Full Electromagnetic Field Research Articles

Autoencoder-extended Conditional Invertible Neural Networks for Unfolding Signal Traces

The reconstruction of cosmic ray-induced air showers from measurements of radio waves constitutes a major challenge. In this work, we focus on recovering the full three-dimensional electromagnetic field from two recorded signal traces of an antenna station covering two horizontal polarization directions. The simulated field is folded by a direction and frequency-dependent characteristic antenna response pattern, resulting in voltage signal traces as a function of time. Both signal traces are contaminated by simulated background noise. We use conditional Invertible Neural Networks (cINNs) to learn posterior distributions, from which the most likely electromagnetic field given a measured signal trace can be inferred. To improve robustness, we extend the method with an autoencoder by reducing the parameter phase space and decoupling the cINN from specific data shapes. Thereby, each signal trace is condensed into a small number of abstract parameters in the latent space on which the cINN operates. The presented method shows promising results and can be transferred to other unfolding problems where the recovery of the pre-measurement state is of interest.

Magnetic quadri-dipolar stars rotating in vacuum

ABSTRACT Main-sequence stars and compact objects such as white dwarfs and neutron stars are usually embedded in magnetic fields that strongly deviate from a pure dipole located right at the stellar centre. An off-centred dipole can sometimes better adjust existing data and offer a simple geometric picture to include multipolar fields. However, such configurations are usually to restrictive, limiting multipolar components to strength less than the underlying dipole. In this paper, we consider the most general lowest order multipolar combination given by a dipole and a quadrupole magnetic field association in vacuum. Following the general formalism for multipolar field computations, we derive the full electromagnetic field outside a rotating quadridipole. Exact analytical expressions for the Poynting flux and the electromagnetic kick are given. Such geometry is useful to study the magnetosphere of neutron stars for which more and more compelling observations reveals hints for at least quadridipolar fields. We also show that for sufficiently high quadrupole components at the stellar surface, the electromagnetic kick imprinted to a neutron star can reach thousands of km s−1 for a millisecond period at birth.

Ultra-intense laser pulse characterization using ponderomotive electron scattering

We present a new analytical solution for the equation of motion of relativistic electrons in the focus of a high-intensity laser pulse. We approximate the electron’s transverse dynamics in the averaged field of a long laser pulse focused to a Gaussian transverse profile. The resultant ponderomotive scattering is found to feature an upper boundary of the electrons’ scattering angles, depending on the laser parameters and the electrons’ initial state of motion. In particular, we demonstrate the angles into which the electrons are scattered by the laser scale as a simple relation of their initial energy to the laser’s amplitude. We find two regimes to be distinguished in which either the laser’s focusing or peak power are the main drivers of ponderomotive scattering. Based on this result, we demonstrate how the intensity of a laser pulse can be determined from a ring-shaped pattern in the spatial distribution of a high-energy electron beam scattered from the laser. We confirm our analysis by means of detailed relativistic test particle simulations of the electrons’ averaged ponderomotive dynamics in the full electromagnetic fields of the focused laser pulse.

Spectral Numerical Mode Matching Method for Metasurfaces

A novel 3-D spectral numerical mode-matching (SNMM) method is proposed and developed as a highly efficient rigorous solver for metasurfaces on the interface of a half-space with Bloch (Floquet) periodic boundary conditions. In addition to full electromagnetic (EM) fields, the SNMM solver can provide with ease the characteristics of metasurfaces including their absorptance, anomalous reflection/refraction, and surface plasmon polaritons (SPPs). The SNMM method is a semianalytical solver: it solves for the Bloch eigenmodes in the horizontal directions by using the mixed spectral-element method (MSEM) numerically, but determines the scattering in the vertical direction analytically through eigenmode propagation. As there is no need for discretization in the vertical direction, it can efficiently and accurately simulate EM wave interactions with metasurfaces. Numerical experiments indicate that the SNMM method is efficient and accurate for the metasurfaces compared with the well-developed 3-D finite-element method (FEM). Applications to homogeneous isotropic/anisotropic, inhomogeneous isotropic, and the gradient metasurfaces are demonstrated. Typically, the computational speed of the proposed solver is one to two orders of magnitude higher than the FEM as implemented in a commercial software package.

Ray-based method for simulating cascaded diffraction in high-numerical-aperture systems.

The electric field at the output of an optical system is in general affected by both aberrations and diffraction. Many simulation techniques treat the two phenomena separately, using a geometrical propagator to calculate the effects of aberrations and a wave-optical propagator to simulate the effects of diffraction. We present a ray-based simulation method that accounts for the effects of both aberrations and diffraction within a single framework. The method is based on the Huygens-Fresnel principle, is entirely performed using Monte Carlo ray tracing, and, in contrast to our previously published work, is able to calculate the full electromagnetic field. The method can simulate the effects of multiple diffraction in systems with a high numerical aperture.

Beating Rayleigh's Curse by Imaging Using Phase Information.

Every imaging system has a resolution limit, typically defined by Rayleigh's criterion. Given a fixed number of photons, the amount of information one can gain from an image about the separation between two sources falls to zero as the separation drops below this limit, an effect dubbed "Rayleigh's curse." Recently, in a quantum-information-inspired proposal, Tsang and co-workers found that there is, in principle, infinitely more information present in the full electromagnetic field in the image plane than in the intensity alone, and suggested methods for extracting this information and beating the Rayleigh limit. In this Letter, we experimentally demonstrate a simple scheme that captures most of this information, and show that it has a greatly improved ability to estimate the distance between a pair of closely separated sources, achieving near-quantum-limited performance and immunity to Rayleigh's curse.

Backward spoof surface wave in plasmonic metamaterial of ultrathin metallic structure.

Backward wave with anti-parallel phase and group velocities is one of the basic properties associated with negative refraction and sub-diffraction image that have attracted considerable interest in the context of photonic metamaterials. It has been predicted theoretically that some plasmonic structures can also support backward wave propagation of surface plasmon polaritons (SPPs), however direct experimental demonstration has not been reported, to the best of our knowledge. In this paper, a specially designed plasmonic metamaterial of corrugated metallic strip has been proposed that can support backward spoof SPP wave propagation. The dispersion analysis, the full electromagnetic field simulation and the transmission measurement of the plasmonic metamaterial waveguide have clearly validated the backward wave propagation with dispersion relation possessing negative slope and opposite directions of group and phase velocities. As a further verification and application, a contra-directional coupler is designed and tested that can route the microwave signal to opposite terminals at different operating frequencies, indicating new application opportunities of plasmonic metamaterial in integrated functional devices and circuits for microwave and terahertz radiation.

The electromagnetic response in a layered vertical transverse isotropic medium: A new look at an old problem

ABSTRACTWe determined that the electromagnetic vertical transverse isotropic response in a layered earth can be obtained by solving two equivalent scalar equations, which were for the vertical electric field and for the vertical magnetic field, involving only a scalar global reflection coefficient. Besides the complete derivation of the full electromagnetic response, we also developed the corresponding computer code called EMmod, which models the full electromagnetic fields including internal multiples in the frequency-wavenumber domain and obtains the frequency-space domain solutions through a Hankel transformation by computing the Hankel integral using a 61-point Gauss-Kronrod integration routine. The code is able to model the 3D electromagnetic field in a 1D earth for diffusive methods such as controlled source electromagnetics as well as for wave methods such as ground penetrating radar. The user has complete freedom to place the source and the receivers in any layer. The modeling is illustrated with ...

Analysis of a Denisov Launcher for 420 GHz High Order Mode Gyrotron

A Denisov launcher operating at 420 GHz with TE17,4 mode is designed and optimized through theoretical analysis and numerical simulations. The launcher consists of a mode converted waveguide and a helical cut. Instead of using the conventional perturbations, a sinusoid type perturbation is used in this converted waveguide. Operations of the launcher are analyzed by the coupled mode theory. The field at the output aperture of the launcher has a nice Gaussian profile distribution. The conversion efficiency is more than 98.1 %. The parasitic modes in the launcher and the aperture field profile were analyzed. The wall field of the launcher is analyzed in case that the input wave is rotating left-hand. A full 3D electromagnetic field analysis of the integrated launcher has been done. The results are in good agreement with the numerical calculation results.

The electromagnetic response in a layered vertical transverse isotropic medium: A new look at an old problem

We determined that the electromagnetic vertical transverse isotropic response in a layered earth can be obtained by solving two equivalent scalar equations, which were for the vertical electric field and for the vertical magnetic field, involving only a scalar global reflection coefficient. Besides the complete derivation of the full electromagnetic response, we also developed the corresponding computer code called EMmod, which models the full electromagnetic fields including internal multiples in the frequency-wavenumber domain and obtains the frequency-space domain solutions through a Hankel transformation by computing the Hankel integral using a 61-point Gauss-Kronrod integration routine. The code is able to model the 3D electromagnetic field in a 1D earth for diffusive methods such as controlled source electromagnetics as well as for wave methods such as ground penetrating radar. The user has complete freedom to place the source and the receivers in any layer. The modeling is illustrated with three examples, which aim to present the different capabilities of EMmod, while assessing its correctness.

Relativistic Wigner functions

Original definition of the Wigner function can be extended in a natural manner to relativistic domain in the framework of quantum field theory. Three such generalizations are described. They cover the cases of the Dirac particles, the photon, and the full electromagnetic field.

Off-resonant transitions in the collective dynamics of multi-level atomic ensembles

We study the contributions of off-resonant transitions to the dynamics of a system of N multi-level atoms sharing one excitation and interacting with the quantized vector electromagnetic field. The rotating wave approximation significantly simplifies the derivation of the equations of motion describing the collective atomic dynamics, but it leads to an incorrect expression for the dispersive part of the atom–atom interaction terms. For the case of two-level atoms and a scalar electromagnetic field, it turns out that the atom–atom interaction can be recovered correctly if integrals over the photon mode frequencies are extended to incorporate negative values. We explicitly derive the atom–atom interaction for multi-level atoms, coupled to the full vector electromagnetic field, and we recover also in this general case the validity of the results obtained by the extension to negative frequencies of the formulas derived with the rotating wave approximation.

Laser-seeding dynamics with few-cycle pulses: Maxwell-Bloch finite-difference time-domain simulations of terahertz quantum cascade lasers

We implement a Maxwell-Bloch simulation for a two-level system within the finite-difference time-domain method to simulate the seeding of lasers by broadband pulse injection. The model does not make the slowly varying envelope approximation, and the full electromagnetic field is simulated so that we are able to obtain time-resolved seeding by few-cycle pulses. The model is compared to recent results on seeding of THz quantum cascade lasers to aid in the interpretation of their complex signals. The simulations are found to be in good agreement with the data when gain recovery times of 15 ps are used. Furthermore, we find that the emission from the laser depends only weakly on the seed used to initiate laser action. The model is readily applicable to any seeded laser system where few-cycle seed pulses are used.

Electromagnetic field model for the numerical computation of voltages induced on buried pipelines by high voltage overhead power lines

This paper proposes an innovative, generally applicable numerical model for the calculation of the three-dimensional (3D) electromagnetic field generated by high voltage (HV) overhead power transmission lines (OHL) on the buried metallic structures (e.g., pipeline networks). The numerical analysis is based on a coupled finite element-boundary element model (FEM-BEM) designed to calculate the induced potential on buried pipelines for complex geometrical structures of HV OHL networks working on normal or fault conditions. The one-dimensional (1D) FEM technique based on pipe elements is used to discretize the mathematical model that describes the interior of the pipe and is coupled with the mathematical model that describes the exterior of the pipe using 3D-BEM integral equations. The full electromagnetic field model gives the flexibility to calculate the potential distribution in any point of the soil, providing useful information for the step and touching voltages. The computation accuracy of the numerical algorithm implemented is verified through two test problems by comparing the numerical results with those obtained using a software package based on the Transmission Line Method (TLM) and CIGRE formulae. Last part of the paper presents calculations of the induced potential on buried pipeline in the vicinity of a complex HV OHL working on normal and fault condition. The influence of the currents’ direction and magnitude flowing on the HV OHL on the induced pipeline potential distribution is analyzed.

Numerical approximation of the Vlasov–Maxwell–Fokker–Planck system in two dimensions

A numerical method is developed for solving the Vlasov–Maxwell–Fokker–Planck system in two spatial dimensions. This system of equations is a model for a collisional plasma in the presence of a self consistent electromagnetic field. The numerical procedure is a type of deterministic particle method and is an extension to include the full electromagnetic field of the approximation method of Wollman and Ozizmir [S. Wollman, E. Ozizmir, Numerical approximation of the Vlasov–Poisson–Fokker–Planck system in two dimensions, J. Comput. Phys. 228 (2009) 6629–6669]. In addition, the long time asymptotic behavior of solutions is studied. It is determined that the solution to the Vlasov–Maxwell–Fokker–Planck system converges to the same steady state solution as that for the Vlasov–Poisson–Fokker–Planck system.

A Vlasov–Fokker–Planck code for high energy density physics

OSHUN is a parallel relativistic 2D3P Vlasov–Fokker–Planck code, developed primarily to study electron transport and instabilities pertaining to laser-produced—including laser-fusion—plasmas. It incorporates a spherical harmonic expansion of the electron distribution function, where the number of terms is an input parameter that determines the angular resolution in momentum-space. The algorithm employs the full 3D electromagnetic fields and a rigorous linearized Fokker–Planck collision operator. The numerical scheme conserves energy and number density. This enables simulations for plasmas with temperatures from MeV down to a few eV and densities from less than critical to more than solid. Kinetic phenomena as well as electron transport physics can be recovered accurately and efficiently.

Near‐field optical microscopy simulations using graphics processing units

AbstractScanning near‐field optical microscopy (NSOM) is a perspective method for local analysis of optical parameters of nanostructures used in electro‐optics, photonics and many other field of research. Due to complicated nature of topography‐related effects in the optical signal, which can influence or completely alter the NSOM data, the practical use of this method for quantitative analysis is still limited. One of the approaches to determine the effects of topography on optical data is to simulate NSOM images using an electromagnetic field modeling method, e.g. finite difference in time domain method. However, this approach is extremely slow, as full electromagnetic field propagation calculation must be run for each pixel of the resulting simulated image. In this paper, we present a fast method for computing simulated NSOM images based on the use of a graphics processing unit (GPU or computer graphics card) in order to speed up the computations. We will show that the large speedup using this approach (more than 40 times) allows us to simulate images in a time comparable to their real measurement, which increases the possibilities of quantitative analysis using NSOM. Data simulated by a GPU will be compared with those of real measurements as well. Copyright © 2010 John Wiley & Sons, Ltd.

Planar Far-Field Lensing with Plasmonic Nano-Slit Arrays

W ith the advent of nanofabrication techniques, such as electron-beam lithography and focused ion beam milling, it is now possible to pattern metallic structures at the nano scale. is enables the exploitation of the plasmonic behavior of metals to create ultra-compact photonic devices for sensing, guiding or focusing.1 In this context, we recently reported on the experimental demonstration of far-fi eld lensing using a plasmonic nano-slit array.2 Refractive lenses are widely used in applications ranging from imaging to concentrating light. e miniaturization of lenses has been essential for the development of modern solid-state image sensors and might have important implications for other opto-electronic applications, including solid-state displays and lighting. e focusing ability of conventional, dielectric-based micro-lenses, however, deteriorates as their physical dimensions are reduced toward a singlewavelength scale. Moreover, the small size limits scientists’ ability to implement devices that are fl exible enough to properly focus light incident at oblique angles directly below the lens. e nano-patterning of optically thick metallic fi lms off ers an alternative that alleviates some of the shortcomings of refractive lenses. In this approach, a “plasmonic” lens is formed by an array of nano-scale slits in a planar metallic fi lm. For a light incident upon such a structure, the phase shift experienced by light as it passes through each individual slit is sensitive to the width.3 With the adjustment of the properties of individual slits, it is possible to create a curved phase front for the transmitted fi eld and thus to achieve focusing behavior. We recently implemented such far-fi eld planar nano-slit lenses using a combination of thin-fi lm deposition methods and focused ion beam milling. We demonstrated with confocal scanning optical microscopy that these structures can act as far-fi eld cylindrical lenses for light at optical frequencies.2 Moreover, we showed excellent agreement between the full electromagnetic fi eld simulations of the design, which include both evanescent and propagating modes, and the far-fi eld, diff ractionlimited confocal measurements. is demonstration is a crucial step in the realization of this potentially important technology, which off ers a range of processing and integration advantages over conventional shaped-based dielectric lenses. For example, the planar geometry facilitates integration using industrystandard semiconductor technology, and the geometry-based design off ers fl exibility that is not easily achieved with refractive lenses at wavelength-size scales. is makes planar “plasmonic” lenses attractive for integrated optoelectronic applications that require angle-compensating micro-lenses. We have shown already that, by introducing position-dependent phase Planar fareld “plasmonic” lens based on nano-slit array in metallic lm. (a) Geometry of the lens consisting of a 400-nm-thick gold lm with air slits of widths between 80 and 150 nm (light blue), sitting on a fused silica substrate (dark blue). (b) Scanning electron micrograph of the structure viewed from the air-side. (c) Focusing pattern measured with a confocal scanning optical microscope (CSOM). (d) Finite-difference frequency-domain simulated focusing pattern of the eld intensity through the center of the slits. (a) (c) (d) 0

A generalized measurement equation and van Cittert-Zernike theorem for wide-field radio astronomical interferometry

We derive a generalized van Cittert-Zernike (vC-Z) theorem for radio astronomy that is valid for partially polarized sources over an arbitrarily wide field of view (FoV). The classical vC-Z theorem is the theoretical foundation of radio astronomical interferometry, and its application is the basis of interferometric imaging. Existing generalized vC-Z theorems in radio astronomy assume, however, either paraxiality (narrow FoV) or scalar (unpolarized) sources. Our theorem uses neither of these assumptions, which are seldom fulfiled in practice in radio astronomy, and treats the full electromagnetic field. To handle wide, partially polarized fields, we extend the two-dimensional (2D) electric field (Jones vector) formalism of the standard ‘Measurement Equation’ (ME) of radio astronomical interferometry to the full three-dimensional (3D) formalism developed in optical coherence theory. The resulting vC-Z theorem enables full-sky imaging in a single telescope pointing, and imaging based not only on standard dual-polarized interferometers (that measure 2D electric fields) but also electric tripoles and electromagnetic vector-sensor interferometers. We show that the standard 2D ME is easily obtained from our formalism in the case of dual-polarized antenna element interferometers. We also exploit an extended 2D ME to determine that dual-polarized interferometers can have polarimetric aberrations at the edges of a wide FoV. Our vC-Z theorem is particularly relevant to proposed, and recently developed, wide FoV interferometers such as Low Frequency Array (LOFAR) and Square Kilometer Array (SKA), for which direction-dependent effects will be important.

Optimal Polarized Beampattern Synthesis Using a Vector Antenna Array

Using polarized waveforms increases the capacity of communication systems and improves the performance of active sensing systems such as radar. We consider the optimal synthesis of a directional beam with full polarization control using an array of electromagnetic vector antennas (EMVA). In such an array, each antenna consists of p ges 2 orthogonal electric or magnetic dipole elements. The control of polarization and spatial power patterns is achieved through carefully designing the amplitudes and phases of the weights of these dipole antennas. We formulate the problem in a convex form, which is thus efficiently solvable by existing solvers such as the interior point method. Our results indicate that vector antenna arrays not only enable full polarization control of the beampattern, but also improve the power gain of the main beam (over the sidelobes), where the gain is shown numerically to be linearly proportional to vector antenna dimensionality p. This implies that EMVA not only offers the freedom to control the beampattern polarization, but also virtually increases the array size by exploiting the full electromagnetic (EM) field components. We also study the effect of polarization on the spatial power pattern. Our analysis shows that for arrays consisting of pairs of electrical and magnetic dipoles, the spatial power pattern is independent of the mainbeam polarization constraint.