Showing papers on "Plane wave published in 2014"

Surface Wave Methods for Near-Surface Site Characterization

Abstract: Overview of surface wave methods Seismic waves Test methodology Historical perspective Challenges of surface wave methods Typical applications Advantages and limitations Linear wave propagation in verticallyinhomogeneous continua Basic notions of wave propagation Rayleigh waves in homogeneous elastic half-spaces Existence of Love waves Surface waves in vertically inhomogeneouselastic continua Surface waves in vertically inhomogeneous, inelastic continua Measurement of surface waves Seismic data acquisition The wave field as a signal in time and space Acquisition of digital seismic signals Acquisition of surface waves Equipment Dispersion analysis Phase and group velocity Steady-state method Spectral analysis of surface waves Multi-offset phase analysis Spatial autocorrelation Transform-based methods Group velocity analysis Errors and uncertainties in dispersion analyses Attenuation analysis Attenuation of surface waves Univariate regression of amplitude versus offset data Transfer function technique and complex wavenumbers Multichannel multimode complex wavenumber estimation Other simplified approaches Uncertainty in the attenuation measurement Inversion Conceptual issues Forward modeling Surface wave inversion by empirical methods Surface wave inversion by analytical methods Uncertainty Case histories Comparison among processing techniques with active-source methods Comparison among inversion strategies Examples for determining Vs and Ds profiles Dealing with higher modes Surface wave inversion of seismic reflection data Advanced surface wave methods Love waves Offshore and nearshore surface wave testing Joint inversion with other geophysical data Passive seismic interferometry Multicomponent surface wave analysis, polarization studies, and horizontal-to-vertical spectral ratio References Index

High-contrast ultrafast imaging of the heart

Abstract: Noninvasive ultrafast imaging of intrinsic waves such as electromechanical waves or remotely induced shear waves in elastography imaging techniques for human cardiac applications remains challenging. In this paper, we propose ultrafast imaging of the heart with adapted sector size by coherently compounding diverging waves emitted from a standard transthoracic cardiac phased-array probe. As in ultrafast imaging with plane wave coherent compounding, diverging waves can be summed coherently to obtain high-quality images of the entire heart at high frame rate in a full field of view. To image the propagation of shear waves with a large SNR, the field of view can be adapted by changing the angular aperture of the transmitted wave. Backscattered echoes from successive circular wave acquisitions are coherently summed at every location in the image to improve the image quality while maintaining very high frame rates. The transmitted diverging waves, angular apertures, and subaperture sizes were tested in simulation, and ultrafast coherent compounding was implemented in a commercial scanner. The improvement of the imaging quality was quantified in phantoms and in one human heart, in vivo. Imaging shear wave propagation at 2500 frames/s using 5 diverging waves provided a large increase of the SNR of the tissue velocity estimates while maintaining a high frame rate. Finally, ultrafast imaging with 1 to 5 diverging waves was used to image the human heart at a frame rate of 4500 to 900 frames/s over an entire cardiac cycle. Spatial coherent compounding provided a strong improvement of the imaging quality, even with a small number of transmitted diverging waves and a high frame rate, which allows imaging of the propagation of electromechanical and shear waves with good image quality.

Optical Huygens' Metasurfaces with Independent Control of the Magnitude and Phase of the Local Reflection Coefficients

Abstract: Implementation of abrupt phase discontinuities along a surface has been the theme of recent research on electromagnetic metasurfaces. Simple functionalities such as reflecting, refracting, or focusing plane waves have been demonstrated with devices featuring phase discontinuities, but optical surfaces allowing independent magnitude and phase control on the scattered waves have yet to emerge. In this paper, we propose the first true optical Huygens’ surface, which explicitly utilizes orthogonal electric and magnetic responses to realize total control on an optical surface’s local reflection coefficients. This extends the functionality of metasurfaces to an unprecedented level. We first demonstrate that a nanorod gap-surface plasmon resonator can act as a Huygens’ source. Thereafter, by properly tuning and rotating these resonators, we realize arbitrary reflection optical metasurfaces—surfaces for which the local reflection coefficients can be independently tailored in both magnitude and phase. We demonstrate the versatility of this approach through designs of a metasurface that asymmetrically reflects two copolarized beams and a Dolph-Tschebyscheff optical reflectarray.

Reflection and transmission of plane waves at surfaces carrying material properties and embedded in second-gradient materials

Abstract: In this paper reflection and transmission of compression and shear waves at structured interfaces between second-gradient continua is investigated. Two semi-infinite spaces filled with the same sec...

Nanostructure arrays in free-space: optical properties and applications

Abstract: Dielectric and metallic gratings have been studied for more than a century. Nevertheless, novel optical phenomena and fabrication techniques have emerged recently and have opened new perspectives for applications in the visible and infrared domains. Here, we review the design rules and the resonant mechanisms that can lead to very efficient light–matter interactions in sub-wavelength nanostructure arrays. We emphasize the role of symmetries and free-space coupling of resonant structures. We present the different scenarios for perfect optical absorption, transmission or reflection of plane waves in resonant nanostructures. We discuss the fabrication issues, experimental achievements and emerging applications of resonant nanostructure arrays.

Scintillations of optical plane and spherical waves in underwater turbulence.

Abstract: The scintillation indices of optical plane and spherical waves propagating in underwater turbulent media are evaluated by using the Rytov method, and the variations in the scintillation indices are investigated when the rate of dissipation of mean squared temperature, the temperature and salinity fluctuations, the propagation distance, the wavelength, the Kolmogorov microscale length, and the rate of dissipation of the turbulent kinetic energy are varied. Results show that as in the atmosphere, also in underwater media the plane wave is more affected by turbulence as compared to the spherical wave. The underwater turbulence effect becomes significant at 5-10 m for a plane wave and at 20-25 m for a spherical wave. The turbulence effect is relatively small in deep water and is large at the surface of the water. Salinity-induced turbulence strongly dominates the scintillations compared to temperature-induced turbulence.

Plasma Physics: An Introduction

Abstract: Introduction What is Plasma? Brief History of Plasma Physics Fundamental Parameters Plasma Frequency Debye Shielding Plasma Parameter Collisions Magnetized Plasmas Plasma Beta DeBroglie Wavelength Exercises Charged Particle Motion Introduction Motion in Uniform Fields Method of Averaging Guiding Center Motion Magnetic Drifts Invariance of Magnetic Moment Poincar'e Invariants Adiabatic Invariants Magnetic Mirrors Van Allen Radiation Belts Equatorial Ring Current Second Adiabatic Invariant Third Adiabatic Invariant Motion in Oscillating Fields Exercises Collisions Introduction Collision Operator Two-Body Elastic Collisions Boltzmann Collision Operator Collisional Conservation Laws Boltzmann H-Theorem Two-Body Coulomb Collisions Rutherford Scattering Cross-Section Landau Collision Operator Coulomb Logarithm Rosenbluth Potentials Collision Times Exercises Plasma Fluid Theory Introduction Moments of Distribution Function Moments of Collision Operator Moments of Kinetic Equation Fluid Equations Entropy Production Fluid Closure Chapman-Enskog Closure Normalization of Neutral Gas Equations Braginskii Equations Normalization of Braginskii Equations Cold-Plasma Equations MHD Equations Drift Equations Closure in Collisionless Magnetized Plasmas Langmuir Sheaths Exercises Waves in Cold Plasmas Introduction Plane Waves in homogeneous Plasmas Cold-Plasma Dielectric Permittivity Cold-Plasma Dispersion Relation Wave Polarization Cutoff and Resonance Waves in Unmagnetized Plasmas Low-Frequency Wave Propagation Parallel Wave Propagation Perpendicular Wave Propagation Exercises Wave Propagation Through Inhomogeneous Plasmas Introduction WKB Solutions Cutoffs Resonances Resonant Layers Collisional Damping Pulse Propagation Ray Tracing Ionospheric Radio Wave Propagation Exercises Magnetohydrodynamic Fluids Introduction Magnetic Pressure Flux Freezing MHD Waves Solar Wind Parker Model of Solar Wind Interplanetary Magnetic Field Mass and Angular Momentum Loss MHD Dynamo Theory Homopolar Disk Dynamo Slow and Fast Dynamos Cowling Anti-Dynamo Theorem Ponomarenko Dynamo Magnetic Reconnection Linear Tearing Mode Theory Nonlinear Tearing Mode Theory Fast Magnetic Reconnection MHD Shocks Parallel MHD Shocks Perpendicular MHD Shocks Oblique MHD Shocks Exercises Waves in Warm Plasmas Introduction Landau Damping Physics of Landau Damping Plasma Dispersion Function Ion Acoustic Waves Waves in Magnetized Plasmas Parallel Wave Propagation Perpendicular Wave Propagation Electrostatic Waves Velocity-Space Instabilities Counter-Propagating Beam Instability Current-Driven Ion Acoustic Instability Harris Instability Exercises Bibliography Index

Vacuum refractive indices and helicity flip in strong-field QED

Abstract: Vacuum birefringence is governed by the amplitude for a photon to flip helicity or polarization state in an external field. Here, we calculate the flip and nonflip amplitudes in arbitrary plane wave backgrounds, along with the induced spacetime-dependent refractive indices of the vacuum. We compare the behavior of the amplitudes in the low energy and high energy regimes, and analyze the impact of pulse shape and energy. We also provide the first lightfront-QED derivation of the coefficients in the Heisenberg-Euler effective action.

Acoustic interaction forces between small particles in an ideal fluid

Abstract: We present a theoretical expression for the acoustic interaction force between small spherical particles suspended in an ideal fluid exposed to an external acoustic wave. The acoustic interaction force is the part of the acoustic radiation force on one given particle involving the scattered waves from the other particles. The particles, either compressible liquid droplets or elastic microspheres, are considered to be much smaller than the acoustic wavelength. In this so-called Rayleigh limit, the acoustic interaction forces between the particles are well approximated by gradients of pair-interaction potentials with no restriction on the interparticle distance. The theory is applied to studies of the acoustic interaction force on a particle suspension in either standing or traveling plane waves. The results show aggregation regions along the wave propagation direction, while particles may attract or repel each other in the transverse direction. In addition, a mean-field approximation is developed to describe the acoustic interaction force in an emulsion of oil droplets in water.

Tunneling of obliquely incident waves through PT -symmetric epsilon-near-zero bilayers

Abstract: We show that obliquely incident, transversely magnetic-polarized plane waves can be totally transmitted (with zero reflection) through epsilon-near-zero (ENZ) bilayers characterized by balanced loss and gain with parity-time ($\mathcal{PT}$) symmetry. This tunneling phenomenon is mediated by the excitation of a surface wave localized at the interface separating the loss and gain regions. We determine the parameter configurations for which the phenomenon may occur and, in particular, the relationship between the incidence direction and the electrical thickness. We show that, below a critical threshold of gain and loss, there always exists a tunneling angle which, for moderately thick (wavelength-sized) structures, approaches a critical value dictated by the surface-wave phase-matching condition. We also investigate the unidirectional character of the tunneling phenomenon, as well as the possible onset of spontaneous symmetry breaking, typical of $\mathcal{PT}$-symmetric systems. Our results constitute an interesting example of a $\mathcal{PT}$-symmetry-induced tunneling phenomenon, and may open up intriguing venues in the applications of ENZ materials featuring loss and gain.

Real-time vector velocity assessment through multigate doppler and plane waves

Abstract: Several ultrasound (US) methods have been recently proposed to produce 2-D velocity vector fields with high temporal and spatial resolution. However, the real-time implementation in US scanners is heavily hampered by the high calculation power required. In this work, we report a real-time vector Doppler imaging method which has been integrated in an open research system. The proposed approach exploits the plane waves transmitted from two sub-arrays of a linear probe to estimate the velocity vectors in 512 sample volumes aligned along the probe axis. The method has been tested for accuracy and reproducibility through simulations and in vitro experiments. Simulations over a 0° to 90° angle range of a 0.5 m/s peak parabolic flow have yielded 0.75° bias and 1.1° standard deviation for direction measurement, and 0.6 cm/s bias with 3.1% coefficient of variation for velocity assessment. In vitro tests have supported the simulation results. Preliminary measurements on the carotid artery of a volunteer have highlighted the real-time system capability of imaging complex flow configurations in an intuitive, easy, and quick way, as shown in a sample supplementary movie. These features have allowed reproducible peak velocity measurements to be obtained, as needed for quantitative investigations on patients.

Directional fluorescence emission by individual V-antennas explained by mode expansion.

Abstract: Specially designed plasmonic antennas can, by far-field interference of different antenna elements or a combination of multipolar antenna modes, scatter light unidirectionally, allowing for directional light control at the nanoscale. One of the most basic and compact geometries for such antennas is a nanorod with broken rotational symmetry, in the shape of the letter V. In this article, we show that these V-antennas unidirectionally scatter the emission of a local dipole source in a direction opposite the undirectional side scattering of a plane wave. Moreover, we observe high directivity, up to 6 dB, only for certain well-defined positions of the emitter relative to the antenna. By employing a rigorous eigenmode expansion analysis of the V-antenna, we fully elucidate the fundamental origin of its directional behavior. All findings are experimentally verified by measuring the radiation patterns of a scattered plane wave and the emission pattern of fluorescently doped PMMA positioned in different regions a...

Rogue wave modes for a derivative nonlinear Schrödinger model.

Abstract: Rogue waves in fluid dynamics and optical waveguides are unexpectedly large displacements from a background state, and occur in the nonlinear Schrodinger equation with positive linear dispersion in the regime of positive cubic nonlinearity. Rogue waves of a derivative nonlinear Schrodinger equation are calculated in this work as a long-wave limit of a breather (a pulsating mode), and can occur in the regime of negative cubic nonlinearity if a sufficiently strong self-steepening nonlinearity is also present. This critical magnitude is shown to be precisely the threshold for the onset of modulation instabilities of the background plane wave, providing a strong piece of evidence regarding the connection between a rogue wave and modulation instability. The maximum amplitude of the rogue wave is three times that of the background plane wave, a result identical to that of the Peregrine breather in the classical nonlinear Schrodinger equation model. This amplification ratio and the resulting spectral broadening arising from modulation instability correlate with recent experimental results of water waves. Numerical simulations in the regime of marginal stability are described.

Photons that travel in free space slower than the speed of light

Abstract: That the speed of light in free space is constant is a cornerstone of modern physics. However, light beams have finite transverse size, which leads to a modification of their wavevectors resulting in a change to their phase and group velocities. We study the group velocity of single photons by measuring a change in their arrival time that results from changing the beam's transverse spatial structure. Using time-correlated photon pairs we show a reduction of the group velocity of photons in both a Bessel beam and photons in a focused Gaussian beam. In both cases, the delay is several microns over a propagation distance of the order of 1 m. Our work highlights that, even in free space, the invariance of the speed of light only applies to plane waves. Introducing spatial structure to an optical beam, even for a single photon, reduces the group velocity of the light by a readily measurable amount.

Enhanced Magnetic and Electric Fields via Fano Resonances in Metasurfaces of Circular Clusters of Plasmonic Nanoparticles

Abstract: We investigate for the first time the capacity of a two-dimensional periodic array (a metasurface) of circular nanoclusters (CNCs) of plasmonic nanoparticles to support magnetic Fano resonances. These resonances are characterized by narrow angular and/or spectral features in the reflection/transmission/absorption coefficients associated with a circular disposition of nanoparticles’ dipole moments (forming a current loop) under oblique TE-polarized plane wave incidence illumination. We find that these narrow resonant features are either array-induced or single-CNC-induced, as shown by using a theoretical analysis based on the single dipole approximation and full-wave simulations, leading to enhanced magnetic and electric fields. In particular, array-induced resonances are narrower than single-CNC-induced ones and also provide even larger field enhancements, in particular generating a magnetic field enhancement of about 10-fold and an electric field enhancement of about 40-fold for a representative metasurf...

Tailoring Reflections From Thin Composite Metamirrors

Abstract: We propose an effective route to fully control the phase of plane waves reflected from electrically (optically) thin composite sheets. This becomes possible using engineered artificial full-reflection layers (metamirrors) formed by arrays of electrically small resonant bi-anisotropic particles. In this scenario, fully reflecting mirrors do not contain any continuous ground plane, but only arrays of small particles. Bi-anisotropic omega coupling is required to get asymmetric response in reflection phase for plane waves incident from the opposite sides of the composite mirror. It is shown that with this concept one can independently tailor the phase of electromagnetic waves reflected from both sides of the mirror array.

The tunneling model of laser-induced ionization and its failure at low frequencies

Abstract: The tunneling model of ionization applies only to longitudinal fields: quasistatic electric fields that do not propagate. Laser fields are transverse: plane wave fields that possess the ability to propagate. Although there is an approximate connection between the effects of longitudinal and transverse fields in a useful range of frequencies, that equivalence fails completely at very low frequencies. Insight into this breakdown is given by an examination of radiation pressure, which is a unique transverse-field effect whose relative importance increases rapidly as the frequency declines. Radiation pressure can be ascribed to photon momentum, which does not exist for longitudinal fields. Two major consequences are that the near-universal acceptance of a static electric field as the zero frequency limit of a laser field is not correct; and that the numerical solution of the dipole-approximate Schrodinger equation for laser effects is inapplicable as the frequency declines. These problems occur because the magnetic component of the laser field is very important at low frequencies, and hence the dipole approximation is not valid. Some experiments already exist that demonstrate the failure of tunneling concepts at low frequencies.

The tunnelling model of laser-induced ionization and its failure at low frequencies

Abstract: The tunnelling model of ionization applies only to longitudinal fields: quasistatic electric fields that do not propagate. Laser fields are transverse: plane wave fields that possess the ability to propagate. Although there is an approximate connection between the effects of longitudinal and transverse fields in a useful range of frequencies, that equivalence fails completely at very low frequencies. Insight into this breakdown is given by an examination of radiation pressure, which is a unique transverse-field effect whose relative importance increases rapidly as the frequency declines. Radiation pressure can be ascribed to photon momentum, which does not exist for longitudinal fields. Two major consequences are that the near-universal acceptance of a static electric field as the zero frequency limit of a laser field is not correct; and that the numerical solution of the dipole-approximate Schrodinger equation for laser effects is inapplicable as the frequency declines. These problems occur because the magnetic component of the laser field is very important at low frequencies, and hence the dipole approximation is not valid. Some experiments already exist that demonstrate the failure of tunnelling concepts at low frequencies.

Plane wave simulation of elastic-viscoplastic single crystals

Abstract: Despite the large amount of research that has been performed to quantify the high strain rate response of Aluminum, few studies have addressed effects of crystal orientation and subsequent crystal-level microstructure evolution on its high strain rate response. To study orientation effects in single crystal Al, both a constitutive model and novel numerical method have been developed. A plane wave formulation is developed so that materials undergoing anisotropic viscoplastic deformation can be modeled in a thermodynamically consistent framework. Then, a recently developed high strain rate viscoplastic model is extended to include single crystal effects by incorporating higher order crystal-based thermoelasticity, anisotropic plasticity kinetics, and distinguishing influences of forest and parallel dislocation densities. Steady propagating shock waves are simulated for [100], [110], and [111] oriented single crystals and compared to existing experimental wave profile and strength measurements. Finally, influences of initial orientation and peak pressure ranging from 0 to 30 GPa are quantified. Results indicate that orientation plays a significant role in dictating the high rate response of both the wave profile and the resultant microstructure evolution of Al. The plane wave formulation can be used to evaluate microstructure-sensitive constitutive relations in a computationally efficient framework.

Ultrarelativistic electron states in a general background electromagnetic field.

Abstract: The feasibility of obtaining exact analytical results in the realm of QED in the presence of a background electromagnetic field is almost exclusively limited to a few tractable cases, where the Dirac equation in the corresponding background field can be solved analytically. This circumstance has restricted, in particular, the theoretical analysis of QED processes in intense laser fields to within the plane wave approximation even at those high intensities, achievable experimentally only by tightly focusing the laser energy in space. Here, within the Wentzel-Kramers-Brillouin approximation, we construct analytically single-particle electron states in the presence of a background electromagnetic field of general space-time structure in the realistic assumption that the initial energy of the electron is the largest dynamical energy scale in the problem. The relatively compact expression of these states opens, in particular, the possibility of investigating analytically strong-field QED processes in the presence of spatially focused laser beams, which is of particular relevance in view of the upcoming experimental campaigns in this field.

Possible second-order nonlinear interactions of plane waves in an elastic solid.

Abstract: There exist ten possible nonlinear elastic wave interactions for an isotropic solid described by three constants of the third order. All other possible interactions out of 54 combinations (triplets) of interacting and resulting waves are prohibited, because of restrictions of various kinds. The considered waves include longitudinal and two shear waves polarized in the interacting plane and orthogonal to it. The amplitudes of scattered waves have simple analytical forms, which can be used for experimental setup and design. The analytic results are verified by comparison with numerical solutions of initial equations. Amplitude coefficients for all ten interactions are computed as functions of frequency for polyvinyl chloride, together with interaction and scattering angles. The nonlinear equation of motion is put into a general vector form and can be used for any coordinate system.

High-frame-rate 2-D vector blood flow imaging in the frequency domain.

Abstract: Conventional ultrasound Doppler techniques estimate the blood velocity exclusively in the axial direction to produce the sonograms and color flow maps needed for diagnosis of cardiovascular diseases. In this paper, a novel method to produce bi-dimensional maps of 2-D velocity vectors is proposed. The region of interest (ROI) is illuminated by plane waves transmitted at the pulse repetition frequency (PRF) in a fixed direction. For each transmitted plane wave, the backscattered echoes are recombined offline to produce the radio-frequency image of the ROI. The local 2-D phase shifts between consecutive speckle images are efficiently estimated in the frequency domain, to produce vector maps up to 15 kHz PRF. Simulations and in vitro steady-flow experiments with different setup conditions have been conducted to thoroughly evaluate the method's performance. Bias is proved to be lower than 10% in most simulations and lower than 20% in experiments. Further simulations and in vivo experiments have been made to test the approach's feasibility in pulsatile flow conditions. It has been estimated that the computation of the frequency domain algorithm is more than 50 times faster than the computation of the reference 2-D cross-correlation algorithm.

Effect of Periodicity in the Resonant Scattering of Light by Finite Sparse Configurations of Many Silver Nanowires

Abstract: We consider the two-dimensional (2-D) scattering of the H- and E-polarized plane waves by several discrete configurations made of M> > 1 periodically arranged circular cylindrical silver wires. To find the scattered field, we use the field Fourier expansions in local coordinates and addition theorems for cylindrical functions. Resulting M × M block-type matrix equation is cast to the Fredholm second-kind form that guarantees convergence of numerical solution when each block is truncated to finite dimensions and truncation order is taken larger. The scattering and absorption cross-sections and the near-field patterns are computed. The interplay of plasmon and grating-type resonances is studied for finite in-line and stacked arrays, corners, and crosses made of nano-size silver wires in the visible range of wavelengths, with the refractive index of silver taken from the experimental data.

Tailoring reflections from thin composite metamirrors

Abstract: We propose an effective route to fully control the phase of plane waves reflected from electrically (optically) thin sheets. This becomes possible using engineered artificial full-reflection layers (metamirrors) as arrays of electrically small resonant bi-anisotropic particles. In this scenario, fully reflecting mirrors do not contain any continuous ground plane, but only arrays of small particles. Bi-anisotropic omega coupling is required to get asymmetric response in reflection phase for plane waves incident from the opposite sides of the composite mirror. It is shown that with this concept one can independently tailor the phase of electromagnetic waves reflected from both sides of the mirror array.

Nonreciprocal electromagnetic wave propagation in one-dimensional ternary magnetized plasma photonic crystals

Abstract: In this paper, we propose a one-dimensional ternary photonic crystal based on magnetized plasma to obtain nonreciprocal propagation. By employing the transfer matrix method, the transmission spectra of the counterpropagating plane waves incident from air upon either end of the periodic structure are calculated. Our results reveal that there is a significant contrast between the transmittance of the waves propagated in opposite directions. This means that the structure shows nonreciprocal effects. It is shown that the bandwidth at which nonreciprocity is observed depends on the external magnetic field. The effects of the incident angle and the number of elementary cells on the nonreciprocal behaviors are studied. We demonstrate that nonreciprocity disappears in very small angles of incidence. The designed structure shows nonreciprocal response even in the case of a small number of layers. It is also demonstrated that nonreciprocal effects become stronger when increasing the plasma density and the wavelength of the incident wave.