Showing papers on "Electromagnetic field published in 2012"

Event-by-event generation of electromagnetic fields in heavy-ion collisions

Abstract: We compute the electromagnetic fields generated in heavy-ion collisions by using the HIJING model. Although after averaging over many events only the magnetic field perpendicular to the reaction plane is sizable, we find very strong electric and magnetic fields both parallel and perpendicular to the reaction plane on the event-by-event basis. We study the time evolution and the spatial distribution of these fields. Especially, the electromagnetic response of the quark-gluon plasma can give non-trivial evolution of the electromagnetic fields. The implications of the strong electromagnetic fields on the hadronic observables are also discussed.

Experimental Realization of a Magnetic Cloak

Abstract: Invisibility to electromagnetic fields has become an exciting theoretical possibility. However, the experimental realization of electromagnetic cloaks has only been achieved starting from simplified approaches (for instance, based on ray approximation, canceling only some terms of the scattering fields, or hiding a bulge in a plane instead of an object in free space). Here, we demonstrate, directly from Maxwell equations, that a specially designed cylindrical superconductor-ferromagnetic bilayer can exactly cloak uniform static magnetic fields, and we experimentally confirmed this effect in an actual setup.

Observation and differentiation of unique high-Q optical resonances near zero wave vector in macroscopic photonic crystal slabs.

Abstract: We demonstrate and distinguish experimentally the existence of a special type of Fano resonances at k≈0 in a macroscopic two-dimensional photonic crystal slab. We fabricate a square lattice array of holes in a silicon nitride layer and perform an angular resolved spectral analysis of the various Fano resonances. We elucidate their radiation behavior using temporal coupled-mode theory and symmetry considerations. The unique simplicity of this system whereby an ultralong lifetime delocalized electromagnetic field can exist above the surface and consequently easily interact with added matter, provides exciting new opportunities for the study of light and matter interaction.

Chiral anomaly and local polarization effect from the quantum kinetic approach.

Abstract: A power expansion scheme is set up to determine the Wigner function that satisfies the quantum kinetic equation for spin-$1/2$ charged fermions in a background electromagnetic field. Vector and axial-vector current induced by magnetic field and vorticity are obtained simultaneously from the Wigner function. The chiral magnetic and vortical effect and chiral anomaly are shown as natural consequences of the quantum kinetic equation. The axial-vector current induced by vorticity is argued to lead to a local polarization effect along the vorticity direction in heavy-ion collisions.

Wireless energy transfer

Abstract: Electromagnetic energy transfer is facilitated. In accordance with an example embodiment, a first resonator transmits electromagnetic energy using an electromagnetic wave, based on frequency matching and alignment of an electromagnetic field with a second resonator within one wavelength of the electromagnetic wave distance from the first resonator. An electromagnetic energy reflector adjacent the first resonator redirects reflected portions of the electromagnetic wave back towards the first resonator circuit.

A Review of Field-to-Transmission Line Coupling Models With Special Emphasis to Lightning-Induced Voltages on Overhead Lines

Abstract: We discuss the transmission line (TL) theory and its application to the problem of lightning electromagnetic field coupling to TLs. We start with the derivation of the general field-to-TL coupling equations for the case of a single-wire line above a perfectly conducting ground. The derived equations are solely based on the thin-wire approximation and they do take into account high-frequency radiation effects. Under the TL approximation, the general equations reduce to the Agrawal et al. field-to-TL coupling equations. After a short discussion on the underlying assumptions of the TL theory, three seemingly different but completely equivalent approaches that have been proposed to describe the coupling of electromagnetic fields to TLs are presented. The derived equations are then extended to deal with the presence of losses and multiple conductors and expressions for the line parameters, including the ground impedance and admittance, are presented. The time-domain representation of the field-to-TL coupling equations, which allows for a straightforward treatment of nonlinear phenomena as well as the variation in the line topology, is also described. Solution methods in the frequency domain and in the time domain are given and application examples with reference to lightning-induced voltages are presented and discussed. Specifically, the effects of ground losses and corona are illustrated and discussed. When the traveling voltage and current waves are originated from lumped excitation sources located at a specific location along a TL (direct lightning strike), both the corona phenomenon and ground losses result, in general, in an attenuation and dispersion of propagating surges along TLs. However, when distributed sources representing the action of the electromagnetic field from a nearby lightning illuminating the line are present, ground losses and corona phenomenon could result in important enhancement of the induced voltage magnitude.

Chiral electromagnetic fields generated by arrays of nanoslits.

Abstract: Using a modal matching theory, we demonstrate the generation of short-range, chiral electromagnetic fields via the excitation of arrays of staggered nanoslits that are chiral in two dimensions. The electromagnetic near fields, which exhibit a chiral density greater than that of circularly polarized light, can enhance the chiroptical interactions in the vicinity of the nanoslits. We discuss the features of nanostructure symmetry required to obtain the chiral fields and explicitly show how these structures can give rise to detection and characterization of materials with chiral symmetry.

Non-Trivial Non-Radiating Excitation as a Mechanism of Resonant Transparency in Toroidal Metamaterials

Abstract: We demonstrate theoretically and confirm experimentally a new mechanism of resonant electromagnetic transparency, which yields extremely narrow isolated symmetric Lorentzian lines of full transmission in metamaterials. It exploits the long sought non-trivial non-radiating charge-current excitation based on toroidal dipole moment, predicted to generate waves of gauge-irreducible vector potential in the complete absence of scattered electromagnetic fields.

Electromagnetic energy transport in nanoparticle chains via dark plasmon modes.

Abstract: Using light to exchange information offers large bandwidths and high speeds, but the miniaturization of optical components is limited by diffraction. Converting light into electron waves in metals allows one to overcome this problem. However, metals are lossy at optical frequencies and large-area fabrication of nanometer-sized structures by conventional top-down methods can be cost-prohibitive. We show electromagnetic energy transport with gold nanoparticles that were assembled into close-packed linear chains. The small interparticle distances enabled strong electromagnetic coupling causing the formation of low-loss subradiant plasmons, which facilitated energy propagation over many micrometers. Electrodynamic calculations confirmed the dark nature of the propagating mode and showed that disorder in the nanoparticle arrangement enhances energy transport, demonstrating the viability of using bottom-up nanoparticle assemblies for ultracompact opto-electronic devices.

Regular models with quadratic equation of state

Abstract: We provide new exact solutions to the Einstein–Maxwell system of equations which are physically reasonable. The spacetime is static and spherically symmetric with a charged matter distribution. We utilise an equation of state which is quadratic relating the radial pressure to the energy density. Earlier models, with linear and quadratic equations of state, are shown to be contained in our general class of solutions. The new solutions to the Einstein–Maxwell are found in terms of elementary functions. A physical analysis of the matter and electromagnetic variables indicates that the model is well behaved and regular. In particular there is no singularity in the proper charge density at the stellar centre unlike earlier anisotropic models in the presence of the electromagnetic field.

Resource Letter EM-1: Electromagnetic Momentum

Abstract: This Resource Letter surveys the literature on momentum in electromagnetic fields, including the general theory, the relation between electromagnetic momentum and vector potential, “hidden” momentum, the 4/3 problem for electromagnetic mass, and the Abraham–Minkowski controversy regarding the field momentum in polarizable and magnetizable media.

Self-organized electromagnetic field structures in laser-produced counter-streaming plasmas

Abstract: Stable structures can self-assemble in plasmas flowing at supersonic speeds, as evident in many astronomical objects. But now it is also seen in the laboratory using two plasmas travelling in opposite directions, each created by ablating a plastic disc with high-power lasers.

Bulk synthesis of Janus objects and asymmetric patchy particles

Abstract: This review article highlights the most recent advances with respect to the preparation of Janus objects by bulk phase processes. Historically most of the concepts developed for generating asymmetric particles have been based on the use of interfaces or surfaces, which are necessary to break the symmetry. This restricts in many cases the amount of produced particles, due to the two-dimensional nature of the approaches. Therefore the bulk synthesis of such asymmetric micro- and nanoobjects is of primary importance for their production at an industrial scale and helps to open up the field to commercial applications. We summarize here the different alternative concepts, spanning a wide range from sophisticated polymer chemistry to the use of external electromagnetic fields, that have been proposed in recent years in order to break the symmetry in true bulk processes.

Intersubband polaritons in the electrical dipole gauge

Abstract: We provide a theoretical description of the coupling between the electromagnetic field and the intersubband excitations of a bidimensional electron gas. Our theory, based on the electrical dipole gauge, applies generally to an arbitrary quantum heterostructure embedded in a multilayer waveguide or in a microcavity. We show that the dipole gauge Hamiltonian takes into account the Coulomb interactions in the system, without the need of adding extra terms to the Hamilitonian. Furthermore, the bright excitations of the system appear as many-body collective plasmon modes, interacting between each other and with the light field through dipole coupling. The electrical dipole gauge therefore provides a suitable framework for the study of solid-state quantum electrodynamics phenomena that occur at very high electronic densities, such as the ultrastrong light-matter interaction.

Emergent electromagnetism in solids

Abstract: The electromagnetic field (EMF) is the most fundamental field in condensed-matter physics. Interaction between electrons, electron–ion interaction and ion–ion interaction are all of electromagnetic origin, while the other three fundamental forces, i.e. the gravitational force and weak and strong interactions, are irrelevant in the energy/length scales of condensed-matter physics. Also the physical properties of condensed matter, such as transport, optical, magnetic and dielectric properties, are almost described as their electromagnetic responses. In addition to this EMF, it often happens that the gauge fields appear as the emergent phenomenon in the low-energy sector due to the projection of the electronic wavefunctions onto the curved manifold of the Hilbert sub-space. These emergent EMFs play important roles in many places in condensed-matter physics including the quantum Hall effect, strongly correlated electrons and also in non-interacting electron systems. In this paper, we describe the fundamental idea behind it and some of its applications studied recently.

Equivalent Circuit for Electromagnetic Interaction and Transmission Through Graphene Sheets

Abstract: An equivalent circuit for the analysis of the interaction between an electromagnetic field and a thin graphene sheet is derived, based on a local anisotropic model of the graphene conductivity, valid at frequencies below the THz range. Due to the anisotropic properties, the equivalent circuit is a four-port network which couples the fundamental TE and TM polarizations. The possible effects of electrostatic and/or magnetostatic bias are included and the equivalent circuit is next used to investigate the shielding properties of graphene layers against impinging plane waves. While the possibility of tuning the graphene conductivity leads to interesting properties of electronic control of shielding perfomance, the proposed equivalent circuit represents a very simple tool for the relevant analysis and design.

Statistical aspects of ultracold resonant scattering

Abstract: Compared to purely atomic collisions, ultracold collisions involving molecules have the potential to support a much larger number of Fano-Feshbach resonances due to the huge amount of ro-vibrational states available. In order to handle such ultracold atom-molecule collisions, we formulate a theory that incorporates the ro-vibrational Fano-Feshbach resonances in a statistical manner while treating the physics of the long-range scattering, which is sensitive to such things as hyperfine states, collision energy, and any applied electromagnetic fields, exactly within multichannel quantum defect theory. Uniting these two techniques, we can assess the influence of highly resonant scattering in the threshold regime, and in particular its dependence on the hyperfine state selected for the collision. This allows us to explore the onset of Ericson fluctuations in the regime of overlapping resonances, which are well known in nuclear physics but completely unexplored in the ultracold domain.

Electric–magnetic symmetry and Noether's theorem

Abstract: In the absence of charges, Maxwell's equations are highly symmetrical. In particular, they place the electric and magnetic fields on equal footing. In light of this electric–magnetic symmetry, we introduce a variational description of the free electromagnetic field that is based upon the acknowledgement of both electric and magnetic potentials. We use our description, together with Noether's theorem, to demonstrate that electric–magnetic symmetry is, in essence, an expression of the conservation of optical helicity. The symmetry associated with the conservation of Lipkin's zilches is also identified. We conclude by considering, with care, the subtle separation of the rotation and boost angular momenta of the field into their 'spin' and 'orbital' contributions.

Quantum magnetomechanics with levitating superconducting microspheres.

Abstract: We show that by magnetically trapping a superconducting microsphere close to a quantum circuit, it is possible to perform ground-state cooling and prepare quantum superpositions of the center-of-mass motion of the microsphere. Due to the absence of clamping losses and time-dependent electromagnetic fields, the mechanical motion of micrometer-sized metallic spheres in the Meissner state is predicted to be very well isolated from the environment. Hence, we propose to combine the technology of magnetic microtraps and superconducting qubits to bring relatively large objects to the quantum regime.

Ideal current patterns yielding optimal signal‐to‐noise ratio and specific absorption rate in magnetic resonance imaging: Computational methods and physical insights

Abstract: At high and ultra-high magnetic field strengths, understanding interactions between tissues and the electromagnetic fields generated by radiofrequency (RF) coils becomes crucial for safe and effective coil design, as well as for insight into limits of performance. In this work we present a rigorous electrodynamic modeling framework, using dyadic Green’s functions, to derive the electromagnetic field in homogeneous spherical and cylindrical samples resulting from arbitrary surface currents in the presence or absence of a surrounding RF shield. We show how to calculate ideal current patterns which result in the highest possible signal to noise ratio (“ultimate intrinsic signal to noise ratio (SNR)”) or the lowest possible RF power deposition (“ultimate intrinsic specific absorption rate (SAR)”) compatible with electrodynamic principles. We identify familiar coil designs within optimal current patterns at low to moderate field strength, thereby establishing and explaining graphically the near-optimality of traditional surface and volume quadrature designs. We also document the emergence of less familiar patterns, e.g. involving substantial electric as well as magnetic dipole contributions, at high field strength. Performance comparisons with particular coil array configurations demonstrate that optimal performance may be approached with finite arrays if ideal current patterns are used as a guide for coil design.

Dirac cone in two- and three-dimensional metamaterials.

Abstract: It is shown by analytical calculation based on the tight-binding approximation that the isotropic Dirac cone in the Brillouin zone center can be created in two- and three-dimensional periodic metamaterials by accidental degeneracy of two modes. In the case of two dimensions, the combination of a doubly degenerate E mode and a non-degenerate A1 mode of the square lattice of the C4v symmetry is examined. For three dimensions, the combination of a triply degenerate T1u mode and a non-degenerate A1g mode of the cubic lattice of the Oh symmetry is examined. The secular equation of the electromagnetic field is derived and solved with detailed analysis of electromagnetic transfer integrals by group theory. This is the first theoretical prediction of the presence of the Dirac cone in the three-dimensional periodic structure.

Reducing the staircasing error in computational dosimetry of low-frequency electromagnetic fields.

Abstract: From extremely low frequencies to intermediate frequencies, the magnitude of induced electric field inside the human body is used as the metric for human protection. The induced electric field inside the body can be computed using anatomically realistic voxel models and numerical methods such as the finite-difference or finite-element methods. The computed electric field is affected by numerical errors that occur when curved boundaries with large contrasts in electrical conductivity are approximated using a staircase grid. In order to lessen the effect of the staircase approximation error, the use of the 99th percentile electric field, i.e. ignoring the highest 1% of electric field values, is recommended in the ICNIRP guidelines. However, the 99th percentile approach is not applicable to localized exposure scenarios where the majority of significant induced electric field values may be concentrated in a small volume. In this note, a method for removing the staircasing error is proposed. Unlike the 99th percentile, the proposed method is also applicable to localized exposure scenarios. The performance of the method is first verified by comparison with the analytical solution in a layered sphere. The method is then applied for six different exposure scenarios in two anatomically realistic human head models. The results show that the proposed method can provide conservative estimates for the 99th percentile electric field in both localized and uniform exposure scenarios.

Observations of electromagnetic whistler precursors at supercritical interplanetary shocks

Abstract: We present observations of electromagnetic precursor waves, identified as whistler mode waves, at supercritical interplanetary shocks using the Wind search coil magnetometer. The precursors propagate obliquely with respect to the local magnetic field, shock normal vector, solar wind velocity, and they are not phase standing structures. All are right-hand polarized with respect to the magnetic field (spacecraft frame), and all but one are right-hand polarized with respect to the shock normal vector in the normal incidence frame. They have rest frame frequencies f(sub ci) < f much < f(sub ce) and wave numbers 0.02 approx < k rho (sub ce) approx <. 5.0. Particle distributions show signatures of specularly reflected gyrating ions, which may be a source of free energy for the observed modes. In one event, we simultaneously observe perpendicular ion heating and parallel electron acceleration, consistent with wave heating/acceleration due to these waves. Al though the precursors can have delta B/B(sub o) as large as 2, fluxgate magnetometer measurements show relatively laminar shock transitions in three of the four events.

Near-field manipulation of spectroscopic selection rules on the nanoscale

Abstract: In conventional spectroscopy, transitions between electronic levels are governed by the electric dipole selection rule because electric quadrupole, magnetic dipole, and coupled electric dipole-magnetic dipole transitions are forbidden in a far field. We demonstrated that by using nanostructured electromagnetic fields, the selection rules of absorption spectroscopy could be fundamentally manipulated. We also show that forbidden transitions between discrete quantum levels in a semiconductor nanorod structure are allowed within the near-field of a noble metal nanoparticle. Atomistic simulations analyzed by an effective mass model reveal the breakdown of the dipolar selection rules where quadrupole and octupole transitions are allowed. Our demonstration could be generalized to the use of nanostructured near-fields for enhancing light-matter interactions that are typically weak or forbidden.

Plasmonic resonances in diffractive arrays of gold nanoantennas: near and far field effects

Abstract: We examine the excitation of plasmonic resonances in arrays of periodically arranged gold nanoparticles placed in a uniform refractive index environment. Under a proper periodicity of the nanoparticle lattice, such nanoantenna arrays are known to exhibit narrow resonances with asymmetric Fano-type spectral line shape in transmission and reflection spectra having much better resonance quality compared to the single nanoparticle case. Using numerical simulations, we first identify two distinct regimes of lattice response, associated with two-characteristic states of the spectra: Rayleigh anomaly and lattice plasmon mode. The evolution of the electric field pattern is rigorously studied for these two states revealing different configurations of optical forces: the first regime is characterized by the concentration of electric field between the nanoparticles, yielding to almost complete transparency of the array, whereas the second regime is characterized by the concentration of electric field on the nanoparticles and a strong plasmon-related absorption/scattering. We present electric field distributions for different spectral positions of Rayleigh anomaly with respect to the single nanoparticle resonance and optimize lattice parameters in order to maximize the enhancement of electric field on the nanoparticles. Finally, by employing collective plasmon excitations, we explore possibilities for electric field enhancement in the region between the nanoparticles. The presented results are of importance for the field enhanced spectroscopy as well as for plasmonic bio and chemical sensing.

The pulsar force‐free magnetosphere linked to its striped wind: time‐dependent pseudo‐spectral simulations

Abstract: Pulsar activity and its related radiation mechanism are usually explained by invoking some plasma processes occurring inside the magnetosphere, be it polar caps, outer/slot gaps or the transition region between the quasi-static magnetic dipole regime and the wave zone, like the striped wind. Despite many detailed local investigations, the global electrodynamics around those neutron stars remains poorly described with only little quantitative studies on the largest scales, i.e. of several light-cylinder radii rL. A better understanding of these compact objects requires a deep and accurate knowledge of their immediate electromagnetic surrounding within the magnetosphere and its link to the relativistic pulsar wind. This is compulsory to make any reliable predictions about the whole electric circuit, energy losses, sites of particle acceleration and the possibly associated emission mechanisms. The aim of this work is to present accurate solutions to the nearly stationary force-free pulsar magnetosphere and its link to the striped wind, for various spin periods and arbitrary inclination. To this end, the time-dependent Maxwell equations are solved in spherical geometry in the force-free approximation using a vector spherical harmonic expansion of the electromagnetic field. An exact analytical enforcement of the divergencelessness of the magnetic part is obtained by a projection method. Special care has been given to designing an algorithm able to look deeply into the magnetosphere with physically realistic ratios of stellar R* to light-cylinder rL radius. However, currently available computational resources allow us only to set R*/rL= 10−1 corresponding to pulsars with a period of 2 ms. The spherical geometry permits a proper and mathematically well-posed imposition of self-consistent physical boundary conditions on the stellar crust. We checked our code against several analytical solutions, like the Deutsch vacuum rotator solution and the Michel monopole field. We also retrieve energy losses comparable to the magnetodipole radiation formula and consistent with previous similar works. Finally, for arbitrary obliquity, we give an expression for the total electric charge of the system. It does not vanish except for the perpendicular rotator. This is due to the often ignored point charge located at the centre of the neutron star. It is questionable if such solutions with huge electric charges could exist in reality except for configurations close to an orthogonal rotator. The charge spread over the stellar crust is not a tunable parameter as often hypothesized.

New measurements of lightning electric fields in Florida: Waveform characteristics, interaction with the ionosphere, and peak current estimates

Abstract: [1] We analyzed wideband electric field waveforms of 265 first and 349 subsequent return strokes in negative natural lightning. The distances ranged from 10 to 330 km. Evolution of first- and subsequent-stroke field waveforms as a function of distance is examined. Statistics on the following field waveform parameters are given: initial electric field peak, opposite-polarity overshoot, ratio of the initial electric field peak to the opposite polarity overshoot, zero-to-peak risetime, initial half-cycle duration, and opposite polarity overshoot duration. The overwhelming majority of both first and subsequent return-stroke field waveforms at 50 to 330 km exhibit an opposite polarity overshoot. At distances greater than 100 km, electric field waveforms, recorded under primarily daytime conditions, tend to be oscillatory. Using finite difference time domain modeling, we interpreted the initial positive half-cycle and the opposite-polarity overshoot as the ground wave and the second positive half-cycle as the one-hop ionospheric reflection. The observed difference in arrival times of these two waves for subsequent strokes is considerably smaller than for first strokes, suggesting that the first-stroke electromagnetic field caused a descent of the ionospheric D-layer. We speculate that there may be cumulative effect of multiple strokes in lowering the ionospheric reflection height. Return-stroke peak currents estimated from the empirical formula, I = 1.5–0.037DE (where I is considered negative and in kA, E is the electric field peak considered positive and in V/m, and D is distance in km), are compared to those reported by the NLDN.

Bayesian inversion of marine CSEM data with a trans‐dimensional self parametrizing algorithm

Abstract: SUMMARY The posterior distribution of earth models that fit observed geophysical data convey information on the uncertainty with which they are resolved. From another perspective, the non-uniqueness inherent in most geophysical inverse problems of interest can be quantified by examining the posterior model distribution converged upon by a Bayesian inversion. In this work we apply a reversible jump Markov chain Monte Carlo method to sample the posterior model distribution for the anisotropic 1-D seafloor conductivity constrained by marine controlled source electromagnetic data. Unlike conventional gradient based inversion approaches, our algorithm does not require any subjective choice of regularization parameter, and it is self parametrizing and trans-dimensional in that the number of interfaces with a resistivity contrast at depth is variable, as are their positions. A synthetic example demonstrates how the algorithm can be used to appraise the resolution capabilities of various electromagnetic field components for mapping a thin resistive reservoir buried beneath anisotropic conductive sediments. A second example applies the method to survey data collected over the Pluto gas field on the Northwest Australian shelf. A benefit of our Bayesian approach is that subsets of the posterior model probabilities can be selected to test various hypotheses about the model structure, without requiring further inversions. As examples, the subset of model probabilities can be viewed for models only containing a certain number of layers, or for models where resistive layers are present between a certain interval as suggested by other geological constraints such as seismic stratigraphy or nearby well logs.