Electromagnetic Beam Research Articles

Theory of Electromagnetic Beams

Abstract The theory of electromagnetic beams is presented in a simple and physical way, with all necessary mathematics explained in the text. The topics covered are in free-space classical electrod...

Fast calculation of tightly focused random electromagnetic beams: controlling the focal field by spatial coherence.

Focusing of a vectorial (electromagnetic) optical beam through a high numerical aperture can be investigated by means of the Richards-Wolf diffraction integral. However, such an integral extends from two-dimensional to four-dimensional, greatly increasing the computation time and therefore limiting the applicability, when light with decreased spatial coherence is considered. Here, we advance an effective protocol for the fast calculation of the statistical properties of a tightly focused field produced by a random electromagnetic beam with arbitrary state of spatial coherence and polarization. The novel method relies on a vectorial pseudo-mode representation and a fast algorithm of the wave-vector space Fourier transform. The procedure is demonstrated for several types of radially (fully) polarized but spatially partially coherent Schell-model beams. The simulations show that the computation time for obtaining the focal spectral density distribution with 512 × 512 spatial points for a low coherence beam is less than 100 seconds, while with the conventional quadruple Richards-Wolf integral more than 100 hours is required. The results further indicate that spatial coherence can be viewed as an effective degree of freedom to govern both the transverse and longitudinal components of a tightly focused field with potential applications in reverse shaping of focal fields and optical trapping control.

Major reshaping of narrow beams by resonant multilayer structures.

Major reshaping of electromagnetic MM-wave beams transmitted at resonant frequencies through high-quality multilayer structures is demonstrated. Beam reshaping emerges due to efficient excitation of intrinsic quasi-optical modes with complex spatial profiles by an incident beam. This leads to a significant increase of width and distortion of shape of transmitted beams, which grow with increasing likelihood of excitation of higher-order modes. The beam shape is extremely sensitive to imperfections of real structures and can become complex and asymmetric even at the normal-angle transmission of symmetric incident beams. The effect is of importance for the resonant MM-wave and THz spectroscopy of low-loss dielectric materials.

Properties of linearly polarized electromagnetic beams

Beams which are linearly polarized in the plane wave limit display surprisingly complex polarization properties in the focal region. We give examples where circular polarization intrudes close to the central linearly polarized region. We explore in detail the generic properties of linearly polarized beams, deriving exact expressions for their energy, momentum and angular momentum per unit length of beam. Results are expressed in terms of the wavenumber weight function which defines the beam. It is also shown that linearly polarized beams have net zero chiral content. All the results given are exact.

Anisotropic turbulence-induced statistical properties evolution of a radially polarized partially coherent pulsed beam on propagation

In this study, we introduce a new kind of partially coherent vector beam with Gaussian distribution in the spatial, temporal and correlation components named a radially polarized partially coherent Gaussian Schell-model pulsed (GSMP) beam as a natural extension of scalar Gaussian Schell-model pulsed beam. Analytical formulas for the two-frequency cross-spectral density matrix elements of this beam are derived based on the extended Huygens–Fresnel theory. Within the frame of extend anisotropic non-Kolmogorov spectrum, the effects of asymmetry anisotropic turbulence eddies on the statistical properties of a radially polarized partially coherent GSMP beam are investigated numerically. Our work improves the classical theory of stochastic electromagnetic beams on propagation and may be useful for applications involving free-space optical communications.

Programmable High-Order OAM-Carrying Beams for Direct-Modulation Wireless Communications

Electromagnetic (EM) beams carrying orbital angular momentum (OAM) are of great importance in modern wireless communications due to their attractive properties of self-reconstruction and multiplexing multiple beams. However, they are hardly deployed in a real-world scenario since they cannot be dynamically generated in a real-time and low-cost way, especially for the case of topological charges more than 2. To resolve this issue, we explore an inexpensive 2-bit programmable coding metasurface working at around 3.2 GHz along with an efficient optimization algorithm for the generation of high-order OAM-carrying beams in a reprogrammable way. Based on the designed metasurface, we can generate OAM EM beams with the electronically controlled vortex centers and topological charges of ${l}=0$ , ±1, ±2, ±3, ±4, ±5, and ±6, which have been experimentally verified. As an illustrative example, we demonstrate an interesting application of the designed 2-bit programmable metasurface in direct-modulation line-of-sight wireless communications. We believe that the developed strategy will bring a fundamentally new perspective on the design of wireless communication architectures at various frequencies.

All-optical comparator based on linear chain of silicon nanoparticles

A simultaneous excitation of two last particles of a finite linear chain of particles by two electromagnetic wave beams with arbitrary amplitudes of the electric field is considered. Using the example of a Si nanosphere chain, that does not support the resonant long-range electromagnetic energy transfer in the visible spectral range, it has been theoretically shown that self-consisted currents excited in a chain composed of a few Si mutually wave interacting nanospheres are related to the amplitude ratio of two incident beams. This technique provides a new way of comparing two optical signals in integrated microphotonic circuits.

Experimental realization of ultra-compact high-efficiency metasurface Luneburg lenses for microwave applications

Metasurfaces have shown the prospect of manipulating electromagnetic beams of various forms within a deeply subwavelength scale, which is especially useful in novel antennas designs and compact microwave devices or systems. In this paper, we present an ultra-compact, high-efficiency, and broadband metasurface Luneburg lens for full-angle operation at microwave frequencies. The design of the Luneburg lens is based on a holly parallel-plate-waveguide, where nonuniform circular holes are etched on the upper metallic plate. The gradient profile of the effective refractive index is achieved by varying the hole diameter. The high efficiency and broadband features of the proposed metasurface Luneburg lens are demonstrated numerically and experimentally. Owing to the symmetry of the Luneburg lens, such a lens is well suited for full-angle operation.

Coding Metasurfaces: Ultrawideband Spin‐Decoupled Coding Metasurface for Independent Dual‐Channel Wavefront Tailoring (Ann. Phys. 3/2020)

In article number 1900472 by Ke Chen, Yijun Feng, and co-workers, a coding methodology is reported that can enable a metasurface with independent 1-bit phase codes for orthogonal spins. The proposed method is verified by two spin-decoupled meta-devices for either electromagnetic beam deflecting or generating multi-beams carrying desired orbital angular momentums.

Coding Programmable Metasurfaces Based on Deep Learning Techniques

Programmable metasurfaces have recently been proposed to dynamically manipulate electromagnetic (EM) waves in both temporal and spatial dimensions. With active components integrated into unit cells of the metasurface, states of the unit cells can be adjusted by digital codes. The metasurface can then construct complex spatial and temporal electromagnetic beams. Given the main parameters of the beam, the optimal codes can be computed by nonlinear optimization algorithms, such as genetic algorithm, particle swarm optimization, etc. The high computational complexity of these algorithms makes it very challenging to compute the codes in real time. In this study, we applied deep learning techniques to compute the codes. A deep convolutional neural network is designed and trained to compute the required element codes in milliseconds, given the requirement of the waveform. The average accuracy of the prediction reaches more than 94 percent. This scheme is validated on a 1-bit programmable metasurface and both experimental and numerical results agree with each other well. This study shows that machines may “learn” the physics of modulating electromagnetic waves with the help of the good generalization ability in deep convolutional neural networks. The proposed scheme may provide us with a possible solution for real-time complex beamforming in antenna arrays, such as the programmable metasurface.

Characterization of the electromagnetic Gaussian Schell-model beam using first-order interference.

We propose a method for the characterization of electromagnetic Gaussian Schell-model (EMGSM) beams. This method utilizes the first-order interference consisting of polarization-state projections along with the two-point (generalized) Stokes parameters. The second-order field correlations employed in this method enable us to determine both the magnitude and the argument of the complex degree of electromagnetic coherence. We experimentally demonstrate this method by characterizing an EMGSM beam, which is synthesized using a laser beam passing through a rotating ground glass diffuser. This beam-characterization method is expected to be potentially useful for probing the partially coherent and partially polarized beams, and have tremendous applications in broad areas of optical communication and beam propagation.

Model of the propagation of very low-frequency beams in the Earth–ionosphere waveguide: principles of the tensor impedance method in multi-layered gyrotropic waveguides

Abstract. The modeling of very low-frequency (VLF) electromagnetic (EM) beam propagation in the Earth–ionosphere waveguide (WGEI) is considered. A new tensor impedance method for modeling the propagation of electromagnetic beams in a multi-layered and inhomogeneous waveguide is presented. The waveguide is assumed to possess the gyrotropy and inhomogeneity with a thick cover layer placed above the waveguide. The influence of geomagnetic field inclination and carrier beam frequency on the characteristics of the polarization transformation in the Earth–ionosphere waveguide is determined. The new method for modeling the propagation of electromagnetic beams allows us to study the (i) propagation of the very low-frequency modes in the Earth–ionosphere waveguide and, in perspective, their excitation by the typical Earth–ionosphere waveguide sources, such as radio wave transmitters and lightning discharges, and (ii) leakage of Earth–ionosphere waveguide waves into the upper ionosphere and magnetosphere. The proposed approach can be applied to the variety of problems related to the analysis of the propagation of electromagnetic waves in layered gyrotropic and anisotropic active media in a wide frequency range, e.g., from the Earth–ionosphere waveguide to the optical waveband, for artificial signal propagation such as metamaterial microwave or optical waveguides.

Spectral and coherent properties of partially polarized pulsed electromagnetic beams upon turbulent atmosphere propagation for different source conditions

The spectral and coherent properties of partially polarized pulsed electromagnetic beams propagating in a horizontal homogeneous and isotropic turbulent atmosphere channel for different source conditions are investigated in detail. Based on the extended Huygens–Fresnel principle and von Karman power spectrum, the analytical expressions for the elements of the two-frequency cross spectral density matrix, the average spectral intensity, the beam width, and the spectral degree of electromagnetic coherence are derived in turn. Finally, with the help of numerical calculations, the effects of different source conditions on the changes in the beam width, spectral and coherent properties of partially polarized pulsed Gaussian Schell-model electromagnetic beams are studied. Some theoretical results in this paper will be useful for applications involving polarized pulsed electromagnetic beams propagating through atmospheric turbulence.

A Piezo-Electromagnetic Coupling Multi-Directional Vibration Energy Harvester Based on Frequency Up-Conversion Technique

Harvesting vibration energy to power wearable devices has become a hot research topic, while the output power and conversion efficiency of a vibration energy harvester with a single electromechanical conversion mechanism is low and the working frequency band and load range are narrow. In this paper, a new structure of piezoelectric electromagnetic coupling up-conversion multi-directional vibration energy harvester is proposed. Four piezoelectric electromagnetic coupling cantilever beams are installed on the axis of the base along the circumferential direction. Piezoelectric plates are set on the surface of each cantilever beam to harvest energy. The permanent magnet on the beam is placed on the free end of the cantilever beam as a mass block. Four coils for collecting energy are arranged on the base under the permanent magnets on the cantilever beams. A bearing is installed on the central shaft of the base and a rotating mass block is arranged on the outer ring of the bearing. Four permanent magnets are arranged on the rotating mass block and their positions correspond to the permanent magnets on the cantilever beams. The piezoelectric cantilever is induced to vibrate at its natural frequency by the interaction between the magnet on cantilever and the magnets on the rotating mass block. It can collect the nonlinear impact vibration energy of low-frequency motion to meet the energy harvesting of human motion.

Broadband transmission-type 1-bit coding metasurface for electromagnetic beam forming and scanning

Coding metasurfaces make it possible to manipulate electromagnetic (EM) waves digitally by means of several discrete particles. Hence, there have been rapid advances in this field recently. Here we propose a novel design of a broadband transmission-type coding metasurface, which is valid to both x - and y -polarized EM incidences from 8.1-12.5 GHz while satisfies the requirements of 1-bit coding without changing the polarization. Two types of multi-layer coding particles with different geometrical parameters are adopted to represent the digital states “0” and “1”, which are easily promoted to terahertz and optics through modifying the size scale. To verify the ability to manipulate the EM waves, we first adopt the coding metasurface to achieve broadband beam forming by converting spherical waves to plane waves and realize high-directivity pencil beam in far field with low side lobes. We further arrange the particles according to the coding sequence 010101… to steer two symmetrical beams in different directions controlled by frequencies with the maximum range of the scanning angle of 30°-50.5°. The good agreements between the simulated and measured results validate the proposed broadband coding metasurface, indicating its huge potential in communication and radar imaging systems.

Non-Dimensional Analysis of Thermal Effect on Skin Exposure to an Electromagnetic Beam

We consider the problem of inducing withdrawal reflex on a test subject by exposing the subject’s skin to an electromagnetic beam. Heat-sensitive nociceptors in the skin are activated wherever the temperature is above the activation temperature. Withdrawal reflex occurs when the activated volume reaches a threshold. We non-dimensionalize the problem to write the temperature as the product of a parameter-free function of non-dimensional variables and a function of beam parameters. This formulation allows studying beam parameters without knowing skin material parameters. We examine the effects of spot size, total power and distribution type of the electromagnetic beam on 3 quantities at reflex: 1) the time to reflex, 2) the maximum temperature increase, and 3) the total energy consumption. We find that the flat-top beam is the best, with the lowest energy consumption and the smallest maximum temperature increase. The Super-Gaussian beam is only slightly inferior to the flat-top. The Gaussian beam has by far the worst performance among these three.

A Concise Model and Analysis for Heat-Induced Withdrawal Reflex Caused by Millimeter Wave Radiation

In this study, we consider the heat-induced withdrawal reflex caused by exposure to an electromagnetic beam. We propose a concise dose-response relation for predicting the occurrence of withdrawal reflex from a given spatial temperature profile. Our model is distilled from sub-step components in the ADT CHEETEH-E model developed at the Institute for Defense Analyses. Our model has only two parameters: the activation temperature of nociceptors and the critical threshold on the activated volume. When the spatial temperature profile is measurable, the two parameters can be determined from test data. We connect this dose-response relation to a temperature evolution model for electromagnetic heating. The resulting composite model governs the process from the electromagnetic beam deposited on the skin to the binary outcome of subject’s reflex response. We carry out non-dimensionalization in the time evolution model. The temperature solution of the non-dimensional system is the product of the applied power density and a parameter-free function. The effects of physical parameters are contained in non-dimensional time and depth. Scaling the physical temperature distribution into a parameter-free function greatly simplifies the analytical solution, and helps to pinpoint the effects of beam spot area and applied power density. With this formulation, we study the theoretical behaviors of the system, including the time of reflex, effect of heat conduction, biological latency in observed reflex, energy consumption by the time of reflex, and the strategy of selecting test conditions in experiments for the purpose of inferring model parameters from test data.

Terahertz coding metasurface based vanadium dioxide

Terahertz (THz) has the characteristics of non ionization, penetration, water absorption, high resolution, etc. It has shown an important application prospect in many fields, such as non-destructive testing, imaging and communication. However, THz is in the transition frequency band ranges from macro-electronics to micro-photonics, so, it belongs to the interdisciplinary field, forming the “terahertz gap” in electromagnetic wave. In recent years, with the continuous development and improvement of THz radiation source and detection technology, the THz modulation technology has gradually aroused the interest of researchers. Metamaterials with many properties that natural materials do not possess provide a common way to control THz. The two-dimensional structure of a metamaterial is called a metasurface. The coding metasurface encodes the phase digitally so that the electromagnetic wave can be regulated. It is proposed that it is first in the microwave band and then extended to the THz band. In the microwave band, the number, direction and amplitude of the far-field beams can be changed dynamically by programming, which is connected with the integrated circuit such as FPGA by using diodes, but due to the limitation of diode size and micro-nano manufacturing technology, the programmable metasurface in microwave band cannot be well used in THz band. In order to improve the flexibility of THz coding metasurface, in this paper we choose the phase change material vanadium dioxide (VO<sub>2</sub>) to active modulation coding metasurface. In this paper, we analyze the VO<sub>2</sub>’s insulating state before the phase transformation and metallic state after the phase transformation. Then designing an active control 1 bit coding metasurface by using the influence of the two states on the amplitude and phase of the unit structure, which is composed of VO<sub>2</sub>, polyimide and aluminum, can not only realize the basic function of coding metasurface adjusting the electromagnetic wave beams, but alsoimplement the switching of two kinds of far-field beams at 1.1 THz for the same coding sequence by thermal stimulated VO<sub>2</sub>. The coding metasurface also realizes the switching between two near-field focal points at 1.1 THz for the same coding sequence. Based on the effect of VO<sub>2</sub> on the phase, this coding metasurface provides a new way to adjust and control the THz wave flexibly, and will have a great application prospect in THz transmission, imaging and communication.

N* Experiments and what they tell us about Strong QCD Physics

I give an overview on experimental studies of the spectrum and the structure of the excited states of the nucleon and what we can learn about their in ternal structure. One focus is on the efforts to obtain a more complete picture of the light-quark baryon exci tation spectrum employing electromagnetic beams that will allow us to draw some conclusions on the symme tries underlying the spectrum. For the higher mass ex citations, the full employment of coupled channel ap proaches is essential when searching for new excited states in the large amounts of data already accumulated in different channels involving a variety of polarization observables. The other focus is on the study of transition form factors and helicity amplitudes and their de pendences on Q2, especially on some of the more promi nent resonances, especially Δ(1232)3/2+, N(1440)1/2+, and negative parity states N(1535)1/2-, and N(1675)5/2-.These were obtained in pion and eta electroproduction experi ments off proton targets and have already led to further insights in the active degrees-of-freedom as a function of the distance scale involved.

Снижение донного сопротивления и создание подъемной силы с использованием микроволнового разряда в сверхзвуковом потоке

Results of experimental studies of deeply subcritical microwave discharge force action on bodies placed in a supersonic air stream are presented. The discharge has been ignited in a quasi-optical electromagnetic beam.