Direction Of Electromagnetic Wave Propagation Research Articles

Far field ring beam generation based on 3-bit encoded metasurface

Abstract The encoding metasurface establishes a bridge between the physical and digital worlds, ushering in a new era of manipulating electromagnetic waves and realizing programmable metamaterials through digital coding sequences. This "digital metasurface," relying on binary logic, significantly simplifies the design process, thereby enhancing the flexibility and efficiency of controlling electromagnetic waves. While most encoding metasurfaces control beamforming for pencil-shaped beams, we propose a 3-bit encoding metasurface with a "well structure" in the microwave band.The 3-bit encoding metasurface features a more extensive encoding sequence, offering increased degrees of freedom and flexibility in manipulating the direction of electromagnetic wave propagation. Its symmetric design features polarization-insensitive characteristics, suitable for generating annular beams by varying gradient-encoded numbers radially. This approach enables the production of linearly polarized omnidirectional radiation within the desired elevation angle range. Additionally, adjustments in quantity and orientation of the produced annular beams are achieved through Fourier convolution theorem. This type of annular beams holds promise for applications in various fields, including wireless radio broadcasting and wireless local area networks.

Asymmetric transmission for linearly polarized terahertz waves without polarization conversion

In this study, a novel metamaterial (MM) architecture is presented, which comprises cut wires and split-ring resonators (SRRs) and demonstrates asymmetric transmission characteristics. Unlike previously reported multi-layer chiral structures, spatial asymmetry is introduced by aligning the cut wires and SRRs with the propagation direction of electromagnetic waves. This innovative alignment effectively disrupts time-reversal symmetry, enabling asymmetric transmission. A thorough analysis of the electromagnetic field distribution and surface current patterns offers insights into the fundamental mechanisms underlying this asymmetric transmission. Notably, the ability of this MM design to maintain the polarization state of transmitted waves positions it as a promising candidate for terahertz systems applications.

Parameter Extraction of Accelerated Moving Targets under Non-Quasi-Axial Incidence Conditions Based on Vortex Electromagnetic Wave Radar

Vortex electromagnetic wave radar carrying orbital angular momentum can compensate for the deficiency of planar electromagnetic wave radar in detecting motion parameters perpendicular to the direction of electromagnetic wave propagation, thus providing more information for target recognition, which has become a hot research field in recent years. However, existing research makes it difficult to obtain the acceleration and rotation centers of targets under non-quasi-axial incidence conditions of vortex electromagnetic waves. Based on this, this article proposes a variable speed motion target parameter extraction method that combines single element and total element echoes. This method can achieve three-dimensional information extraction of radar targets based on a uniform circular array (UCA). Firstly, we establish a non-quasi-axis detection echo model for variable-speed moving targets and extract echoes from different array elements. Then, a single element echo is used to extract the target’s range slow time profile and obtain the target’s rotation center z coordinate. We further utilize the target linear, angular Doppler frequency shift extremum, and median information to extract parameters such as target acceleration, tilt angle, rotation radius, and rotation center x and y coordinates. We analyzed the impact of different signal-to-noise ratios and motion states on parameter extraction. The simulation results have verified the effectiveness of the proposed algorithm.

Full‐Space Janus Meta‐Lens for Shared‐Aperture Transmission‐Reflection‐Independent Focusing of Electromagnetic Wave

Janus metasurfaces emerge as a promising platform for implementing multiple wave functionalities by fully exploiting the inherent propagation direction of electromagnetic waves. Their out‐of‐plane asymmetric structures enable different wave functions depending on the propagation direction. Herein, a multiplexed Janus metasurface is proposed, which operates in the microwave region to flexibly manipulate the transmission and reflection wavefronts for same linearly polarized (LP) incidence propagating along the two opposite directions. A meta‐lens is constructed to validate the concept of full‐space shared‐aperture transmission‐reflection‐independent focusing of electromagnetic (EM) waves, exhibiting four distinct focusing performances. Experiments are conducted in the microwave region that agree well with the simulation results. The proposed full‐space Janus metasurface may provide a platform for asymmetric imaging, multichannel information processing, and encrypted communication.

Investigation on high EMI shielding effectiveness and shielding mechanism of spherical Ti3C2Tx microfilm prepared by spray-freezing

Due to the explosive growth of the number of miniature electronic devices in the 5 G era, higher requirements have been raised for electromagnetic pollution protection materials. The production of effective electromagnetic (EM) interference (EMI) shielding materials is still challenging. In this research, Ti3C2Tx microspheres were successfully synthesized by the self-developed spray freezing technology and 43 µm-thick spherical Ti3C2Tx microfilm was prepared by vacuum filtration. The spherical film can block 5 G communication signals and its EMI shielding effectiveness (SE) is 51 dB, which is 50% higher than that of the 2D Ti3C2Tx film of approximately the same thickness. Additionally, the effects of transmitting X-band EM waves through a 43 µm-thick spherical Ti3C2Tx sheet were simulated employing Comsol Multiphsics. It is found that an annular current traveling perpendicular to the direction of electromagnetic wave propagation will be induced in the spherical Ti3C2Tx, which leads an extra larger EM loss. This discovery provides a novel avenue for the structural design of EMI shielding materials.

Analysis of Information Transmission Characteristics Based on Adaptive Ground Electrode Current Field

The information transmission mechanism of the ground electrode current field uses a very low-frequency electrical signal, which is applied to the two electrodes driven into the soil layer or the collapsed body of the tunnel to form a current field in the rock layer or soil layer. Signal detection is created via the strong penetration of wireless information transmission. This research focuses on various electromagnetic effects, such as polarization, magnetization, and the transmission of electromagnetic waves under the influence of different media, such as rock, sand, reinforced concrete, and air voids. The influence of these adaptive electromagnetic effects on the transmission of electromagnetic waves is mainly reflected in the reflection, refraction, and attenuation of electromagnetic wave signals. The inhomogeneity of the earth medium, the influence of topographic features, and multi-path transmission all cause signal distortion, fading, or changes in the direction of electromagnetic wave propagation. By studying the three physical quantities of magnetic permeability, permittivity, and conductivity, the electromagnetic characteristics of the earth medium are described to research the information transmission characteristics of the earth electrode current field.

Theoretical research on the transverse spin of structured optical fields inside a waveguide

Structured optical fields inside a waveguide possess the transverse spin, i.e., the spin angular momentum perpendicular to the direction of the waveguide. The physical origin of the transverse spin can be attributed to the presence of an effective rest mass of photons in guided waves, or equivalently, to the existence of a longitudinal field component, such that the transverse and longitudinal fields together form an elliptical polarization plane. In contrary to the traditional viewpoint, the transverse spin of photons in guided waves is also quantized, and its quantization form is related to the ellipticity of the polarization ellipse. The direction of the transverse spin depends on the propagation direction of electromagnetic waves along the waveguide, such a spin-momentum locking may have important applications in spin-dependent unidirectional optical interfaces. By means of a coupling between the transverse spin of guided waves and some physical degrees of freedom, one can develop an optical analogy of spintronics, i.e., spinoptics.

Photonic band gaps and waveguide slow-light propagation in Bravais–Moiré two-dimensional photonic crystals

Photonic band gap widths and slow-light optical guided modes are theoretically investigated for Bravais–Moiré (BM) photonic crystals (PCs) made of cylindrical dielectric cores which are formed from the combination of two square Bravais lattices. The Moiré pattern forms due to a commensurable rotation of one of these lattices with respect to the other. The analysis of gap maps is made versus the radii of dielectric cores—both rotated and unrotated—contained in the BM unit cell (UC). Guided modes are considered within the framework of coupled-resonator optical waveguides (CROWs), built from the generation of a point defect chain along the direction of electromagnetic wave propagation. For the analyzed structures, rather wide photonic band gaps were found. It was noticed that changing the core radii can significantly affect the dielectric contrast in the UC, leading to wider gaps. In addition, due to the kind of crystal cell structure considered, guided modes with group velocities smaller than those typically observed in PCs with simple square lattices were found for the investigated CROWs.

Using Improved XGBoost Algorithm to Obtain Modified Atmospheric Refractive Index

Atmospheric refraction is a special meteorological phenomenon mainly caused by gas molecules and aerosol particles in the atmosphere, which can change the propagation direction of electromagnetic waves in the atmospheric environment. Atmospheric refractive index, an index to measure atmospheric refraction, is an important parameter for electromagnetic wave. Given that it is difficult to obtain the atmospheric refractive index of 100 meters (m)–3000°m over the ocean, this paper proposes an improved extreme gradient boosting (XGBoost) algorithm based on comprehensive learning particle swarm optimization (CLPSO) operator to obtain them. Finally, the mean absolute percentage error (MAPE) and root mean-squared error (RMSE) are used as evaluation criteria to compare the prediction results of improved XGBoost algorithm with backpropagation (BP) neural network and traditional XGBoost algorithm. The results show that the MAPE and RMSE of the improved XGBoost algorithm are 39% less than those of BP neural network and 32% less than those of the traditional XGBoost. Besides, the improved XGBoost algorithm has the strongest learning and generalization capability to calculate missing values of atmospheric refractive index among the three algorithms. The results of this paper provide a new method to obtain atmospheric refractive index, which will be of great reference significance to further study the atmospheric refraction.

Off/on switchable smart electromagnetic interference shielding aerogel

Summary Smart electromagnetic interference (EMI) shielding materials with tunable EM wave response characteristics are attractive for future EM devices. Here, we report an off/on switchable EMI shielding material that can be tuned from EM wave transmission to shielding and vice versa by mechanical compression and decompression. This smart material is prepared by filling conductive carbon nanoparticles into wood-derived lamellar carbon aerogel, showing reversible compressibility and strain-sensitive conductivity. The original aerogel has good impedance matching and low dielectric loss, allowing the EM wave to pass through. Mechanical compression enables the carbon nanoparticles to establish highly conductive pathways in the aerogel, which significantly increases the conductivity of aerogel and thus activates its EMI shielding performance. This function switch is reversible by repeatedly compressing and decompressing the aerogel. The availability of such a never-before-realized off/on switchable EMI shielding material provides an opportunity for the development of advanced smart EM devices.

A New Method to Determine the Spatial Sensitivity of Time Domain Reflectometry Probes Based on Three-Dimensional Weighting Theory

The time domain reflectometry (TDR) method has been widely used to measure soil water content for agriculture and engineering applications. Quick design and optimization of the probe is crucial to achieving practical utilization. Generally, the two-dimensional weighting theory, calculation of the spatial sensitivity of TDR probes in the plane transverse to the direction of electromagnetic wave propagation, and relevant numerical simulation techniques can be used to solve any issues. However, it is difficult to tackle specific problems such as complex probe shape, end effect, and so forth. In order to solve these issues, a method including a three-dimensional weighting theory and the relevant numerical simulation technique was proposed and verified to confirm the feasibility of this method by means of comparing the existing experimental results and the computational values. First, a shaft probe was used to determine the impact of the shaft on the effective dielectric constant of the probe. Then, three-rod probes were calibrated by a sample with a special shape and water-level variations around the probe using the proposed method to determine the values of the apparent dielectric constant. Besides, model boundary size and end effect were also considered in the computation of dielectric constants. Results showed that compared with the experimental and computational data, the newly proposed method calculated the measurement sensitivity of the shaft probes well. In addition, it was observed that experiment dielectric constant values were slightly different from computational ones, not only using a vertical probe but also horizonal probe. Moreover, it was also found that there was a slight influence of sample shape and end effect on the apparent dielectric constant, but model boundary size has a certain impact on the values. Overall, the new method can provide benefits in the design and optimization of the probe.

Multiphysics simulations of adaptive metasurfaces at the meta-atom length scale

Abstract Adaptive metasurfaces (MSs) provide immense control over the phase, amplitude and propagation direction of electromagnetic waves. Adopting phase-change materials (PCMs) as an adaptive medium allows us to tune functionality of MSs at the meta-atom length scale providing full control over MS (re-)programmability. Recent experimental progress in the local switching of PCM-based MSs promises to revolutionize adaptive photonics. Novel possibilities open new challenges, one of which is a necessity to understand and be able to predict the phase transition behavior at the sub-micrometer scale. A meta-atom can be switched by a local deposition of heat using optical or electrical pulses. The deposited energy is strongly inhomogeneous and the resulting phase transition is spatially non-uniform. The drastic change of the material properties during the phase transition leads to time-dependent changes in the absorption rate and heat conduction near the meta-atom. These necessitate a self-consistent treatment of electromagnetic, thermal and phase transition processes. Here, a self-consistent multiphysics description of an optically induced phase transition in MSs is reported. The developed model is used to analyze local tuning of a perfect absorber. A detailed understanding of the phase transition at the meta-atom length scale will enable a purposeful design of programmable adaptive MSs.

Study on the velocity band gap characteristics of photonic crystal under the relativistic conditions

The periodic dielectric slabs are investigated. The reflection and transmission characteristics of electromagnetic wave are researched when photonic crystals are moved at a uniform velocity close to the light speed under the relativistic conditions. When the motion direction of photonic crystal is perpendicular to the propagation direction of electromagnetic wave, the electromagnetic band gap is formed alternately by the reflection spectrum of photonic crystal changed with velocity. The gap is different from the traditional electromagnetic band gap, which is not a function of the spectrum but corresponding to the velocity. The peculiar phenomenon generated by electromagnetic wave at different velocity band is called velocity band gap. The velocity band gap is affected by the frequency of electromagnetic wave, period of photonic crystal, permittivity and other parameters. It varies periodically in the whole velocity range and decreases with the increasing velocity. The velocity band gap is expanded and moved to the low velocity interval with the period of photonic crystal increasing. When the difference in permittivity of the two adjacent dielectric slabs becomes higher and the frequency of electromagnetic wave increases, the velocity band gap is changed accordingly. The change is similar to the state in which a spring fixed at one end is stretched, but the change in velocity band gap is not uniform.

A Broadband Tunable Coaxial Attenuator Based on Graphene

A dynamically tunable coaxial attenuator based on graphene is presented in this paper. The attenuator is achieved by inserting a graphene sandwich structure (GSS) inside the dielectric of coaxial line along the propagation direction of electromagnetic waves to dissipate the electromagnetic field. The mechanism of attenuator is theoretically analyzed, and an approximate formula to calculate attenuation with different parameters is proposed. We experimentally demonstrate that attenuation of the attenuator can be tuned from 3 dB to 14.5 dB with good flatness in broad operation frequency from 2 GHz to 9 GHz when graphene impedance is changed by different bias voltage. The theoretical calculation and simulation results have a good agreement with experimental results, while the $\vert S_{11}\vert $ can always remain low level under -15 dB.

Rough Ocean Surface Effects on Evaporative Duct Atmospheric Refractivity Inversions Using Genetic Algorithms

AbstractRadar performance is impacted by variations in atmospheric refractivity because it changes the direction of electromagnetic wave propagation. Because direct measurement and simulation of atmospheric refractivity at high resolution over large distances is challenging, inversion techniques have developed to address this knowledge gap in predicting refractivity. Here we use synthetic radar data to solve refractivity inversion problems, focusing on evaporation ducts. Unlike most prior “refractivity from clutter” studies, this study examines inversions based on point‐to‐point propagation rather than sea clutter. This approach removes complexities associated with uncertainties in surface reflection coefficients; however, the sea surface still plays a role in the forward scattering and reflection (multipath) of electromagnetic waves. Using synthetic data that permit quantitative evaluation of inverse solution accuracy and detailed knowledge regarding the sea state conditions used to produce the data, we evaluate the impact various representations of the sea surface have on the accuracy of refractivity inversions. These representations include neglect of the sea surface, use of the same phase‐resolved surface, and use of a statistically equivalent sea surface. Results show that neglect of the sea surface and use of a statistically equivalent sea surface significantly decreases the accuracy of inverse solutions, and this decrease primarily results from discrepancies in the multipath pattern that generate larger variations in the propagation than does the refractivity. We show that averaging propagation over several different phases of the sea surface aids in washing out the multipath pattern to increase sensitivity of the inversion to the features of the refractivity profile.

A Study on Characteristics of Electromagnetic Waves Propagating Through the Space Between Overlapped Metal Plates

The characteristics of electromagnetic (EM) waves propagating through the space between overlapped metal (conductive) plates and electrically conductive parts, which usually exists around a metal shielded box, are discussed in this study. The authors developed a method for measuring the current density distribution on the interface between the overlapped and energized metal plates. Using this method, the authors estimated that the general area of the electrically conductive parts is less than about 10 × 10 mm2. Thus, the space between overlapped metal plates and electrically conductive parts is frequently expected to be extremely thin for the width and short in the direction of EM wave propagation. In this case, it was confirmed that the extremely thin and short space can be regarded as a rectangular waveguide for the propagation of EM waves.

Design and experimental verification of single-layer high-efficiency transmissive phase-gradient metasurface

Polarization characteristic is an important feature of electromagnetic (EM) wave. Manipulating polarization state and controlling propagation direction of EM wave by phase-gradient metasurface (PGM) have become a research hotspot in recent years. However, using transmissive PGM for polarization manipulation often suffers a low efficiency. To alleviate this problem, multilayered structure was utilized. However, it often suffered bulky volume and design complexity. Therefore, engineering a thin high-efficiency transmissive PGM with polarization manipulation is a pressing and challenging issue. In this paper, a single-layer high-efficiency transmissive PGM with cross-polarization conversion and anomalous refraction is designed. To illustrate the working mechanism, the PGM is comprehensively investigated through theoretical analysis, EM simulations and experimental measurements. The unit cell evolving from an electric-field-coupled resonator is carefully designed to exhibit a Pancharatnam-Berry phase gradient. Each rotated element irradiated separately by the normally-incident left-handed circularly polarized (LHCP)and right-handed circularly polarized (RHCP) waves is simulated in CST microwave studio. The results show that the cross-polarization transmission magnitude keeps over 0.9 and does not change as the rotation angle varies. Moreover, the phase shift is twice the rotation angles and the direction of refracted beam is opposite under the above two different polarizations. In addition, the cross-polarization conversion ratio is above 0.9 from 14 GHz to 15.8 GHz. On the premise of high transmission magnitude, the phase of the cross-polarized transmission can be freely manipulated via varying axis orientation. By spatially arranging six unit cells in rotation angle steps of 30, a PGM with a phase difference of 60 between adjacent unit cells is designed. As is well known, linearly-polarized (LP) EM waves can be decomposed into LHCP and RHCP waves with equal amplitudes. Therefore, an LP wave through the PGM will be separated into two counterpropagating CP waves. The high-efficiency anomalous refraction of the PGM is verified from simulated near-field electric field distributions and far field normalized power patterns. The simulated refracted angle is 33.5, which is in accordance with the theoretical designed value (33.75). Moreover, the transmissive power intensity spectrum under the normally-incident LP waves is simulated and measured. The simulated and measured results are in good agreement with each other, showing that the transmitted wave is perfectly split into two counterpropagating waves from 14.9 GHz to 15.3 GHz. Compared with the available transmissive PGMs, our proposed PGM features high efficiency and thin structure with only single layer, making the proposed PGM a promising alternative to manipulating propagation and polarization of EM waves.

Transmission performance of 1 × 2 type photonic crystal power splitter with ring resonators

The structure of 1×2 type photonic crystal power splitter with ring resonators is designed in a two-dimension square-lattice dielectric-rod photonic crystal structure, and it consists of one input and two outputs. Its transmission performance is investigated by using the finite-difference time-domain (FDTD) method. The results show that the structure has high transmission efficiency in the third communication window (wavelength 1550nm), can split pass band electromagnetic waves equally. It is compact with low loss and design flexibility in propagation direction of electromagnetic waves.

Effects of SiC shape and oxidation on the infrared emissivity properties of ZrB2–SiC ceramics

ZrB2–SiC ceramics were prepared by hot-pressing sintering using SiC whiskers and SiC particles and the effects of SiC shape and oxidation on the infrared emissivity properties of ZrB2–SiC ceramics were investigated systematically. SiC whiskers and SiC particles affected the spectral emissivity of ceramics by changing the propagation direction of electromagnetic waves. Compared with SiC particles, SiC whiskers had smaller surface area at the same volume ratio and caused relatively higher infrared emissivity in ceramics. After oxidation at 1300°C for different time, the total emissivity of ZrB2–SiC ceramics varied in 0.70–1.00. The oxide layer on sample surface slowed down the increase rate of infrared emissivity and the substrate dominated the radiant behavior of ceramics. The investigation also illustrated that the exposed ZrO2 on the samples surface degenerated the infrared emissivity properties of ZrB2–SiC ceramics.

Numerical analysis of propagation characteristics of electromagnetic wave in lossy left-handed material media

The propagation characteristics of electromagnetic wave in lossy left-handed materials (LHM) are studied using finite-difference time-domain (FDTD) method base on auxiliary differential equation (ADE) technology. The LHM medium is realized with lossy Drude models for both the negative electric permittivity and the negative magnetic permeability. The discretized ADE-FDTD equations are derived in detail. The incident wave used in the simulation is a multiple cycle m-n-m pulses source. The term of Poynting's vector ExHy was calculated. These numerical results demonstrate conclusively that the phase velocity direction of electromagnetic wave propagation and the direction of the Poynting vectors are anti-parallel in LHM. The amplitude of electric field is reduced with the enhancive distance of LHM slab. It is also demonstrated that the energy of electromagnetic wave in the LHM slab is obviously attenuated, and the attenuation of energy becomes stronger with the angular plasma frequency ωp increasing. These results indicate that LHM stealth is effective in theory, and reasonable selection of the large negative index of refraction can greatly enhance its effectiveness.