Electromagnetic Wave Packet Research Articles

Current drive by high intensity, pulsed, electron cyclotron wave packets

The nonlinear interaction of electrons with a high intensity, spatially localized, Gaussian, electro-magnetic wave packet, or beam, in the electron cyclotron range of frequencies is described by the relativistic Lorentz equation. There are two distinct sets of electrons that result from wave-particle interactions. One set of electrons is reflected by the ponderomotive force due to the spatial variation of the wave packet. The second set of electrons are energetic enough to traverse across the wave packet. Both sets of electrons can exchange energy and momentum with the wave packet. The trapping of electrons in plane waves, which are constituents of the Gaussian beam, leads to dynamics that is distinctly different from quasilinear modeling of wave-particle interactions. This paper illustrates the changes that occur in the electron motion as a result of the nonlinear interaction. The dynamical differences between electrons interacting with a wave packet composed of ordinary electromagnetic waves and electrons interacting with a wave packet composed of extraordinary waves are exemplified.

Energy, linear momentum, and angular momentum exchange between an electromagnetic wave-packet and a small particle

Invoking Maxwell's classical electrodynamics in conjunction with expressions for the electromagnetic (EM) energy, momentum, force, and torque, we use a few simple examples to demonstrate the nature of linear and angular momentum exchange between a wave-packet and a small spherical particle. The linear and angular momenta of the EM field, when absorbed by the particle, will be seen to elicit different responses from the particle.

Theory of Goos-Hänchen shift in graphene: Energy-flux method

We present a theoretical study of Goos-Hänchen shift and associated time delay of a p-polarized electromagnetic wave packet for total internal reflection at a plane interface between two media of different permittivity, with graphene at their interface. The study is based on the energy-flux method which takes into account the energy flux in graphene in addition to the energy flux in the incident, totally reflected, coupled incident-totally reflected and evanescent wave packets.

Surfatron acceleration of weakly relativistic electrons by electromagnetic wave packet in space plasma

Resonant interactions between an electromagnetic wave packet and charged particles based on numerical calculations are investigated. The strong surfatron acceleration of weakly relativistic electrons by an electromagnetic wave packet in space plasma is studied. In the central area of the wave packet, the electric field amplitude is above a threshold value and this makes it possible to capture particles in the surfing mode. The work is carried out by exact solving of second order nonlinear, nonstationary equations for the wave packet phase on the particle's trajectory at the carrying frequency. Numerical modeling shows that the trapping of weakly relativistic electrons in strong acceleration mode occurs immediately for a wide enough range of favorable initial wave phase values (80% and more). Furthermore it has been demonstrated that the combination of ranges of the particle's initial parameters corresponding to the capturing in surfatron acceleration is large enough. Temporal dynamics of velocity components for accelerated particles and the particularities of their trajectory, taking into account cyclotron rotation at the initial stage and phase plane structure for numerically solved nonlinear equations, are considered. Simulation results allow us to draw conclusions about the possibility of surfatron acceleration of weakly relativistic charged particles in space plasma by an electromagnetic wave packet.

Abruptly Focusing and Defocusing Needles of Light and Closed-Form Electromagnetic Wavepackets

Fourier optics enforces a tradeoff between length and narrowness in electromagnetic wavepackets, so that a narrow spatial focus diffracts at a large divergence angle, and only infinitely wide beams can remain non-diffracting. We show that it is possible to bypass this tradeoff between the length and narrowness of intensity hotspots, and find a family of electromagnetic wavepackets that abruptly focus to and defocus from high-intensity regions of any aspect ratio. Such features are potentially useful in scenarios where one would like to avoid damaging the surrounding environment, for instance, to target tumors very precisely in cancer treatment, drill holes of very precise dimensions in laser machining, or trigger nonlinear processes in a well-defined region. In the process, we also construct the first closed-form solutions to Maxwell's equations for finite-energy electromagnetic pulses. These pulses also exhibit intriguing physics, with an on-axis intensity peak that always travels at the speed of light despite inherent diffraction.

Interplay between diffraction and the Pancharatnam-Berry phase in inhomogeneously twisted anisotropic media

We discuss the propagation of an electromagnetic field in an inhomogeneously anisotropic material where the optic axis is rotated in the transverse plane but is invariant along the propagation direction. In such a configuration, the evolution of an electromagnetic wavepacket is governed by the Pancharatnam-Berry phase (PBP), responsible for the appearance of an effective photonic potential. In a recent paper [A. Alberucci et al., "Electromagnetic confinement via spin-orbit interaction in anisotropic dielectrics", ACS Photonics \textbf{3}, 2249 (2016)] we demonstrated that the effective potential supports transverse confinement. Here we find the profile of the quasi-modes and show that the photonic potential arises from the Kapitza effect of light. The theoretical results are confirmed by numerical simulations, accounting for the medium birefringence. Finally, we analyze in detail a configuration able to support non-leaky guided modes.

ДИНАМИКА ЗАХВАТА И ПОСЛЕДУЮЩЕГО СЕРФОТРОННОГО УСКОРЕНИЯ ЭЛЕКТРОНОВ ЭЛЕКТРОМАГНИТНЫМИ ВОЛНАМИ В КОСМИЧЕСКОЙ ПЛАЗМЕ

The research of dynamics of the capture and subsequent surfatron acceleration of electrons by electromagnetic waves, propagating perpendicular to the magnetic field in cosmic plasma is conducted on the basis of nonlinear numerical calculations. The optimal conditions for the realization of ultra-relativistic electron acceleration by wave packets in cosmic plasma including favorable initial phase of the wave on the particle trajectory, the sign of electron initial impulse in the direction of the wave front, the magnitude of the phase velocity of the wave are formulated. The asymptotic behavior of the electron characteristics under hard acceleration (relativistic factor, the impulse components and the captured particle velocity, position of the bottom of the effective potential well, etc.) is obtained. It is shown that the trajectories have the form of a spiral on the phase plane for trapped electrons and gradually approach the singular point of the type of stable focus. The bottom position of effective non-stationary potential well for the trapped electrons is different because it depends on the charge sign of the accelerated particle. For trapped electrons the wave phase graphics on the particle trajectory correspond to oscillations with an increasing period and decreasing amplitude with the increase of energy of the particle. The bottom of an effective non-stationary potential well is achieved by trapped particle asymptotically. Thus, the trapped electrons with different initial wave phases in their trajectory are gradually condensed on the bottom of the effective non-stationary potential well. It is important to emphasize that the packets of electromagnetic waves in the vicinity of relatively quiet stars (like the Sun) can be local sources of additional acceleration of the spectrum of cosmic rays with initial energies of the order of GeV up to the energies of hundreds of GeV and a dozen of TeV, which provides the observed variation of the spectrum of cosmic rays in this area depending on the space weather. Much more additional acceleration of cosmic rays can be in the plasma of local interstellar clouds at distances from the Sun of about parsecs. The analysis of surfatron generation mechanism of relativistic particles charged with electromagnetic waves in the relatively tranquil cosmic plasma is important for understanding the causes of the observed variability of the energy spectra of cosmic rays and the appearance of various features in their energy spectrum. This allows giving a correct interpretation of the experimental data. It is important to note that the flows of cosmic rays can significantly affect the vertical profiles of atmospheric temperature, precipitation, and the dynamics of large-scale tropical cyclogenesis

Superluminal light attenuated by strong dispersion of complex refractive index

The propagation of narrow packets of electromagnetic waves (EMWs) in frequency dispersive medium with the consideration of the complex refractive index is studied. It is shown that counting in the dispersion of the complex refractive index within the context of the conventional expression of the group velocity of narrow wave packets of EMWs propagating in a dispersive medium results in the appearance of additional constraints on the group velocity, which dictates that the physically acceptable group velocity can only be realized in the case of a negligible imaginary part of the group index. In this paper, the conditions that allow one to realize the physically acceptable group velocity are formulated and analyzed numerically for the relevant model of the refractive index of a system of two-level atoms in the optical frequency range. It is shown that in the frequency band where superluminal light propagation is expected, there is a strong dispersion of the refractive index that is accompanied with strong absorption, resulting in a strongly attenuated superluminal light.

Semianalytical study of the propagation of an ultrastrong femtosecond laser pulse in a plasma with ultrarelativistic electron jitter

The interaction of a multi-petawatt, pancake-shaped laser pulse with an unmagnetized plasma is studied analytically and numerically in a regime with ultrarelativistic electron jitter velocities, in which the plasma electrons are almost completely expelled from the pulse region. The study is applied to a laser wakefield acceleration scheme with specifications that may be available in the next generation of Ti:Sa lasers and with the use of recently developed pulse compression techniques. A set of novel nonlinear equations is derived using a three-timescale description, with an intermediate timescale associated with the nonlinear phase of the electromagnetic wave and with the spatial bending of its wave front. They describe, on an equal footing, both the strong and the moderate laser intensity regimes, pertinent to the core and to the edges of the pulse. These have fundamentally different dispersive properties since in the core the electrons are almost completely expelled by a very strong ponderomotive force, and the electromagnetic wave packet is imbedded in a vacuum channel, thus having (almost) linear properties. Conversely, at the pulse edges, the laser amplitude is smaller, and the wave is weakly nonlinear and dispersive. New nonlinear terms in the wave equation, introduced by the nonlinear phase, describe without the violation of imposed scaling laws a smooth transition to a nondispersive electromagnetic wave at very large intensities and a simultaneous saturation of the (initially cubic) nonlocal nonlinearity. The temporal evolution of the laser pulse is studied both analytically and by numerically solving the model equations in a two-dimensional geometry, with the spot diameter presently used in some laser acceleration experiments. The most stable initial pulse length is estimated to exceed ≳1.5–2 μm. Moderate stretching of the pulse in the direction of propagation is observed, followed by the development of a vacuum channel and of a very large electrostatic wake potential, as well as by the bending of the laser wave front.

Observation of wave packet distortion during a negative-group-velocity transmission.

In Physics, causality is a fundamental postulation arising from the second law of thermodynamics. It states that, the cause of an event precedes its effect. In the context of Electromagnetics, the relativistic causality limits the upper bound of the velocity of information, which is carried by electromagnetic wave packets, to the speed of light in free space (c). In anomalously dispersive media (ADM), it has been shown that, wave packets appear to propagate with a superluminal or even negative group velocity. However, Sommerfeld and Brillouin pointed out that the “front” of such wave packets, known as the initial point of the Sommerfeld precursor, always travels at c. In this work, we investigate the negative-group-velocity transmission of half-sine wave packets. We experimentally observe the wave front and the distortion of modulated wave packets propagating with a negative group velocity in a passive artificial ADM in microwave regime. Different from previous literature on the propagation of superluminal Gaussian packets, strongly distorted sinusoidal packets with non-superluminal wave fronts were observed. This result agrees with Brillouin's assertion, i.e., the severe distortion of seemingly superluminal wave packets makes the definition of group velocity physically meaningless in the anomalously dispersive region.

Surface angular momentum of light beams

Traditionally, the angular momentum of light is calculated for "bullet-like" electromagnetic wave packets, although in actual optical experiments "pencil-like" beams of light are more commonly used. The fact that a wave packet is bounded transversely and longitudinally while a beam has, in principle, an infinite extent along the direction of propagation, renders incomplete the textbook calculation of the spin/orbital separation of the angular momentum of a light beam. In this work we demonstrate that a novel, extra surface part must be added in order to preserve the gauge invariance of the optical angular momentum per unit length. The impact of this extra term is quantified by means of two examples: a Laguerre-Gaussian and a Bessel beam, both circularly polarized.

Shape-Preserving Accelerating Electromagnetic Wave Packets in Curved Space

Wave packets of light have been made to travel in a curved space along geodesic paths, generating optical analogues of general-relativity phenomena. A new analysis of the curved-space generalization of the Maxwell equations shows that wave packets can also travel along nongeodesic paths while changing and recovering their shapes periodically.

The momentum of an electromagnetic wave inside a dielectric

The problem of assigning a momentum to an electromagnetic wave packet propagating inside an insulator has become known under the name of the Abraham–Minkowski controversy. In the present paper we re-examine this issue making the hypothesis that the forces exerted on an insulator by an electromagnetic field do not distinguish between polarization and free charges. Under this assumption we show that the Abraham expression for the radiation mechanical momentum is highly favored.

Group velocity of the electromagnetic wave packet in a relativistic dispersive medium

This paper presents the fully relativistic, general expression for the group velocity Vg of the electromagnetic wave packet in a relativistic dispersive medium. Motion of the medium is accounted for with the relativistic index of refraction. The general formula for Vg is specialized to a few particular situations that are briefly discussed. The explicit, exact, fully relativistic expression for Vg is relevant to all issues where the electromagnetic wave packet travels through a medium in motion, especially where the velocity of the medium is high or utmost accuracy is essential, e.g. it pertains to the slow light phenomenon, pulsar radiation, GPS, radar, and LIDAR technologies.

Superluminal electromagnetic solitary waves in electron-positron plasmas

The propagation of an electromagnetic wave packet in an electron-positron plasma, in the form of coupled localized electromagnetic excitations, is investigated, from first principles. By means of the Poincaré section method, a special class of superluminal localized nonlinear stationary solutions, existing along a separatrix curve, are proposed as intrinsic electromagnetic modes in a relativistic electron-positron plasma. The ratio of the envelope time scale to the carrier wave time scale of these envelope solitary waves critically depends on the carrier's phase velocity. In the strongly superluminal regime, vph/c ≫ 1, the large difference between the envelope and carrier time scales enables us to carry out a multiscale perturbative analysis resulting in an analytical form of the solution envelope. The analytical prediction thus obtained is shown to be in agreement with the solution obtained via a direct numerical integration.

Numerical investigation of the surfatron acceleration efficiency of charged particles by wave packets in space plasma

A theoretical study of the efficiency of the relativistic acceleration of charged particles by a finite amplitude electromagnetic wave packet in space plasma is presented. The effect of surfatron mechanism particle acceleration is investigated by numerical analysis of the second-order, non-stationary, nonlinear equation for the wave packet phase at the particle trajectory. The influence of the phase and group velocities of the wave packet at the wave packet carrying frequency on the acceleration efficiency is studied. The optimal conditions for weakly relativistic particles captured by electromagnetic wave packets with the following highly relativistic charge acceleration are determined. The particle energy growth rate for the regime of surfatron relativistic acceleration is determined. The temporal dynamics of particle acceleration are investigated. The conclusion about the possibilities of ultrarelativistic surfatron acceleration of charges by a wave packet with a smooth amplitude envelope is given.

Goos–Hänchen and Imbert–Fedorov shifts of an electromagnetic wave packet by a moving object

We present a solution to the problem of reflection and refraction of a polarized Gaussian beam at the interface between the transparent medium and the moving object based on Minkowski’s constitutive relations. The Goos–Hanchen (GH) and Imbert–Fedorov (IF) effects are discussed in detail when the object moves parallel to the interface. It is shown that the IF shifts of reflection and transmission beams can reach several wavelengths of the incident beam at some certain incident angles and moving velocities of the object. The transitions from the positive IF shift to the negative IF shift have been observed with the increase of the velocity of the moving object. Furthermore, our calculations show that some unusual GH effects can also be caused by the movement of the medium. The physical origins for these phenomena have been analyzed. It is well known that the IF shift is directly related to the spin-Hall effects of light. Thus, these findings can also provide an alternative pathway for controlling the spin-Hall effects of light.

Infrared limit in external field scattering

Scattering of electrons/positrons by external classical electromagnetic wave packet is considered in infrared limit. In this limit, the scattering operator exists and produces physical effects, although the scattering cross-section is trivial.

Slow packets of electromagnetic waves in magnetized plasma with ferrite grains

The propagation of narrow Gaussian electromagnetic wave packets in the collisionless magnetized plasma with ferrite grains (MPFG) is studied. In this system, in the frequency range close to the frequency of the electron cyclotron resonance, which coincides with the frequency of the ferromagnetic resonance, the dispersion of the electric permittivity (electron subsystem) and magnetic permeability (ferrite grain subsystem) must be taken into account simultaneously. The refractive index η of MPFG can be negative and with increase in frequency becomes positive passing through zero. The strong dispersion of η and the group velocity vg results in the spreading of the wave packets, which propagate with velocity much smaller than the speed of light. Their phase velocity is negative if η<0 and positive if η>0. For η<0, the group velocity can have a maximum at some frequency. The wave packets centered at this frequency are weakly spreading. The existence of slow waves in the frequency range, where η≤0, may be a general property of left handed media.

Influence of the shape of the cross section of a plasma-dielectric interface on the dispersion properties of high-frequency azimuthal surface modes

Theoretical study of the propagation of a packet of surface electromagnetic surface waves with a zero axial wavenumber in a circular-cross-section cylindrical metal waveguide within the frequency range that is higher than upper hybrid resonance is carried out. The waveguide is partially filled by plasma and immersed into axial magnetic field. The cross section of the plasma column is assumed to differ from circular shape. The effect of this shape on the dispersion properties of azimuthal surface modes is investigated by the method of successive approximations. The fields of the waves and their eigenfrequencies are determined up to terms of the second order in the deviation of the plasma cross section shape from the ring one. The correction to the eigenfrequency of azimuthal surface modes caused by this feature of the plasma column section is proved to increase with decreasing the external magnetic field and increasing the value of the dielectric constant of the dielectric, that separates the plasma from the metal wall of the waveguide. The spectral composition of the wave packet, in the form of which these modes propagate, is studied. The amplitudes of the satellite harmonics of these modes are found to increase with increasing the plasma density and decreasing the external magnetic field.