Showing papers on "Group velocity published in 2016"
TL;DR: In this paper, a linear wave theory for the Madden-Julian oscillation (MJO) is extended upon in this study, where column moisture is the only prognostic variable and the horizontal wind is diagnosed as the forced Kelvin and Rossby wave responses to an equatorial heat source/sink.
Abstract: A linear wave theory for the Madden–Julian oscillation (MJO), previously developed by Sobel and Maloney, is extended upon in this study. In this treatment, column moisture is the only prognostic variable and the horizontal wind is diagnosed as the forced Kelvin and Rossby wave responses to an equatorial heat source/sink. Unlike the original framework, the meridional and vertical structure of the basic equations is treated explicitly, and values of several key model parameters are adjusted, based on observations. A dispersion relation is derived that adequately describes the MJO’s signal in the wavenumber–frequency spectrum and defines the MJO as a dispersive equatorial moist wave with a westward group velocity. On the basis of linear regression analysis of satellite and reanalysis data, it is estimated that the MJO’s group velocity is ~40% as large as its phase speed. This dispersion is the result of the anomalous winds in the wave modulating the mean distribution of moisture such that the moistur...
262 citations
TL;DR: It is shown that, with helical-structured acoustic metamaterials, it is now possible to implement dispersion-free sound deceleration and to turn a normally incident plane wave into a self-accelerating beam on the prescribed parabolic trajectory.
Abstract: The ability to slow down wave propagation in materials has attracted significant research interest. A successful solution will give rise to manageable enhanced wave-matter interaction, freewheeling phase engineering and spatial compression of wave signals. The existing methods are typically associated with constructing dispersive materials or structures with local resonators, thus resulting in unavoidable distortion of waveforms. Here we show that, with helical-structured acoustic metamaterials, it is now possible to implement dispersion-free sound deceleration. The helical-structured metamaterials present a non-dispersive high effective refractive index that is tunable through adjusting the helicity of structures, while the wavefront revolution plays a dominant role in reducing the group velocity. Finally, we numerically and experimentally demonstrate that the helical-structured metamaterials with designed inhomogeneous unit cells can turn a normally incident plane wave into a self-accelerating beam on the prescribed parabolic trajectory. The helical-structured metamaterials will have profound impact to applications in explorations of slow wave physics.
224 citations
TL;DR: The interplay between spin-orbit torques and the relativistic dynamics of antiferromagnetic domain walls leads to the efficient manipulation of antifierromagnetic spin textures and paves the way for the generation of high frequency signals from antiferromeagnets.
Abstract: We theoretically investigate the dynamics of antiferromagnetic domain walls driven by spin-orbit torques in antiferromagnet-heavy-metal bilayers. We show that spin-orbit torques drive antiferromagnetic domain walls much faster than ferromagnetic domain walls. As the domain wall velocity approaches the maximum spin-wave group velocity, the domain wall undergoes Lorentz contraction and emits spin waves in the terahertz frequency range. The interplay between spin-orbit torques and the relativistic dynamics of antiferromagnetic domain walls leads to the efficient manipulation of antiferromagnetic spin textures and paves the way for the generation of high frequency signals from antiferromagnets.
216 citations
TL;DR: In this article, a pseudo-high-order GVLVS with a two-humped pulse along one polarization and a single-hump pulse along the orthogonal polarization was obtained.
Abstract: Manipulation of group-velocity-locked vector solitons (GVLVSs) is numerically proposed and experimentally demonstrated. A pseudo-high-order GVLVS could be generated from a fundamental GVLVS with the help of a polarization-resolved system. Specifically, a pseudo-high-order GVLVS with a two-humped pulse along one polarization and a single-humped pulse along the orthogonal polarization could be obtained. The phase difference between the two humps could be 180°.
198 citations
TL;DR: In this paper, the dispersion relation between wave frequency and wave number is derived for single-walled carbon nanotubes, based on the formulated equation of wave motion, and the closed-form dispersion relations between the wave frequency (or phase velocity) and the wave number are derived.
159 citations
TL;DR: It is shown that epsilon-near-zero materials with a very low intrinsic dielectric loss do necessarily possess avery low group velocity of electromagnetic wave propagation, which leads to the loss function being singular and causes high nonradiative damping of the optical resonators and emitters embedded into them or placed at their surfaces.
Abstract: From the fundamental principle of causality we show that epsilon-near-zero (ENZ) materials with a very low (asymptotically zero) intrinsic dielectric loss do necessarily possess a very low (asymptotically zero) group velocity of electromagnetic wave propagation. This leads to the loss function being singular and causes high nonradiative damping of the optical resonators and emitters (plasmonic nanoparticles, quantum dots, chromophore molecules) embedded into them or placed at their surfaces. Rough ENZ surfaces do not exhibit hot spots of local fields suggesting that surface modes are overdamped. Reflectors and waveguides also show very large losses both for realistic and idealized ENZ materials.
131 citations
TL;DR: In this paper, the authors considered the reflection and transmission of waves at a plane interface between a homogeneous elastic half-space and a halfspace of elastic material that is periodically laminated.
Abstract: This work is concerned with the reflection and transmission of waves at a plane interface between a homogeneous elastic half-space and a half-space of elastic material that is periodically laminated. The lamination is always in the direction of the x1-coordinate axis and the displacement is always longitudinal shear, so that the only non-zero displacement component is u 3 ( x 1 , x 2 , t ) . After an initial discussion of Floquet–Bloch waves in the laminated material, brief consideration is given to the reflection–transmission problem, when the interface between the two media is the plane x 1 = 0 . Nothing unusual emerges: there are just a single reflected wave and a single transmitted wave, undergoing positive group-velocity refraction. Then, the problem is considered when the interface between the two media is the plane x 2 = 0 . The periodic structure of the interface induces an infinite set of reflected waves and an infinite set of transmitted waves. All need to be taken into account, but most decay exponentially away from the interface. It had previously been recognized that, if the incident wave had appropriate frequency and angle of incidence, a propagating transmitted wave would be generated that would undergo negative group-velocity refraction – behaviour usually associated with a metamaterial. It is established by an example in this work that there is, in addition, a propagating transmitted wave with smaller wavelength but larger group velocity that undergoes positive group-velocity refraction. The work concludes with a brief discussion of this finding, including its implications for the utility (or not) of “effective medium” theory.
65 citations
TL;DR: It is confirmed that using the ACT/LDV system and implementing simple Snell's law method is highly sensitive and effective in characterizing the dispersion curves of Lamb waves in composite structures as well as its angular dependency.
64 citations
TL;DR: In this article, the authors considered the fiber-optical analog of the event horizon, where intense light pulses in fibers establish horizons for probe light, and they found that the Hawking radiation is peaked around group-velocity horizons in which the speed of the pulse matches the group velocity of the probe light.
Abstract: Hawking radiation has been regarded as a more general phenomenon than in gravitational physics, in particular in laboratory analogs of the event horizon. Here we consider the fiber-optical analog of the event horizon, where intense light pulses in fibers establish horizons for probe light. Then, we calculate the Hawking spectrum in an experimentally realizable system. We found that the Hawking radiation is peaked around group-velocity horizons in which the speed of the pulse matches the group velocity of the probe light. The radiation nearly vanishes at the phase horizon where the speed of the pulse matches the phase velocity of light.
63 citations
TL;DR: The properties of the chaotic wave fields generated in the frame of the Kundu-Eckhaus equation (KEE) are studied and it is showed that the probability of rogue wave occurrence significantly depends on the quintic and the Raman-effect nonlinear terms of the KEE.
Abstract: In this paper we study the properties of the chaotic wave fields generated in the frame of the Kundu-Eckhaus equation (KEE). Modulation instability results in a chaotic wave field which exhibits small-scale filaments with a free propagation constant, $k$. The average velocity of the filaments is approximately given by the average group velocity calculated from the dispersion relation for the plane-wave solution; however, direction of propagation is controlled by the $\ensuremath{\beta}$ parameter, the constant in front of the Raman-effect term. We have also calculated the probabilities of the rogue wave occurrence for various values of propagation constant $k$ and showed that the probability of rogue wave occurrence depends on $k$. Additionally, we have showed that the probability of rogue wave occurrence significantly depends on the quintic and the Raman-effect nonlinear terms of the KEE. Statistical comparisons between the KEE and the cubic nonlinear Schr\"odinger equation have also been presented.
60 citations
TL;DR: In this paper, a Bayesian Lamb wave-based damage localization approach is proposed for structural health monitoring of anisotropic composite materials. But the approach does not take into account the effects of the anisotropy on group velocity.
Abstract: This article focuses on Bayesian Lamb wave-based damage localization in structural health monitoring of anisotropic composite materials. A Bayesian framework is applied to take account of uncertainties from experimental time-of-flight measurements and angle-dependent group velocity within the composite material. An original parametric analytical expression of the direction dependence of group velocity is proposed and validated numerically and experimentally for anisotropic composite and sandwich plates. This expression is incorporated into time-of-arrival (ellipse-based) and time-difference-of-arrival (hyperbola-based) Bayesian damage localization algorithms. This way, the damage location and the group velocity profile are estimated jointly and a priori information is taken into consideration. The proposed algorithm is general as it allows us to take into account uncertainties within a Bayesian framework, and to model effects of anisotropy on group velocity. Numerical and experimental results obtained wit...
TL;DR: In this article, the authors review the relation between the model wavenumber buildup and the inversion process in full waveform inversion (FWI) and identify their individual influence on the data.
Abstract: A model of the earth can be described using a Fourier basis represented by its wavenumber content. In full-waveform inversion (FWI), the wavenumber description of the model is natural because our Born-approximation-based velocity updates are made up of wavefields. Our objective in FWI is to access all the model wavenumbers available in our limited aperture and bandwidth recorded data that are not yet accurately present in the initial velocity model. To invert for those model wavenumbers, we need to locate their imprint in the data. Thus, I review the relation between the model wavenumber buildup and the inversion process. Specifically, I emphasize a focus on the model wavenumber components and identified their individual influence on the data. Missing the energy for a single vertical low-model wavenumber from the residual between the true Marmousi model and some initial linearly increasing velocity model produced a worse least-squares fit to the data than the initial model itself, in which all the...
TL;DR: In this article, a Love-wave waveform inversion was used to estimate near-surface S-wave velocity of earth models, which can be applied to any kind of earth model.
Abstract: High-frequency surface-wave techniques are widely used to estimate S-wave velocity of near-surface materials. Surface-wave methods based on inversions of dispersion curves are only suitable to laterally homogeneous or smoothly laterally varying heterogeneous earth models due to the layered-model assumption during calculation of dispersion curves. Waveform inversion directly fits the waveform of observed data, and it can be applied to any kinds of earth models. We have used the Love-wave waveform inversion in the time domain to estimate near-surface S-wave velocity. We used the finite-difference method as the forward modeling method. The source effect was removed by the deconvolution technique, which made our method independent of the source wavelet. We defined the difference between the deconvolved observed and calculated waveform as the misfit function. We divided the model into different sizes of blocks depending on the resolution of the Love waves, and we updated the S-wave velocity of each blo...
TL;DR: A theoretical approach is taken into consideration to investigate Love wave propagation in a transversely isotropic piezoelectric layer on a piezomagnetic half-space and the numerical results clearly show the influence of different stacking sequences on dispersion curves and on magneto-electromechanical coupling factor.
TL;DR: In this article, the effects of fluid properties on the wave velocity and frequency of gas-liquid annular two-phase flow were investigated in 26 and 16-mm inner pipe diameters.
TL;DR: In this article, it is shown that the transverse expansion of the target may dominate over the group velocity effect in the case of tightly focused laser pulses and thin foils, and that the off-normal incidence of the laser on the target, due either to the experimental setup, or to the deformation of a target, will also lead to a limit on the maximum ion energy.
Abstract: Radiation pressure acceleration (RPA) is a highly efficient mechanism of laser-driven ion acceleration, with with near complete transfer of the laser energy to the ions in the relativistic regime. However, there is a fundamental limit on the maximum attainable ion energy, which is determined by the group velocity of the laser. The tightly focused laser pulses have group velocities smaller than the vacuum light speed, and, since they offer the high intensity needed for the RPA regime, it is plausible that group velocity effects would manifest themselves in the experiments involving tightly focused pulses and thin foils. However, in this case, finite spot size effects are important, and another limiting factor, the transverse expansion of the target, may dominate over the group velocity effect. As the laser pulse diffracts after passing the focus, the target expands accordingly due to the transverse intensity profile of the laser. Due to this expansion, the areal density of the target decreases, making it transparent for radiation and effectively terminating the acceleration. The off-normal incidence of the laser on the target, due either to the experimental setup, or to the deformation of the target, will also lead to establishing a limit on maximum ion energy.
TL;DR: In this article, a nonlocal strain gradient theory developed from nonlocal theory was proposed for carbon nanotube in which axial velocity and the velocity gradient effect were separately considered on the basis of newly proposed nonlocal gradient theory.
TL;DR: In this article, a new optical method for determining flow-velocity vector fields that allows investigations of unsteady and fast processes is described, based on measuring the displacements of turbulent structures, which are visualized in a light sheet, within a fixed time interval between consecutive video frames.
Abstract: A new optical method for determining flow-velocity vector fields that allows investigations of unsteady and fast processes is described. The method is based on measuring the displacements of turbulent structures, which are visualized in a light sheet, within a fixed time interval between consecutive video frames. The method was tested using measurements of pulsating-flow velocity. It was shown that the difference between the mean flow velocity in the investigated flow measured using this method and a hot-wire anemometer does not exceed 1%, while the deviation of the rms velocity pulsations is within 4.5%.
TL;DR: In this article, it is shown that the transverse expansion of the target may dominate over the group velocity effect in the case of tightly focused laser pulses and thin foils, and that the off-normal incidence of the laser on the target, due either to the experimental setup, or to the deformation of a target, will also lead to a limit on the maximum ion energy.
Abstract: Radiation pressure acceleration (RPA) is a highly efficient mechanism of laser-driven ion acceleration, with near complete transfer of the laser energy to the ions in the relativistic regime. However, there is a fundamental limit on the maximum attainable ion energy, which is determined by the group velocity of the laser. The tightly focused laser pulses have group velocities smaller than the vacuum light speed, and, since they offer the high intensity needed for the RPA regime, it is plausible that group velocity effects would manifest themselves in the experiments involving tightly focused pulses and thin foils. However, in this case, finite spot size effects are important, and another limiting factor, the transverse expansion of the target, may dominate over the group velocity effect. As the laser pulse diffracts after passing the focus, the target expands accordingly due to the transverse intensity profile of the laser. Due to this expansion, the areal density of the target decreases, making it transparent for radiation and effectively terminating the acceleration. The off-normal incidence of the laser on the target, due either to the experimental setup, or to the deformation of the target, will also lead to establishing a limit on maximum ion energy.
TL;DR: In this article, a volumetric 3-component velocimetry measurement system was developed to investigate the three-dimensional three-component flow field inside a 35mm mini-hydrocyclone.
TL;DR: The fifth order Korteweg--de Vries (KdV) equation is shown to be a universal model of Eulerian hydrodynamics with higher order dispersive effects and the long-time behavior of solutions for step-like initial data is classified.
Abstract: Dissipationless hydrodynamics regularized by dispersion describe a number of physical media including water waves, nonlinear optics, and Bose-Einstein condensates. As in the classical theory of hyperbolic equations where a non-convex flux leads to non-classical solution structures, a non-convex linear dispersion relation provides an intriguing dispersive hydrodynamic analogue. Here, the fifth order Korteweg-de Vries (KdV) equation, also known as the Kawahara equation, a classical model for shallow water waves, is shown to be a universal model of Eulerian hydrodynamics with higher order dispersive effects. Utilizing asymptotic methods and numerical computations, this work classifies the long-time behavior of solutions for step-like initial data. For convex dispersion, the result is a dispersive shock wave (DSW), qualitatively and quantitatively bearing close resemblance to the KdV DSW. For non-convex dispersion, three distinct dynamic regimes are observed. For small jumps, a perturbed KdV DSW with positive polarity and orientation is generated, accompanied by small amplitude radiation from an embedded solitary wave leading edge, termed a radiating DSW or RDSW. For moderate jumps, a crossover regime is observed with waves propagating forward and backward from the sharp transition region. For jumps exceeding a critical threshold, a new type of DSW is observed we term a translating DSW or TDSW. The TDSW consists of a traveling wave that connects a partial, non-monotonic, negative solitary wave at the trailing edge to an interior nonlinear periodic wave. Its speed, a generalized Rankine-Hugoniot jump condition, is determined by the far-field structure of the traveling wave. The TDSW is resolved at the leading edge by a harmonic wavepacket moving with the linear group velocity. The non-classical TDSW exhibits features common to both dissipative and dispersive shock waves.
TL;DR: In this paper, a generalized non-autonomous nonlinear Schrodinger equation with both external potential and phase modulation was investigated, which can be used to describe the nonautonomous soliton in the optical fiber.
DOI•
30 Nov 2016TL;DR: In this article, the influence of different parameters of the zero crossing technique on measurement of the phase velocity of A 0 mode of the Lamb waves is investigated in more details, and the optimal parameters and necessary steps for phase velocity measurement are presented also.
Abstract: The delay time estimation using zero-crossing technique is widely used in ultrasonic measurements. The measurements of the phase velocity of guided waves are more complicated due to the dispersion. Application of zero-crossing technique for the measurements of the phase velocities is more complicated due to changes of the waveform of the signals and limited ranges of the measurements base. The objective of the work presented was to investigate in more details the influence of different parameters of the zero – crossing technique on measurement of the phase velocity of A 0 mode of the Lamb waves. Using the signals obtained from the finite element modelling of Lamb waves in a 2 mm thickness aluminium plate it was demonstrated that an insufficient sampling frequency can lead to the errors in the phase velocity and corresponding frequency estimations. On the other hand it was shown also, that in order to obtain a higher equivalent sampling frequency it is reasonable to exploit interpolation. The optimal parameters of the zero-crossing technique and necessary steps for phase velocity measurement are presented also.
TL;DR: In this article, the effect of the decay rate of the cavity photons on the group velocity is studied and it is demonstrated that the light propagation switches from sub-luminal to superluminous just by changing the Rabi frequency of the pump field.
Abstract: The propagation of a probe laser field in a cavity optomechanical system with a Bose-Einstein condensate is studied. The transmission properties of the system are investigated and it is shown that the group velocity of the probe pulse field can be controlled by Rabi frequency of the pump laser field. The effect of the decay rate of the cavity photons on the group velocity is studied and it is demonstrated that for small values of the decay rates, the light propagation switches from subluminal to superluminal just by changing the Rabi frequency of the pump field. Then, the gain-assisted superluminal light propagation due to the cross-Kerr nonlinearity is established in cavity optomechanical system with a Bose-Einstein condensate. Such behavior can not appear in the pump-probe two-level atomic systems in the normal phase. We also find that the amplification is achieved without inversion in the population of the quantum energy levels.
TL;DR: In the presented work, analytical expressions are shown for the determination of the k-ω location of the zero group velocity Lamb modes as a function of the Poisson's ratio and an inverse problem is formulated to determine the unknown wave velocities in plates of known thickness.
TL;DR: In this paper, the authors investigated the effectiveness of using traffic noise for passive subsurface imaging and found that traffic-induced vibrations dominate the ambient seismic noise field at frequencies between 3 and 15 Hz.
Abstract: Using a dense seismic array in Long Beach, California, we have investigated the effectiveness of using traffic noise for passive subsurface imaging. Spectral analysis revealed that traffic-induced vibrations dominate the ambient seismic noise field at frequencies between 3 and 15 Hz. Using the ambient-noise crosscorrelation technique at these frequencies, we have extracted fundamental- and first-order-mode Rayleigh waves generated by Interstate 405 and local roads. We picked group traveltimes associated with the fundamental mode and used them in a straight-ray tomography procedure to produce group velocity maps at 3.0 and 3.5 Hz. The velocity trends in our results corresponded to shallow depths and coincided well with lithologies outlined in a geologic map of the survey area. The most prominent features resolved in our velocity maps were the low velocities to the north corresponding to less-consolidated materials, high velocities to the south corresponding to more-consolidated materials, a low-vel...
TL;DR: In this article, the authors studied linear wave propagation in nonlinear hexagonal lattices capable of undergoing large deformations, under different levels of pre-load, and derived analytical expressions for the stiffness of a unit cell in the deformed configuration.
Abstract: We study linear wave propagation in nonlinear hexagonal lattices capable of undergoing large deformations, under different levels of pre-load. The lattices are composed of a set of masses connected by linear axial and angular springs, with the nonlinearity arising solely from geometric effects. By applying different levels of pre-load, the small amplitude linear wave propagation response can be varied from isotropic to highly directional. Analytical expressions for the stiffness of a unit cell in the deformed configuration are derived and they are used to analyze the dispersion surfaces and group velocity variation with pre-load. Numerical simulations on finite lattices demonstrate the validity of our unit cell predictions and illustrate the wave steering potential of our lattice.
TL;DR: In this paper, the influence of point defect modes on the group velocity of flexural waves in a phononic crystal Timoshenko beam was investigated using the transfer matrix method with a supercell technique.
TL;DR: In this article, the authors investigated coherent spin-wave generation by focused ultrashort laser pulse irradiation for a permalloy thin film at micrometer scale using an all-optical space and time-resolved magneto-optically Kerr effect.
Abstract: Coherent spin-wave generation by focused ultrashort laser pulse irradiation was investigated for a permalloy thin film at micrometer scale using an all-optical space and time-resolved magneto-optical Kerr effect. The spin-wave packet propagating perpendicular to magnetization direction was clearly observed, however that propagating parallel to the magnetization direction was not observed. The propagation length, group velocity, center frequency, and packet-width of the observed spin-wave packet were evaluated and quantitatively explained in terms of the propagation of a magneto-static spin-wave driven by ultrafast change of an out-of-plane demagnetization field induced by the focused-pulse laser.
TL;DR: In this paper, the authors theoretically demonstrate that graphene-insulator-graphene tunnel structures can serve as plasmonic gain media due to the possibility of stimulated electron tunneling accompanied by emission of plasmons under application of interlayer voltage.
Abstract: We theoretically demonstrate that graphene-insulator-graphene tunnel structures can serve as plasmonic gain media due to the possibility of stimulated electron tunneling accompanied by emission of plasmons under application of interlayer voltage. The probability of plasmon-assisted tunneling is resonantly large at certain values of frequency and interlayer voltage corresponding to the transitions between chiral electron states with collinear momenta, which is a feature unique to the linear bands of graphene. The plasmon dispersion develops an anticrossing with the resonances in tunnel conductivity and demonstrates negative group velocity in several frequency ranges.