer is less than that in the inner layer.     

Guided modes in the chiral negative refractive index fiber

The novel characteristics of guided modes in the chiral negative refractive index fiber are investigated theoretically in this letter.We derive the characteristic equation of guided modes.Based on two types of chiral metamaterial parameters,we present the dispersion curves,energy flux,and power of guided modes.Some abnormal features,such as the existence of surface mode and dispersion curves with different shapes,intersection of dispersion curves of different guided modes,negative energy flux in the achiral cladding,and zero power at some normalized frequencies,are found in the chiral negative refractive index fiber.     

Comparing guided modal properties of surface waves along single- and double-negative indexed slab waveguides

The guided dispersion characteristics of the fundamental symmetric and asymmetric modes of surface waves along single- and double-negative indexed slab waveguides are investigated, and a comparative analysis made when varying the single- and double-negative permittivity and permeability. While the values of the permittivity and permeability of the slab region are varied to obtain a salient picture of the guided dispersion characteristics, identical absolute product values are used for both slab cases to facilitate a reasonable comparison. In particular, in common ranges where guided mode solutions coexist for both the single- and double-negative indexed slabs, the guided mode characteristics are similar with a lower normalized frequency regime, indicating that the sign of the material parameters has a negligible effect, whereas the characteristics become quite different as the normalized frequency increases. Some other anomalous guided dispersion properties are also discussed and compared with previously reported results.     

Highly dispersive large mode area photonic bandgap fiber

An all-silica photonic bandgap fiber composed of a low-index core surrounded by alternating high- and low-index rings allows us to achieve a large mode area (500 microm(2)) and large chromatic dispersion. Sharp resonances from the even Bragg mode to odd ring modes theoretically lead to 20,000 ps/(nm km) chromatic dispersion when large bends are applied. By nature, sharp resonances are sensitive to inhomogeneities along the fiber length. Under experimental conditions, the resonances are broadened and the dispersion coefficient is decreased to 1000 ps/(nm km). However, to the best of our knowledge, this is the largest dispersion coefficient reported using a large mode area fiber.     

Guided wave dispersion curves for a bar with an arbitrary cross-section,a rod and rail example

Theoretical and experimental issues of acquiring dispersion curves for bars of arbitrary cross-section are discussed. Since a guided wave can propagate over long distances in a structure, guided waves have great potential for being applied to the rapid non-destructive evaluation of large structures such as rails in the railroad industry. Such fundamental data as phase velocity, group velocity, and wave structure for each guided wave mode is presented for structures with complicated cross-sectional geometries as rail. Phase velocity and group velocity dispersion curves are obtained for bars with an arbitrary cross-section using a semi-analytical finite element method. Since a large number of propagating modes with close phase velocities exist, dispersion curves consisting of only dominant modes are obtained by calculating the displacement at a received point for each mode. These theoretical dispersion curves agree in characteristic parts with the experimental dispersion curves obtained by a two-dimensional Fourier transform technique.     

Experimental evidence of one-dimensional plasma modes in superconducting thin wires

We have studied niobium superconducting thin wires deposited onto a SrTiO3 substrate. By measuring the reflection coefficient of the wires, resonances are observed in the superconducting state in the 130 MHz to 4 GHz range. They are interpreted as standing wave resonances of one-dimensional plasma modes propagating along the superconducting wire. The experimental dispersion law, omega versus q, presents a linear dependence over the entire wave vector range. The modes are softened as the temperature increases close the superconducting transition temperature. Very good agreement is obtained between our data and the predicted dispersion relation of one-dimensional plasma modes.     

Modulation instabilities in randomly birefringent two-mode optical fibers

Modulation instabilities in the randomly birefringent two-mode optical fibers(RB-TMFs) are analyzed in detail by accounting the effects of the differential mode group delay(DMGD) and group velocity dispersion(GVD) ratio between the two modes, both of which are absent in the randomly birefringent single-mode optical fibers(RB-SMFs). New MI characteristics are found in both normal and anomalous dispersion regimes. For the normal dispersion, without DMGD,no MI exists. With DMGD, a completely new MI band is generated as long as the total power is smaller than a critical total power value, named by Pcr, which increases significantly with the increment of DMGD, and reduces dramatically as GVD ratio and power ratio between the two modes increases. For the anomalous dispersion, there is one MI band without DMGD. In the presence of DMGD, the MI gain is reduced generally. On the other hand, there also exists a critical total power(Pcr), which increases(decreases) distinctly with the increment of DMGD(GVD ratio of the two modes) but varies complicatedly with the power ratio between the two modes. Two MI bands are present for total power smaller than Pcr, and the dominant band can be switched between the low and high frequency bands by adjusting the power ratio between the two modes. The MI analysis in this paper is verified by numerical simulation.     

Time-frequency analysis of the dispersion of Lamb modes.

Accurate knowledge of the velocity dispersion of Lamb modes is important for ultrasonic nondestructive evaluation methods used in detecting and locating flaws in thin plates and in determining their elastic stiffness coefficients. Lamb mode dispersion is also important in the acoustic emission technique for accurately triangulating the location of emissions in thin plates. In this research, the ability to characterize Lamb mode dispersion through a time-frequency analysis (the pseudo-Wigner-Ville distribution) was demonstrated. A major advantage of time-frequency methods is the ability to analyze acoustic signals containing multiple propagation modes, which overlap and superimpose in the time domain signal. By combining time-frequency analysis with a broadband acoustic excitation source, the dispersion of multiple Lamb modes over a wide frequency range can be determined from as little as a single measurement. In addition, the technique provides a direct measurement of the group velocity dispersion. The technique was first demonstrated in the analysis of a simulated waveform in an aluminum plate in which the Lamb mode dispersion was well known. Portions of the dispersion curves of the A0, A1, S0, and S2 Lamb modes were obtained from this one waveform. The technique was also applied for the analysis of experimental waveforms from a unidirectional graphite/epoxy composite plate. Measurements were made both along and perpendicular to the fiber direction. In this case, the signals contained only the lowest order symmetric and antisymmetric modes. A least squares fit of the results from several source to detector distances was used. Theoretical dispersion curves were calculated and are shown to be in good agreement with experimental results.     

基于类表面等离子体激元的矩形金属光栅色散特性的研究

等离子体激元(surface plasmon polaritons, SPP)因其独特的光学和物理特性, 使其具有诸如透射增强和局域共振等一系列新颖现象, 已成为当前国内外学者研究的热点. 本文对基于类表面等离子体激元(Spoof Surface Plasmons, SSP)的矩形金属光栅色散特性和模式分布进行了研究. 利用本征函数法并结合场匹配条件, 获得了矩形栅表面SSP的场表达式、色散关系和模式分布, 并通过电磁仿真进行了验证. 在此基础上分析了矩形栅各参数对SSP色散及模式分布的影响, 研究结果表明: 由本征函数法获得的SSP色散特性与仿真结果基本符合; 增大金属栅高度或减小排列周期能减小SSP的相速度; 而增大金属栅周期占空比能在一定程度上拓展SSP与电子束互作用的带宽; 改变金属盖板高度对慢波SSP色散模式基本没有影响; 减小金属栅侧面宽度能增大模式之间的间隔, 从而能有效避免模式竞争的发生. 本文对基于SSP的矩形金属光栅色散特性的研究将为进一步研究SSP与电子束的相互作用, 形成高效、宽带的新型太赫兹源奠定良好的理论基础.     

含左手材料对称五层平面波导模场特性

对芯层为左手材料而内外包层都是普通材料的对称五层平面波导进行了探讨.得到了两类TE振荡模的色散方程,且在考虑材料色散的条件下,画出了相关模式的色散曲线.随着模阶数的增加,模色散曲线左移,它们的截止频率变小.其次,发现零阶TE振荡模的存在.除此以外,TE振荡模有三类色散特性：正常色散;反常色散;双模简并.