Showing papers on "Wave propagation published in 2008"

Experimental verification of epsilon-near-zero metamaterial coupling and energy squeezing using a microwave waveguide.

Abstract: Utilizing a microwave setup, we experimentally verify our recently developed theory of energy squeezing and tunneling [Phys. Rev. Lett. 97, 157403 (2006)] through an ultranarrow waveguide channel that mimics zero-permittivity properties. Exploiting the infinite phase velocity supported by a waveguide transition section at cutoff, we test our theory of tunneling in this zero-permittivity region without use of resonant inclusions. This ``supercoupling'' is shown to have unique anomalous properties: an almost uniform phase along the narrow channel and weak dependence over its geometry.

Nonlocal shell model for elastic wave propagation in single- and double-walled carbon nanotubes

Abstract: This paper investigates the transverse and torsional wave in single- and double-walled carbon nanotubes (SWCNTs and DWCNTs), focusing on the effect of carbon nanotube microstructure on wave dispersion. The SWCNTs and DWCNTs are modeled as nonlocal single and double elastic cylindrical shells. Molecular dynamics (MD) simulations indicate that the wave dispersion predicted by the nonlocal elastic cylindrical shell theory shows good agreement with that of the MD simulations in a wide frequency range up to the terahertz region. The nonlocal elastic shell theory provides a better prediction of the dispersion relationships than the classical shell theory when the wavenumber is large enough for the carbon nanotube microstructure to have a significant influence on the wave dispersion. The nonlocal shell models are required when the wavelengths are approximately less than 2.36×10 −9 and 0.95×10 −9 m for transverse wave in armchair (15,15) SWCNT and torsional wave in armchair (10,10) SWCNT, respectively. Moreover, an MD-based estimation of the scale coefficient e 0 for the nonlocal elastic cylindrical shell model is suggested. Due to the small-scale effects of SWCNTs and the interlayer van der Waals interaction of DWCNTs, the phase difference of the transverse wave in the inner and outer tube can be observed in MD simulations in wave propagation at high frequency. However, the van der Waals interaction has little effect on the phase difference of transverse wave.

Tidal variability in the ionospheric dynamo region

Abstract: [1] The seasonal and interannual variability of migrating (Sun-synchronous) and nonmigrating solar atmospheric tides at altitudes between 100 and 116 km are investigated using temperature measurements made with the SABER instrument on the TIMED spacecraft during 2002–2006. Quasi-biennial variations of order ±10–15% in migrating diurnal and semidiurnal tidal amplitudes are found, presumably due to modulation by the quasi-biennial oscillation (QBO) as the tides propagate from their troposphere and stratospheric sources to the lower thermosphere. A number of nonmigrating tidal components are found that have the potential to produce significant longitudinal variability of the total tidal fields. The most prominent of these, i.e., those that appear at amplitudes of order 5–10 K in a 5-year mean climatology, include the zonally symmetric (s = 0) diurnal tide (D0); the eastward propagating diurnal and semidiurnal tides with zonal wave numbers s = −2 (DE2 and SE2) and s = −3 (DE3 and SE3); and the following westward propagating waves: diurnal s = 2 (DW2); semidiurnal s = 1 (SW1), s = 3 (SW3), and s = 4 (SW4); and terdiurnal s = 5 (TW5). These waves can be plausibly accounted for by nonlinear interaction between migrating tidal components and stationary planetary waves with s = 1 or s = 2 or by longitudinal variations of tropospheric thermal forcing. Additional waves that occur during some years or undergo phase cancellation within construction of a 5-year climatology include DW5, SE1, SE4, SW6, TE1, TW1, and TW7. It is anticipated that the winds that accompany all of these waves in the 100–170 km region will impose longitudinal variability in the electric fields produced through the ionospheric dynamo mechanism, thereby modulating vertical motion of the equatorial ionosphere and the concomitant plasma densities. In addition to the wave-4 modulation of the equatorial ionosphere that has recently been discovered and replicated in modeling studies, the waves revealed here will generate wave-1 (SW1, SW3, D0, DW2), wave-2 (SW4, TW1), wave-3 (DE2, SE1), wave-4 (DE3, SE2, DW5, SW6, TE1, TW7), wave-5 (SE3), and wave-6 (SE4) components of this ionospheric variability, depending on year and time of year. However, the absolute and relative efficiencies with which these waves produce electric fields remains to be determined.

Coriolis effect in optics: unified geometric phase and spin-Hall effect.

Abstract: We examine the spin-orbit coupling effects that appear when a wave carrying intrinsic angular momentum interacts with a medium. The Berry phase is shown to be a manifestation of the Coriolis effect in a noninertial reference frame attached to the wave. In the most general case, when both the direction of propagation and the state of the wave are varied, the phase is given by a simple expression that unifies the spin redirection Berry phase and the Pancharatnam-Berry phase. The theory is supported by the experiment demonstrating the spin-orbit coupling of electromagnetic waves via a surface plasmon nanostructure. The measurements verify the unified geometric phase, demonstrated by the observed polarization-dependent shift (spin-Hall effect) of the waves.

Bound states in the continuum in photonic waveguides inspired by defects

Abstract: Photonic crystal with defect layer forms directed continuum for electromagnetic waves. Defect rods in the vicinity of the defect layer interact with the continuum and give rise to scattering of ingoing waves. We derive quantum-mechanical analog of the non-Hermitian Hamiltonian of the open system with complex eigenvalues, which describes a scattering of electromagnetic waves by the defect rods. In this formalism a bound state in the continuum (BIC) can be easily found by the condition that one of the complex eigenvalues becomes real for variation of dielectric constant of the defect rods. We numerically find BICs with discrete frequencies belong to the continuum for different arrangements of the defects and show that they are localized around the defects.

Current-induced spin-wave Doppler shift.

Abstract: Spin transfer appears to be a promising tool for improving spintronics devices. Experiments that quantitatively access the magnitude of the spin transfer are required for a fundamental understanding of this phenomenon. By inductively measuring spin waves propagating along a permalloy strip subjected to a large electrical current, we observed a current-induced spin wave Doppler shift that we relate to the adiabatic spin transfer torque. Because spin waves provide a well-defined system for performing spin transfer, we anticipate that they could be used as an accurate probe of spin-polarized transport in various itinerant ferromagnets.

Source inversion of W phase: speeding up seismic tsunami warning

Abstract: W phase is a long period phase arriving before S wave. It can be interpreted as superposition of the fundamental, first, second and third overtones of spheroidal modes or Rayleigh waves and has a group velocity from 4.5 to 9 km s^−1 over a period range of 100–1000 s. The amplitude of long period waves better represents the tsunami potential of an earthquake. Because of the fast group velocity of W phase, most of W phase energy is contained within a short time window after the arrival of the P wave. At a distance of 50°, W phase energy is contained within 23 min after the origin time which is the distinct advantage of using W phase for rapid tsunami warning purposes. We use a time domain deconvolution method to extract W phases from the broad-band records of global seismic networks. The bandwidth of W phase is approximately from 0.001 to 0.01 Hz, and we bandpass filter the data from 0.001 to 0.005 Hz in most cases. Having extracted W phase from the vertical component records, we perform a linear inversion using a point source to determine Mw and the source mechanism for several large earthquakes including the 2004 Sumatra–Andaman earthquake, the 2005 Nias earthquake, the 2006 Kuril Is. earthquake and the 2007 Sumatra earthquake. W phase inversion yields reliable solutions and holds promise of the use of W phase for rapid assessment of tsunami potential.

Theoretical background for continental‐ and global‐scale full‐waveform inversion in the time–frequency domain

Abstract: SUMMARY We propose a new approach to full seismic waveform inversion on continental and global scales. This is based on the time–frequency transform of both data and synthetic seismograms with the use of time- and frequency-dependent phase and envelope misfits. These misfits allow us to provide a complete quantification of the differences between data and synthetics while separating phase and amplitude information. The result is an efficient exploitation of waveform information that is robust and quasi-linearly related to Earth's structure. Thus, the phase and envelope misfits are usable for continental- and global-scale tomography, that is, in a scenario where the seismic wavefield is spatially undersampled and where a 3-D reference model is usually unavailable. Body waves, surface waves and interfering phases are naturally included in the analysis. We discuss and illustrate technical details of phase measurements such as the treatment of phase jumps and instability in the case of small amplitudes. The Frechet kernels for phase and envelope misfits can be expressed in terms of their corresponding adjoint wavefields and the forward wavefield. The adjoint wavefields are uniquely determined by their respective adjoint-source time functions. We derive the adjoint-source time functions for phase and envelope misfits. The adjoint sources can be expressed as inverse time–frequency transforms of a weighted phase difference or a weighted envelope difference. In a comparative study, we establish connections between the phase and envelope misfits and the following widely used measures of seismic waveform differences: (1) cross-correlation time-shifts; (2) relative rms amplitude differences; (3) generalized seismological data functionals and (4) the L2 distance between data and synthetics used in time-domain full-waveform inversion. We illustrate the computation of Frechet kernels for phase and envelope misfits with data from an event in the West Irian region of Indonesia, recorded on the Australian continent. The synthetic seismograms are computed for a heterogeneous 3-D velocity model of the Australian upper mantle, with a spectral-element method. The examples include P body waves, Rayleigh waves and S waves, interfering with higher-mode surface waves. All the kernels differ from the more familar kernels for cross-correlation time-shifts or relative rms amplitude differences. The differences arise from interference effects, 3-D Earth's structure and waveform dissimilarities that are due to waveform dispersion in the heterogeneous Earth.

Relativistic electron precipitation by EMIC waves from self-consistent global simulations

Abstract: [1] We study the effect of electromagnetic ion cyclotron (EMIC) wave scattering on radiation belt electrons during the large geomagnetic storm of 21 October 2001 with minimum Dst = −187 nT. We use our global physics-based model, which solves the kinetic equation for relativistic electrons and H+, O+, and He+ ions as a function of radial distance in the equatorial plane, magnetic local time, energy, and pitch angle. The model includes time-dependent convective transport and radial diffusion and all major loss processes and is coupled with a dynamic plasmasphere model. We calculate the excitation of EMIC waves self-consistently with the evolving plasma populations. Particle interactions with these waves are evaluated according to quasi-linear theory, using diffusion coefficients for a multicomponent plasma and including not only field-aligned but also oblique EMIC wave propagation. The pitch angle diffusion coefficients increase from 0° to ∼60° during specific storm conditions. Pitch angle scattering by EMIC waves causes significant loss of radiation belt electrons at E ≥ 1 MeV and precipitation into the atmosphere. However, the relativistic electron flux dropout during the main phase at large L ≥ 5 is due mostly to outward radial diffusion, driven by the flux decrease at geosynchronous orbit. We show first results from global simulations indicating significant relativistic electron precipitation within regions of enhanced EMIC instability, whose location varies with time but is predominantly in the afternoon-dusk sector. The precipitating electron fluxes are usually collocated with precipitating ion fluxes but occur at variable energy range and magnitude. The minimum resonant energy increases at low L and relativistic electrons at E ≤ 1 MeV do not precipitate at L < 3 during this storm.

Sound wave propagation in single-walled carbon nanotubes using nonlocal elasticity

Abstract: Based on the Bernoulli–Euler and Timoshenko beam theories, a single-elastic beam model using nonlocal elasticity is developed for the wave propagation in carbon nanotubes (CNTs). The small-scale effect is taken into consideration in the present theory. Frequency equations and modal shape functions of Timoshenko beams structures with some typical boundary conditions are also derived from nonlocal elasticity. In addition, the applicability of the two beam models is explored by numerical simulations. The research work reveals the significance of the small-scale effect on wave propagation in single-walled CNTs.

Wave propagation and instabilities in monolithic and periodically structured elastomeric materials undergoing large deformations

Abstract: Wave propagation in elastomeric materials undergoing large deformations is relevant in numerous application areas, including nondestructive testing of materials and ultrasound techniques, where finite deformations and corresponding stress states can influence wave propagation and hence interpretation of data. In the case of periodically structured hyperelastic solids, the effect of deformation on the propagation of acoustic waves can be even more dramatic. In fact, transformations in the periodic patterns have been observed upon application of load due to microstructural elastic instabilities, providing opportunities for transformative phononic crystals which can switch band-gap structure in a sudden, but controlled manner. Here both analytical and numerical techniques are presented for assessing the influence of finite deformations on the propagation of elastic waves in both monolithic as well as periodically structured elastomeric materials. Both elastic instabilities and propagation of acoustic waves are strongly influenced by geometric pattern, material properties and loading conditions, giving different opportunities for tuning and manipulating the location and presence of instabilities and phononic band gaps

Large-Eddy Simulations and Observations of Atmospheric Marine Boundary Layers above Nonequilibrium Surface Waves

Abstract: Winds and waves in marine boundary layers are often in an unsettled state when fast-running swell generated by distant storms propagates into local regions and modifies the overlying turbulent fields. A large-eddy simulation (LES) model with the capability to resolve a moving sinusoidal wave at its lower boundary is developed to investigate this low-wind/fast-wave regime. It is used to simulate idealized situations with wind following and opposing fast-propagating waves (swell), and stationary bumps. LES predicts momentum transfer from the ocean to the atmosphere for wind following swell, and this can greatly modify the turbulence production mechanism in the marine surface layer. In certain circumstances the generation of a low-level jet reduces the mean shear between the surface layer and the PBL top, resulting in a near collapse of turbulence in the PBL. When light winds oppose the propagating swell, turbulence levels increase over the depth of the boundary layer and the surface drag increases ...

Retrieving effective parameters for metamaterials at oblique incidence

Abstract: We introduce a technique to retrieve effective metamaterial parameters for arbitrary angles of incidence. It employs the complex reflection and/or transmission coefficients of a finite slab. Explicit expressions for both polarizations are derived and the constraints to be met for obtaining unique solutions are discussed. The method is applied to the fishnet structure. It turns out that all retrieved parameters strongly depend on the lateral wave vector component due to the complexity of the metamaterial structure. Thus, these parameters are mere wave parameters rather than global material parameters. The physical effects behind this behavior are very likely anisotropy and spatial dispersion.

Perfect reflection of light by an oscillating dipole.

Abstract: We show theoretically that a directional dipole wave can be perfectly reflected by a single pointlike oscillating dipole Furthermore, we find that, in the case of a strongly focused plane wave, up to 85% of the incident light can be reflected by the dipole Our results hold for the full spectrum of the electromagnetic interactions and have immediate implications for achieving strong coupling between a single propagating photon and a single quantum emitter

Oceanic propagation of a potential tsunami from the La Palma Island

Abstract: [1] The likelihood of a large scale tsunami from the La Palma Island is considered small by most. Nevertheless, the potential catastrophic consequences call for attention. Here we report on numerical simulations of a tsunami that might result from the extreme case of a flank collapse of the Cumbre Vieja volcano at the La Palma Island, done by combining a multimaterial model for the wave generation with Boussinesq models for the far-field propagation. Our simulations show that the slide speed is close to critical, effectively generating an initial wave of several hundred meters height. Our main focus is the wave propagation which is genuinely dispersive. In the far-field, propagation becomes increasingly complex due to the combined effects of dispersion, refraction, and interference in the direction of propagation. Constructive interference of the trailing waves are found to decrease the decay of the maximum amplitude with distance compared to classical asymptotic theory at transatlantic distances. Thus, the commonly used hydrostatic models fail to describe the propagation. Consequences of the La Palma scenario would be largest at the Canary Islands, but our findings also suggests that the whole central Atlantic would face grave consequences. However, the largest surface elevations are smaller than the most pessimistic reports found in literature. We also find undular bores towards the shorelines of America.

Extreme ocean waves

Abstract: Preface.- Freak Waves: Peculiarities of Numerical Simulations.- Rogue Waves in High-Order Nonlinear Schrodinger Models.- Non-Gaussian Properties of Shallow Water Waves in Crossing Seas.- Modeling of Rogue Wave Shapes in Shallow Water.- Runup of Long Irregular Waves on Plane Beach.- Symbolic Computation for Nonlinear Wave Resonances.- Searching for Factors that Limit Observed Extreme Maximum Wave Height Distributions in the North Sea.- Extremes and Decadal Variations of the Northern Baltic Sea Wave Conditions.- Extreme Waves Generated by Cyclones in Guadeloupe.- An Analytical Model of Large Amplitude Internal Solitary Waves.

Transmission-Line Networks Cloaking Objects From Electromagnetic Fields

Abstract: We consider a novel method of cloaking objects from the surrounding electromagnetic fields in the microwave region. The method is based on transmission-line networks that simulate the wave propagation in the medium surrounding the cloaked object. The electromagnetic fields from the surrounding medium are coupled into the transmission-line network that guides the waves through the cloak thus leaving the cloaked object undetected. The cloaked object can be an array or interconnected mesh of small inclusions that fit inside the transmission-line network.

Hinode observations of transverse waves with flows in coronal loops

Abstract: Aims. We report the first evidence for transverse waves in coronal multithreaded loops with cool plasma ejected from the chromosphere flowing along the threads. These observations are good candidates for coronal seismology. Methods. We analyzed observations made with Solar Optical Telescope (SOT) on board the Hinode satellite in the Ca II H line filter. Results. The oscillations are visible for about 3 periods, with a period lasting about 2 min, with weak damping. We see the oscillations in thin threads (∼0.5 �� ) of cool plasma flowing in the coronal loops with speeds in the range 74−123 km s −1 . Conclusions. Observations indicate that the waves exhibit different properties in the various threads. In some threads, the waves are nearly standing fundamental kink modes with a phase speed of about 1250 km s −1 , whereas the dynamics of other threads is consistent with propagating fast magnetosonic waves. Based on the observed wave and loop properties and the assumed active region loop density in the range (1−5) × 10 9 cm −3 , the estimated energy flux is sufficient to heat the loops to coronal temperatures, and the average magnetic field in the threads is estimated as 20 ± 7G .

Physical configuration and performance modeling of all-dielectric metamaterials

Abstract: In this paper, physical concept and performance analysis of all-dielectric metamaterials are presented. Metamaterials with desired material parameters $(\ifmmode\pm\else\textpm\fi{}\ensuremath{\epsilon},\ifmmode\pm\else\textpm\fi{}\ensuremath{\mu})$ are developed by creating electric and magnetic resonant modes. Dielectric disk and spherical particle resonators are considered as the great candidates for establishing the dipole moments (metamaterial alphabet). A full wave finite difference time domain technique is applied to comprehensively obtain the physical insights of dielectric resonators. Near-field patterns are plotted to illustrate the development of electric and magnetic dipole fields. Geometric-polarization control of the dipole moments allows $\ensuremath{\epsilon}$ and $\ensuremath{\mu}$ to be tailored to the application of interest. All-dielectric double negative metamaterials are designed. Engineering concerns, such as loss reduction and bandwidth enhancement are investigated.

Compact dielectric particles as a building block for low-loss magnetic metamaterials.

Abstract: We characterize experimentally a compact dielectric particle that can be used to design very low-loss artificial electromagnetic materials (metamaterials). Focusing on magnetic media, we show that the particle can behave almost identically to the well-known split-ring resonators (SRRs) widely used in present designs, without suffering from the Ohmic losses that can limit the applicability of SRRs especially at high frequencies. We experimentally compare qualitatively and quantitatively the dielectric particle with a typical split-ring resonator of the same size built on a low-loss dielectric substrate and show that at GHz frequencies the quality factor of the dielectric particle is more than 3 times bigger than that of its metallic counterpart. Low-loss and simple geometry are significant advantages compared to conventional metal SRRs.

Higher order asymptotic homogenization and wave propagation in periodic composite materials

Abstract: We present an application of the higher order asymptotic homogenization method (AHM) to the study of wave dispersion in periodic composite materials. When the wavelength of a travelling signal becomes comparable with the size of heterogeneities, successive reflections and refractions of the waves at the component interfaces lead to the formation of a complicated sequence of the pass and stop frequency bands. Application of the AHM provides a long-wave approximation valid in the low-frequency range. Solution for the high frequencies is obtained on the basis of the Floquet–Bloch approach by expanding spatially varying properties of a composite medium in a Fourier series and representing unknown displacement fields by infinite plane-wave expansions. Steadystate elastic longitudinal waves in a composite rod (one-dimensional problem allowing the exact analytical solution) and transverse anti-plane shear waves in a fibre-reinforced composite with a square lattice of cylindrical inclusions (two-dimensional problem) are considered. The dispersion curves are obtained, the pass and stop frequency bands are identified.

The interior penalty discontinuous Galerkin method for elastic wave propagation: grid dispersion

Abstract: SUMMARY Recently, there has been an increased interest in applying the discontinuous Galerkin method (DGM) to wave propagation. In this work, we investigate the applicability of the interior penalty DGM to elastic wave propagation by analysing it’s grid dispersion properties, with particular attention to the effect that different basis functions have on the numerical dispersion. We consider different types of basis functions that naturally yield a diagonal mass matrix. This is relevant to seismology because a diagonal mass matrix is tantamount to an explicit and efficient time marching scheme. We find that the Legendre basis functions that are traditionally used in the DGM introduce numerical dispersion and anisotropy. Furthermore, we find that using Lagrange basis functions along with the Gauss nodes has attractive advantages for numerical wave propagation.

Arbitrarily elliptical–cylindrical invisible cloaking

Abstract: Based on the idea of coordinate transformation (Pendry, Schurig and Smith 2006 Science 312 1780), arbitrarily elliptical–cylindrical cloaks are proposed and designed. The elliptical cloak, which is composed of inhomogeneous anisotropic metamaterials in an elliptical-shell region, will deflect incoming electromagnetic (EM) waves and guide them to propagate around the inner elliptical region. Such EM waves will return to their original propagation directions without distorting the waves outside the elliptical cloak. General formulations of the inhomogeneous and anisotropic permittivity and permeability tensors are derived for arbitrarily elliptical axis ratio k, which can also be used for the circular cloak when k = 1. Hence the elliptical cloaks can make a large range of objects invisible, from round objects (when k approaches 1) to long and thin objects (when k is either very large or very small). We also show that the material parameters in elliptical cloaking are singular at only two points, instead of on the whole inner circle for circular cloaking, which are much easier to be realized in actual applications. Full-wave simulations are given to validate the arbitrarily elliptical cloaking.

Seismic interferometry, surface waves and source distribution

Abstract: SUMMARY Seismic interferometry can be used to estimate interreceiver surface wave signals by cross-correlation of signals recorded at each receiver. The quality of the estimated surface waves is controlled by the distribution of sources exciting the cross-correlated wavefields, and it is commonly thought that only sources at or near the surface are required to generate accurate estimates. We study the role of source distribution in surface wave interferometry for both surface and subsurface sources using surface wave Green's functions for laterally homogeneous media. We solve the interferometric integral using a Rayleigh wave orthogonality relationship combined with a stationary phase approach. Contrary to popular opinion we find that sources at depth do indeed play a role in the recovery of surface waves by interferometry. We find that interferometry performs well when surface sources are distributed homogeneously at the surface of the Earth. However, when this homogeneous distribution is not available amplitude errors are introduced, and when multiple modes are present strong spurious events appear and higher mode surface waves may not be correctly estimated. In order to recover higher mode surface waves we propose an additional step in the processing of surface wave data for seismic interferometry: by separating modes and applying interferometry to each mode individually it is possible to recover the interreceiver surface wave modes, without the artefacts introduced by limited source coverage.

Exothermic reaction waves in multilayer nanofilms

Abstract: Experimental and theoretical studies on heterogeneous exothermic reaction waves in multilayer nanofilms are analysed. Mathematical models for the reaction wave propagation are described. The dynamics of phase and structural transformations during heterogeneous reactions in nanoscale systems is considered. Prospects of the studies of reaction waves for the elucidation of the mechanisms of processes in nanoscale systems and for diverse practical applications (in particular, for the development of new welding and soldering techniques) are demonstrated.