Mode Theory Research Articles

Omnidirectional circularly polarized base station antenna with high gain and wide band

Omnidirectional circularly polarized base station antenna with high gain and wide band

On the Limitations of the Coupled Mode Theory and Parity-Time Symmetry for Near-Field Wireless Power Transfer Research

On the Limitations of the Coupled Mode Theory and Parity-Time Symmetry for Near-Field Wireless Power Transfer Research

Multiple Fano resonances by MIM waveguide coupling whispering gallery mode resonator and application in Refractive index sensing

In this study, a baffled metal–insulator–metal (MIM) waveguide coupled with a whispering gallery mode resonator (WGMR) is proposed. This structure could excite quadruple Fano resonances. The asymmetric Fano resonance transmittance spectrum and the electric field at the resonance peak were numerically simulated using the finite difference time domain method. The obtained data were fitted using the multimode interference coupled mode theory. The number of Fano resonance peaks was tuned by the outer radius of the WGMR. With other geometric parameters unchanged, the number of Fano resonance peaks increased with increasing outer radius of the WGMR. Thus, it achieved multiple Fano resonances by adjusting solely the radius of the WGMR. When the ring width was fixed, the structure could excite multiple Fano resonances within a certain outer radius range. This structure was used for refractive index sensing and its sensitivity and figure of merit (FOM) were 3004 nm/RIU and 3.14 × 104 in a gas environment, respectively. Therefore, the proposed structure can excite multiple Fano resonances and achieve high sensitivity and FOM, providing a theoretical basis for micro and nano applications.

Research on Yaw Stability Control of Front-Wheel Dual-Motor-Driven Driverless Formula Racing Car

In order to improve the yaw stability of a front-wheel dual-motor-driven driverless vehicle, a yaw stability control strategy is proposed for a front-wheel dual-motor-driven formula student driverless racing car. A hierarchical control structure is adopted to design the upper torque distributor based on the integral sliding mode theory, which establishes a linear two-degree-of-freedom model of the racing car to calculate the expected yaw angular velocity and the expected side slip angle and calculates the additional yaw moments of the two front wheels. The lower layer is the torque distributor, which optimally distributes the additional moments to the motors of the two front wheels based on torque optimization objectives and torque distribution rules. Two typical test conditions were selected to carry out simulation experiments. The results show that the driverless formula racing car can track the expected yaw angular velocity and the expected side slip angle better after adding the yaw stability controller designed in this paper, effectively improving driving stability.

Bond analysis in meta- and para-substituted thiophenols: overlap descriptors, local mode analysis, and QTAIM.

This study delves into the chemical nuances of thiophenols and their derivatives through a comprehensive computational analysis, moving beyond traditional energetic perspectives such as bond dissociation enthalpy and S-H dissociation dynamics. By employing the overlap model along with its topological descriptors (OP/TOP), quantum theory of atoms in molecules (QTAIM), and local vibrational mode (LVM) theories, the research provides a deeper understanding of the S-H and C-S bonding scenarios in substituted thiophenols. The investigation follows the electron-donating capacity of S-H substituent variation with the nature and positioning of other ring substituents. Energy profile analyses indicate distinct stability differences in the cis and trans conformations of meta- and para-PhSH systems, influenced by the electron-donating strength of these substituents. The study also uncovers significant variations in S-H bond distances and descriptor values, particularly in para-substituted PhSH, reflecting the influence of electron-donating or withdrawing substituents. In contrast, alterations at the meta-position show minimal effects on C-S bond descriptors, while para-substitutions markedly influence C-S bond characteristics, demonstrating a clear correlation with the electron-donating or withdrawing capabilities of the substituents. This research sheds light on the intricate bond dynamics in aromatic systems with diverse substituents, highlighting the complex interaction between electronic effects and molecular conformation. The study employs the B97X-D/Def2TZVP level of theory for molecular geometries, ensuring accurate characterization of structures as true minima via analytical harmonic frequency determination. The electronic properties of S-H and C-S bonds in variously substituted thiophenols were analyzed using OP/TOP, QTAIM, and LVM methodologies. Computational processes, including conformational scans, geometry optimizations, and vibrational frequency calculations, were conducted using Gaussian 09, with ultra-fine integration grids and tight convergence criteria for the SCF procedure. Bond descriptors were computed utilizing ChemBOS, Multiwfn, and LModeA software, providing a robust and detailed examination of bond properties.

The Determination of Effective Modes of Wind-Bridge Coupling System Based on Stabilization Clustering Method

Modal identification is the core content of the full-bridge aeroelastic model wind tunnel test. Modal consistency is the most direct and effective basis to ensure that the model and the real bridge meet the similarity criteria. Accurate, simple and fast identification of modal parameters has very important practical significance for the evaluation and correction of aeroelastic models. In this paper, the eigensystem realization algorithm (ERA) and stochastic subspace identification (SSI) is improved, and the automatic identification theory of effective modes of bridge aeroelastic model based on stable clustering method is proposed. The stable diagram eliminates the unstable modes by comparing the results of different system orders, and the pedigree clustering further polarizes the stable modes to obtain multi-order and high-precision system modes. Numerical simulation examples and wind tunnel engineering examples show that the method has high accuracy and good adaptability, and can solve the problem of unstable damping identification results to a certain extent. In the wind tunnel test, the stable clustering method is one of the effective methods to identify the modal of the bridge aeroelastic model excited by the wind environment.

Coupled mode theory for plasmonic couplers

Photonic integrated circuits play an increasingly important role in several emerging technologies. Their functionality arises from a combination of integrated components, e.g., couplers, splitters, polarization rotators, and wavelength selective filters. Efficient and accurate simulation of these components is crucial for circuit design and optimization. In dielectric systems, design procedures typically rely on coupled-mode theory (CMT) methods, which then guide subsequent refined full-wave calculations. Miniaturization to deep sub-wavelength scales requires the inclusion of lossy plasmonic (metal) components, making optimization more complicated by the interplay between coupling and absorption. Even though CMT is well developed, there is no consensus as to how to rigorously and quantitatively implement it for lossy systems. Here we present an intuitive coupled-mode theory framework for quantitative analysis of dielectric–plasmonic directional and adiabatic couplers, whose large-scale implementation in 3D is prohibitively slow with full-wave methods. This framework relies on adapting existing coupled mode theory approaches by including loss as a perturbation. This approach will be useful in designing dielectric–plasmonic circuits, providing a first reference point for anyone using techniques such as inverse design and deep learning optimization methods.

Direct Evidence of Drift‐Compressional Wave Generation in the Earth's Magnetosphere Detected by Arase

AbstractWe present the first direct evidence of an in situ excitation of drift‐compressional waves driven by drift resonance with ring current protons in the magnetosphere. Compressional Pc4–5 waves with frequencies of 4–12 mHz were observed by the Arase satellite near the magnetic equator at L ∼ 6 in the evening sector on 19 November 2018. Estimated azimuthal wave numbers (m) ranged from −100 to −130. The observed frequency was consistent with that calculated using the drift‐compressional mode theory, whereas the plasma anisotropy was too small to excite the drift‐mirror mode. We discovered that the energy source of the wave was a drift resonance instability, which was generated by the negative radial gradient in a proton phase space density at 20–25 keV. This proton distribution is attributed to a temporal variation of the electric field, which formed the observed multiple‐nose structures of ring current protons.

Characteristic Mode Solution of Complex-Coefficient Complex-Solution Differential Equations

Computation of complex-coefficient complex-solution differential equations is a problem that arises in various domains of science and engineering. This paper aims at applying the Theory of Characteristic Modes (TCM) approach along with the Method of Moments (MoM) in solving these problems with emphasis on procedures for higher differential equations. Several available methods, known in literatures, are available for solving the problem. The complexity of the available methods differs based on the accuracy of the solution. In this paper, the general method is first presented and then a simplified version of it is proposed to solve high order differential equations. Two examples are illustrated, a third and a fourth order complex-coefficients complex-solution differential equations, to show the simplicity of the proposed method. The proposed approach can be also introduced along with other methods to solve these special occurrences differential equations and other boundary value problems.

Observations of Rogue Seas in the Southern Ocean.

We report direct observations of surface waves from a stereo camera system along with concurrent measurements of wind speed during an expedition across the Southern Ocean in the austral winter aboard the South African icebreaker S.A. Agulhas II. Records include water surface elevation across a range of wave conditions spanning from early stages of wave growth to full development. We give experimental evidence of rogue seas, i.e., sea states characterized by heavy tails of the probability density function well beyond the expectation based on bound mode theory. These conditions emerge during wave growth, where strong wind forcing and high nonlinearity drive wave dynamics. Quasiresonance wave-wave interactions, which are known to sustain the generation of large amplitude rogue waves, capture this behavior. Wave statistics return to normality as the wind forcing ceases and waves switch to a full developed condition.

Computer vision-based measurement of pure- and mixed-gas swelling in polymer membranes: Individual and synergistic effects of configurational free volume and thermal rearrangement

Although polymer swelling (i.e., dilation) data are crucial to assess the long-term stability of polymer membranes and correct sorption isotherms for highly sorbing penetrants, the swelling propensity of new generation membranes exhibiting configurational free volume (i.e, triptycene units) has never been investigated. To fill this gap, in this paper we study pure and mixed-gas swelling of a series of thermally rearranged polybenzoxazoles (TPBOs) and their poly(o-hydroxyimide) precursors exhibiting systematically varied amount of configurational free volume. The individual and synergistic effects of configurational free volume and thermal rearrangement on the polymer swelling propensity are presented and discussed. To do so, we use a newly developed experimental approach based on computer vision, which is capable of correcting most of the limitations of traditional methods used to measure swelling and achieve superior accuracy. We show that thermally rearranged glassy polymers exhibiting configurational free volume exhibit better swelling resistance relative to conventional polymers. The fundamental origin of this behavior resides, as explained by the dual mode theory, on the fact that the Langmuir's mode swelling propensity in thermally rearranged triptycene-based polymers is much lower than in glassy polymers exhibiting only conformational free volume. Differences between pure-gas and mixed-gas induced swelling are also highlighted and discussed. Finally, a thorough investigation of the swelling measurement uncertainty and precision is provided and discussed, including an analysis of the major factors affecting experimental uncertainty.

Effect of surface anchoring energy on a liquid crystal optical waveguide-based polarization rotator

This study reports the effect of the surface anchoring energy of a liquid crystal (LC) cell on the performance of the liquid crystal optical waveguide polarization rotator (LCOW-PR) for the purpose of providing a theoretical reference for practical preparation of the LCOW-PR. First, the expression for the deflection angle of the director at the boundary of the LC cell is derived so that the distributions of the director and dielectric tensor of the LC can be accurately solved under any anchoring energy. On this basis, the correlation between the crucial indicators such as the polarization conversion length (PCL) together with the polarization conversion efficiency (PCE) of the LCOW-PR and the anchoring effect strength is constructed by combining with the existing numerical algorithms. The numerical results show that the maximum variation of the PCL is lower than 0.1 µm as the anchoring effect strength increases from 1×10−6J/m2 to 1×10−3J/m2, while the PCE decreases from 99.72% to 78.33%. This implies that the PCL of the LCOW-PR does not depend on the surface anchoring energy, but the anchoring effect strength of the orientational layer must be controlled to the order of 10−6J/m2 or even lower to achieve high-performance conversion between the polarization modes. Simultaneously, the effectiveness of the calculations in this work is verified with the help of the coupled mode theory as well as a comparison to previous reports.

Assessment of the Madden‐Julian Oscillation in CMIP6 Models Based on Moisture Mode Theory

AbstractThe moist processes of the Madden‐Julian Oscillation (MJO) in the Coupled Model Intercomparison Project Phase 6 models are assessed using moisture mode theory‐based diagnostics over the Indian Ocean (10°S–10°N, 75°E–100°E). Results show that no model can capture all the moisture mode properties relative to the reanalysis. Most models satisfy weak temperature gradient balance but have unrealistically fast MJO propagation and a lower moisture‐precipitation correlation. Models that satisfy the most moisture mode criteria reliably simulate a stronger MJO. The background moist static energy (MSE) and low‐level zonal winds are more realistic in the models that satisfy the most criteria. The MSE budget associated with the MJO is also well‐represented in the good models. Capturing the MJO's moisture mode properties over the Indian Ocean is associated with a more realistic representation of the MJO and thus can be employed to diagnose MJO performance.

Terahertz refractive index sensor based on dual plasmon-induced transparency in a graphene metasurface

A novel graphene metasurface is proposed in this article, which is simulated by the finite difference time domain (FDTD) and found to exhibit a significant dual Plasmon-induced transparency (PIT) phenomenon in the terahertz frequency band. For further research, new transmission spectra are simulated using the coupled mode theory (CMT), and after comparison, it is found that the images were basically consistent and had a high degree of agreement. In addition, through observation and analysis of the PIT window, it is found that the PIT phenomenon can be effectively tuned by adjusting the Fermi level. Finally, the metasurface is proposed in this article, which has excellent sensing properties. After calculation and comparison of different refractive indices of surrounding media, it is found that the maximum sensitivity can reach 1.567 THz RIU−1 at a frequency of f = 4.8716 THz, with a figure of merit (FOM) of 23.855. Compared with other sensors of the same type, the sensor in this paper has great advantages. In summary, the graphene metasurface proposed in this article provides more theoretical support for manufacturing dynamically adjustable graphene sensors.

Adaptive Terminal Sliding Mode Trajectory Tracking Control for Autonomous Vehicles Considering Completely Unknown Parameters and Unknown Perturbation Conditions

In the actual implementation of autonomous vehicle controller and related applications, it is difficult to obtain all the actual parameters of the vehicle. Considering factors such as uneven pavement and different pavement conditions, it is difficult to accurately establish the vehicle dynamic system model. Based on the non-singular terminal sliding mode and adaptive control theory, this paper establishes a trajectory tracking control strategy for an autonomous vehicle with unknown parameters and unknown disturbances. Firstly, the complex trajectory tracking problem is decoupled from the position and heading angle tracking problem, and the preview error equation is established. Secondly, a non-singular terminal sliding mode (NTSM) controller is established to stabilize the trajectory tracking error to the origin in a finite time, and adaptive laws are proposed to estimate the unknown vehicle parameters to adapt to environmental changes. Through the CarSim–Matlab platform, typical working conditions are implemented to verify the proposed controller. Our experimental outcomes affirm that the NTSM controller effectively guarantees the autonomous vehicle’s accurate following of the reference path, ensuring smooth control inputs throughout the entire process.

All-optical neuromorphic XOR and XNOR operation utilizing a photonic spiking neuron based on a passive add-drop microring resonator.

We propose a concise hardware architecture supporting efficient exclusive OR (XOR) and exclusive NOR (XNOR) operations, by employing a single photonic spiking neuron based on a passive add-drop microring resonator (ADMRR). The threshold mechanism and inhibitory dynamics of the ADMRR-based spiking neuron are numerically discussed on the basis of the coupled mode theory. It is shown that a precise XOR operation in the ADMRR-based spiking neuron can be implemented by adjusting temporal differences within the inhibitory window. Additionally, within the same framework, the XNOR function can also be carried out by accumulating the input power over time to trigger an excitatory behavior. This work presents a novel, to the best of our knowledge, and pragmatic technique for optical neuromorphic computing and information processing utilizing passive devices.

Characteristic mode based self isolated MIMO antenna design for millimeter wave 5G communications

The paper introduces a self-isolated quad-port multi input multi output antenna system designed for fifth-generation wireless communication. The reported antenna structure possesses an overall dimension of 12.4 × 12.4 × 0.508 mm3 and is derived from a single-element antenna, featuring a circular patch accompanied by a sectoral slot. To achieve the desired frequency band, an additional circular patch is purposefully placed over the main radiator. The development of each design stage in the antenna structure is investigated using the theory of characteristic mode. This configured multi-element antenna structure achieves a broad operational bandwidth spanning 4.5 GHz, encompassing frequencies from 26.1 GHz to 30.6 GHz, rendering the antenna well-suited for deployment in the millimeter-wave frequency bands of n257, n258, and n261 respectively. A total gain of 6.27 dBi is achieved with port isolation of −24 dB. Furthermore, the antenna undergoes validation for diversity metrics, including envelope correlation coefficient, diversity gain, channel capacity loss, total active reflection coefficient, and mean effective gain. The antenna is fabricated and experimentally validated with the simulated counterpart. Later, a comparative analysis has been conducted which revealed that the proposed antenna demonstrates > 80 % increase in compactness compared to existing antenna designs documented in the literature.

Modeling the Non-Hermitian Infinity-Loop Micro-Resonator over a Free Spectral Range Reveals the Characteristics for Operation at an Exceptional Point

We develop a 4 × 4-matrix model based on temporal coupled mode theory (TCMT) to elucidate the intricate energy exchange within a non-Hermitian, resonant photonic structure, based on the recently described infinity-loop micro-resonator (ILMR). We consider the structure to consist of four coupled resonant modes, with clockwise and counterclockwise propagating optical fields, the interplay between which gives rise to a rich spectral form with both overlapping and non-overlapping resonances within a single free spectral range (FSR). Our model clarifies the precise conditions for exceptional points (EPs) in this system by examining neighboring resonances over the device free spectral range (FSR). We find that the system is robust to the conditions for observing an EP, despite the presence of non-zero coupling of signals, or crosstalk, between the resonant modes.

A four-band coherent perfect absorber of borophene metamaterial operating in the communication band

We propose a coherent perfect absorber based on a borophene metamaterial grating structure. Based on the theoretical analysis of the temporal coupled mode theory and the numerical results of the finite-difference time-domain simulations, the transmission characteristics of the system are investigated. The dynamic tunability of the absorption intensity and resonance wavelength is realized by adjusting the phase difference of the coherent incident beams and carrier density of borophene. Moreover, by optimizing the parameters, the coherent perfect absorption peak (99.99%) is obtained near the wavelength of 1550 nm, that is, the third communication window. Meanwhile, the structure can be operated in ultra-thin gratings as low as 6 nm and angle tolerances as large as 45°, which provides the possibility for applications in highly integrated and wide-angle tolerated devices.

Subarray-based joint source localization in shallow water waveguide via subspace intersection

By the use of an underwater bottom-mounted Horizontal Linear Array (HLA), the two-dimensional joint localization technique and its accuracy are analyzed in this article. The proposed method takes advantage of arrival angles measured by a dual-subarray system consisting of a large-aperture HLA. Then, for the system, expressions for source position estimate and error variation are derived. Analysis of the Geometrical Dilution of Precision, which is a measure of localization error defined in Eq. (4), reveals that the localization error is mainly related to the source bearing, subarray interval and their measurement errors. In order to investigate the impact of the underwater waveguide, numerical simulations were carried out based on the normal mode theory. By using the distribution of acoustic intensity and structure of modal arrival, it is proved that the localization accuracy increases as the grazing angle and the subarray interval increase. However, when the measurement errors of source bearing and subarray interval increase, the localization accuracy will decrease. In addition, the obtained bearing estimation result provides an evident characteristic of beam spreading. When sources are near the end-fire direction of the dual-subarray system, significant bearing estimation error occurs, the negative effect of which is then presented. By applying the Subspace Intersection method, unbiased bearing estimation has been achieved, with relatively reduced computational complexity. Most of all, the improved localization performance is validated by experimental data collected in the South China Sea.