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The interaction of light with micro-scale dielectric structures has garnered significant attention in recent years due to its potential applications in various fields ranging from photonics to biomedical imaging. In this context, this study investigates the generation of twin photonic jets from a dielectric micro-cylinder illuminated by two point-sources. The effect of different parameters on the jet's shape profile and length was analyzed. Those parameters include the cylinder's refractive index and radius, the source distance from the cylinder, and the vertical displacement between the sources. Innovatively, this study introduces the concept of ‘Horn Jets' to characterize the twin photonic jets resulting from the illuminated dielectric micro-cylinder, marking a significant conceptual advancement in the field. Moreover, the generation of whispering gallery modes (WGMs) is investigated. The outcomes obtained through the spectral element method highlight the possibility of generating WGMs by meticulously selecting the cylinder's radius and refractive index, under the scenario of two-point-source illuminations. The results show that the magnitude of the WGMs resonance significantly lies on the vertical displacement due to the resulting interference pattern's dark and bright fringes with the cylinder edges.

Antennas that radiate orbital angular momentum (OAM) modes are usually large and have complex structures. In this paper, in combination with characteristic mode analysis, the ground plane is used as the OAM wave radiator. To excite degenerate modes of the ground plane with phase quadrature by using a single-fed structure, two bent metal strips with grounded ends are introduced around the rectangular ground plane, and a corner of one of the bent strips is set as the feeding point. For ground radiators, a first-order OAM mode is generated at the high-frequency end by synthesizing the degenerate modes. In case of the bent side strip, an inverted-F antenna that operates in the low-frequency mode is formed. This compact antenna has dual-band characteristics and facilitates simple feeding, which makes it suitable for mobile terminal applications.

With the continuous emergence of various advanced packaging technologies such as copper interconnection and 3-D packaging technology, it is essential to efficiently and accurately investigate the thermal analysis of high-performance, high-power and complicated electronic devices to better design heat dissipation structures. However, multiscale transient thermal analysis of complex electronic devices by existing numerical methods is still a challenge. In this work, the 3-D domain decomposition method (DDM) with the adaptive time step for the transient thermal analysis of integrated circuits (ICs) is proposed to tackle this problem. By flexible multiscale mesh generation and automatically time step changes based on posteriori errors, the new method significantly improves computational efficiency. Some illustrative numerical examples are presented to verify the accuracy and efficiency of the proposed method by considering 3-D transient heat transfer with thermal conduction, natural convection and radiation boundaries.

Development of hypersonic aircraft demands relativistic electromagnetic scattering modeling of a high-speed moving dielectric coated object, which is applied to the recognition and tracking of moving stealth object. However, far-zone polarized scatterings from a moving 3D dielectric coated object have not been investigated so far. This paper addresses this problem by combining the finite-difference time-domain method with Lorentz transformation (Lorentz-FDTD). Through Lorentz transformation and the principle of phase invariance, the frequency, propagation direction, magnitude of the incident plane wave and the size of the moving object in the laboratory frame, which is stationary with respect to the free space, are transformed to those in the rest frame that moves with the moving object. The scattered field near the object is solved by the FDTD method with full permittivity and conductivity tensors in the rest frame, then far-zone polarized scattered field is obtained by the implementation of near-to-far field transformation. Through Lorentz transformation for coordinates, the polarized Radar Cross Sections (RCSs) of moving plasma coated objects are solved. Especially, the scattering characteristics of radial radar cross sections of moving objects are discussed. Several numerical experiments are carried out, the efficiency and the accuracy of the proposed method are validated.

Surface acoustic wave (SAW) devices are key components in 5G communication systems, and the design of SAW filters with high frequency, high performance and high reliability is a necessary and challenging task. In this work, SAW resonators with $LiNbO_{3}/SiO_{2}/Si$ multi-layer structure are designed based on the spectral element method (SEM). Normally, however, massive computing resources are needed for solving a whole resonator directly by using full-wave numerical methods. To avoid simulating the large-aperture full structure of any SAW resonator, an equivalent small-aperture 3D structure can be modelled in the direction of the aperture by changing the internal resistance of the driving source to obtain the results of the full-scale aperture model. The one-port SAW resonators are then connected in the form of ladder networks to form band-pass SAW filters. The numerical results show that the SEM is more computationally efficient than the finite element method under the same number of degrees of freedom, and the results of the 3D small-aperture model are consistent with the complete resonator.

A single-layer corporate-fed substrate integrated waveguide slot array antenna is newly proposed and verified. It is characterized by low profile, easy fabrication, low cost, low sidelobes, wideband operation, and high efficiency. A V-band $16\times 16$ -element array with uniform excitation is designed for demonstration. The radiating elements and feeding waveguides are arranged within a single-layer substrate, whose dielectric constant and loss tangent are appropriately selected. By adopting the full-corporate feed, the $16\times 16$ radiating slots are excited by the $16\times 16$ output ports of the feeding circuit, which is composed of cascaded H-plane T-junctions. For further bandwidth improvement, a metal grid plate is introduced on the top of the radiating slots. The test antenna is fabricated by the conventional PCB process. The measured bandwidth for $\vert S_{11}\vert < -10$ dB is 16.2%, fully covering the 57–67 GHz frequency range. Meanwhile, the measured antenna gain higher than 30 dBi associated with the antenna efficiency higher than 45% is achieved over the $58-65$ GHz frequency range.

In this paper, the symmetric Lanczos algorithm and mixed finite element method are combined to improve the efficiency and accuracy of S-parameter simulation in a broad frequency band. The mixed finite element method based on tree-cotree splitting is adopted to overcome the low-frequency (high-density) breakdown problem, thus ensuring the accuracy of low frequency (high integration density) results. The modified symmetric Lanczos algorithm for second order eigenvalue problem is employed for fast frequency sweep by one matrix inversion, then the field in the entire frequency band is obtained rapidly by solving the reduced order matrix equation. The accuracy of the high frequency results is improved by increasing the number of Lanczos vectors. Numerical simulations of two integrated passive filters (IPD filter) and an interconnect model are shown to verify the hybrid method along with commercial software HFSS.

Accurate modeling of relativistic electromagnetic scattering characteristics from high-speed motion of plasma coated objects is crucial for the development of hypersonic aircraft and their applications in the identification and surveillance of moving stealth targets. Nevertheless, a solution for bistatic polarized radar cross sections (RCSs) from a 3-D object with plasma coated layer in motion has yet to be obtained. This manuscript proposes a solution to this problem by employing a combination of the auxiliary differential equation (ADE) method with Lorentz finite-difference time-domain (FDTD) method. Utilizing the Lorentz transformation, this paper presents the transformation of parameters of the incident plane wave and dimensions of the object between the laboratory system that remains static and the rest system that remains stationary relative to the object in high-speed motion. The near-zone electromagnetic fields near the object are computed using the ADE method in the rest system, after which the near-field to far-field (NF-FF) transformation is employed to obtain the far-zone polarized scattered field. By applying Lorentz transformation to the coordinates, this paper presents a solution for the polarized scattering from moving plasma coated objects. Especially, radial components of the polarized scatterings are analyzed. The proposed method is validated through several numerical experiments, demonstrating its efficiency and accuracy.

The spectral element method (SEM) is widely used for analyzing high frequency electromagnetic wave propagation and scattering in complex structures. At low frequencies, however, the primary (direct) fields are usually much larger than the secondary fields caused by scattering, thus making it much more difficult to accurately simulate the secondary (scattered) fields because of the source singularity in the primary fields. To more effectively and accurately simulate the low-frequency secondary fields in subsurface structures, in this work the vector Helmholtz equation for the scattered fields is used to directly obtain the secondary field data in inhomogeneous anisotropic media. The computational method for the secondary electromagnetic fields is based on the mixed spectral element method (mixed SEM) for anisotropic objects in a layered anisotropic background medium. The electromagnetic field is separated into the background fields that are evaluated analytically using the dyadic Green's functions (DGFs) of a layered uniaxially anisotropic medium, and the secondary fields caused by anomalous bodies with arbitrary shapes are numerically computed by the mixed SEM. This avoids the source singularity, and allows the transmitters to be located outside the computational domain. By enforcing Gauss' law as the constraint condition, the mixed SEM efficiently eliminates the low frequency breakdown problem. Numerical results verify the proposed method over the traditional SEM in low-frequency scattering from objects in subsurface layered anisotropic media.

An all-dielectric nanoantenna is expected to greatly boost the nonlinear performance owing to its large mode volume and low material loss. However, it still suffers from the relatively weak near-field enhancement and the inferior far-field directivity. In this work, by successively introducing an upper metal ring and a bottom metal film, the plasmonic resonance and the perfect electric conductor (PEC) mirror effect are employed to significantly enhance and tailor the third harmonic generation (THG) from the high-index silicon nanoantenna. Numerical results show that benefiting from both of these two metal components, not only the THG efficiency is increased by more than 1 and 3.5 orders of magnitude compared with those with only single metal component and without any metal component, respectively, but also an improved radiation directivity can be obtained. The dependences of geometric parameters are discussed as well. This work suggests a route to enhance and manipulate the nonlinear radiation by a hybrid nanoantenna and facilitates its practical applications such as biosensing and nonlinear light sources.

Waveguide problems are fundamental to elastic and acoustic wave propagation, where we are interested in finding the propagation constants and modal patterns of waveguide modes. As the waveguide is assumed uniform in one direction, the original 3-D problem can be converted into a so-called 2.5-D problem by using the Fourier transform in that direction. However, the introduction of elastic metamaterials (EMM) broadens the horizon of this subject, and new features are required in EMM waveguides that cannot be obtained by most traditional waveguide solvers. In this work, a spectral element method (SEM) is developed to simulate the elastic/acoustic waveguide problem in anisotropic media with anisotropic mass density and/or negative index parameters. We design an anisotropic density EMM waveguide with our SEM solver to demonstrate several intriguing phenomena. Comparisons with the traditional finite element method (FEM) for several examples show the significant advantages of the SEM in term of accuracy and efficiency.

Fast electromagnetic (EM) simulation of thin layers, such as geological fractures and faults, is still a challenge since the local mesh refinement near such structures dramatically increases the number of degrees of freedom (DoFs), thus large computational resources are required for modeling their interactions with electromagnetic fields. Therefore, significant attention has been paid to overcome the problem. In this work, we propose the 2.5 dimensional hybrid mixed spectral element method with a surface current boundary condition (the 2.5-D hybrid MSEM-SCBC) to accelerate the simulation and utilize it for the low-frequency solutions of electromagnetic field in subsurface sensing. By introducing SCBC to model the EM field diffraction, the geological fractures and faults are represented geometrically by zero-thickness sheets instead of finite thickness thin layers to avoid local mesh refinement, and to significantly decrease DoFs and computational costs. Numerical results demonstrate that the proposed method is efficient and accurate in simulation of 2.5-D subsurface sensing problems.

The range-angle-dependent beampattern of frequency diverse array (FDA) makes it capable of dealing with range-dependent interference, which cannot be handled by conventional phased array. However, current publications pay more attention to the dot-shaped beam forming, which is not suitable for the range-dependent jamming signal suppression. To address this problem, this paper proposes a weight designing method to control the nulls distribution among range bins in the mainbeam. With the additional degrees of freedom provided by FDA, it is possible to flexibly adjust the positions and notch widths of the range nulls. The performance and effectiveness of the proposed weighted FDA are validated with numerical simulations.

This article presents a double-layer four-corner-fed slotted waveguide sparse array antenna designed for the near-field-focused applications in the X-band. Generally, it is challenging to realize the ideal phase distribution of the near-field focused array antenna, especially for the near-field focusing array antenna with a focal distance less than $5\lambda $ . We propose a four-corner-feed structure associated with appropriate waveguide dimensions to realize approximately ideal phase distribution for a focus at $1.5\lambda $ . The sparse array based on the particle swarm optimization is adopted to suppress the sidelobes and the grating lobes on the focal plane caused by the residual phase errors. A test antenna is designed at the center frequency of 10 GHz, and is fabricated by Direct Metal Laser Sintering technique for demonstration. It is verified by the experiment that we achieve satisfying near-field focusing effects, especially in terms of the focal spot size and sidelobe levels on the focal plane 45 mm away from the antenna aperture.

Due to the divergence characteristics of vortex microwaves, it is difficult to determine orbital angular momentum (OAM) orders through far fields, while the spiral spectrum analysis method based on the area integral of antenna near fields is complicated. In this paper, combined with near-field approximation processing, a spiral spectrum analysis method based on loop integration is proposed. According to the Laguerre–Gaussian vortex beam model, the key parameter estimation formulas suitable for low-order OAM microwave beams are derived, including the position of the reliable observation plane and the size of the annulus area. On this basis, the radius of the integral loop used for the spiral spectrum calculation is provided. Since only field data on the loop are collected in this method, the calculation amount is reduced and the analysis efficiency of the spiral spectrum is greatly improved. Due to the combination of near-field and far-field considerations, this method can fully reflect the spiral spectrum characteristics of antenna vortex waves.

Three-dimensional (3-D) super-resolution microwave imaging of human brain is a typical electromagnetic (EM) inverse scattering problem with high contrast. It is a challenge for the traditional schemes based on deterministic or stochastic inversion methods to obtain high contrast and high resolution, and they require huge computational time. In this work, a dual-module 3-D EM inversion scheme based on deep neural network is proposed. The proposed scheme can solve the inverse scattering problems with high contrast and super-resolution in real time and reduce a huge computational cost. In the EM inversion module, a 3-D full convolution EM reconstruction neural network (3-D FCERNN) is proposed to nonlinearly map the measured scattered field to a preliminary image of 3-D electrical parameter distribution of the human brain. The proposed 3-D FCERNN is completely composed of convolution layers, which can greatly save training cost and improve model generalization compared with fully connected networks. Then, the image enhancement module employs a U-Net to further improve the imaging quality from the results of 3-D FCERNN. In addition, a dataset generation strategy based on the human brain features is proposed, which can solve the difficulty of human brain dataset collection and high training cost. The proposed scheme has been confirmed to be effective and accurate in reconstructing the distribution of 3-D super-resolution electrical parameters distribution of human brain through noise-free and noisy examples, while the traditional EM inversion method is difficult to converge in the case of high contrast and strong scatterers. Compared with our previous work, the training of FCERNN is faster and can significantly decrease computational resources.

In this paper, we numerically and experimentally demonstrate a substrate integrated plasmonic waveguide (SIPW) concept and its application in microwave bandpass filters. This SIPW consists of double arrays of slots etched on the top and bottom metal layers of a substrate integrated waveguide (SIW) to support spoof surface plasmon polariton (SSPP) modes with low and high cutoff frequencies. The simulated results show that by tuning the parameters of the SIPW's unit cell, the dispersion characteristics can be engineered at will. Then, we propose a sharp roll-off microwave bandpass filter based on this SIPW. This filter has a passband from 7.5 to 13.0 GHz with high return loss and low insertion loss. Furthermore, to demonstrate the independent tuning of the passband of the filter, we also design two microwave bandpass filters with passbands of 9.2-13.0 GHz and 7.5-10.5 GHz by decreasing the distance between two rows of via holes and increasing the slot length, respectively. Finally, to experimentally validate the filter designs, we fabricate and measure three prototypes and find that the experimental results are in excellent agreement with the simulations. This SIPW concept may have extensive potential applications in the development of various plasmonic integrated functional devices and circuits.

We numerically demonstrate an ultra-wideband terahertz absorber using dielectric circular truncated cones on a non-structured graphene layer supported by a dielectric spacer on a metal reflector in the bottom. The absorber has an ultra-wideband absorption bandwidth from 0.34 to more than 10 THz with an average absorbance of 95.88% and a relative bandwidth of 186% because of the continuous multimode Fabry-Perot (FP) resonances. The absorber also possesses excellent absorption characteristics with polarization independence under normal incidence and angular stability up to 60°. The proposed ultra-wideband absorber may have promising applications in terahertz trapping, imaging, and detecting.

In this paper, we present a novel Transformation Optics based Finite-difference time-domain (TO-FDTD) algorithm with Square Transformed Region, which is suitable for fast electromagnetic analysis of small structures in a large computational domain. A small structure can be solved with coarse grids in a transformed region, since it is enlarged by transformation optics. No fine grid is applied for the small structure so that computational efficiency can be improved greatly. In addition, the TO-FDTD algorithm can also avoid time-space field interpolations and the late-time instability of the subgridding algorithm. To eliminate the staircasing errors introduced by curved boundaries of its transformed region, we propose the square transformed region instead of the circular one. We parameterize the square boundary in polar coordinates so that it is easy to be implemented. Through coordinate transformation, new anisotropic permittivity and permeability can be obtained in the transformed region. Then we develop a stable anisotropic FDTD algorithm to solve the transformed Maxwell’s Equations. Numerical results on diffraction and scattering of small structures show that our proposed TO-FDTD algorithm has high computational efficiency and accuracy.

In the artificial neural network (ANN), the most time-consuming part for parametric modeling of microwave components is the collection of training datasets from full-wave electromagnetic simulations. However, the reported models for parametric modeling of EM behaviors are based on supervised learning, in which the labeled sampling data from full-wave EM simulations ought to be sufficient for ANN training. Thus, the number of full-wave simulations is the main factor that influences the effectiveness of collecting training and testing samples. Based on the dynamic adjustment kernel extreme learning machine, this paper proposes a semi-supervised learning model lying between supervised learning and unsupervised learning to largely reduce the number of required training samples. The proposed model contains two training processes, the initial training, and the self-training. In the initial training process, a small number of training samples from full-wave simulations are used to make the model rapidly converge. Then, in the self-training process, the model produces unlabeled training datasets to train itself till the testing accuracy is satisfied. Two numerical examples of a microstrip-to-microstrip vertical transition and a dual-band four-pole filter are employed to verify the effectiveness of the semi-supervised learning model.

A proper dielectric coating can reduce the electromagnetic scattering of the conducting object significantly so that it cannot be detected by the radar. However, when the object contains a thin coating, fine grid size is needed to discretize the thin coating in the conventional finite-difference time-domain (FDTD) algorithm, which increases the amount of memory and computational time significantly. To overcome this dilemma, we present a transformation optics-based FDTD (TO-FDTD) algorithm to accelerate the solution of electromagnetic scattering from objects with thin dielectric coatings. Two kinds of novel TO-FDTD models are proposed in this paper for a coated cylinder and a coated arbitrary polygonal cylinder, respectively. Through coordinate transformation, the size of the object remains unchanged while its thin coating is enlarged to a thicker one, meaning that it can be simulated by the FDTD algorithm with uniform coarse grids instead of fine grids. The transformed material parameters become inhomogeneous and anisotropic in the transformed region, which can be obtained by solving a Jacobian transformation matrix. We then develop a stable FDTD algorithm for solving anisotropic Maxwell’s equations. Bistatic scatterings of coated cylinders and a coated polygonal cylinder are solved by the TO-FDTD algorithm proposed in this paper, respectively. The result of the TO-FDTD algorithm matches well with the exact value and the result of the commercial software Comsol. The computational efficiency and accuracy of the proposed TO-FDTD algorithm are validated by numerical experiments. Numerical results show that the TO-FDTD algorithm has higher computational accuracy than the conventional FDTD algorithm that fails to simulate the absorbing property of the coating, when the same coarse grid size is used in the simulation. Under the same level of accuracy, the proposed TO-FDTD method can improve the computational efficiency by 62-63 times than the conventional FDTD method with fine grids in the simulations in the paper.

In rectifying or active antenna systems, the higher order modes can hamper the system efficiency, and thus, the filter antennas are used to eliminate the harmonic radiation. In this paper, the distribution of standing wave fields in patch antenna having been presented and then the characteristic mode theory having been employed, the influences of feeding location and the indentation of patch edges on the suppression of higher order harmonics and their spurious modes are investigated. It is indicated that indenting the radiating edges of the patch element works more effectively on the control of spurious modes while indenting the non-radiating edges can eliminate the harmonics by shifting down the fundamental mode. By selecting feed position and indenting the patch edges, a microstrip patch antenna with both harmonic and spurious mode controls is proposed.

A tunable dual-band bandstop filter (BSF) in the far-infrared region is proposed based on a graphene surface plasmon. The BSF consists of a graphene waveguide and two graphene-ribbon resonators on the SiO2/Si substrate, supporting waveguide mode and edge mode, respectively. The influence of graphene structure parameters on transmission spectra is numerically investigated, which shows that the resonant properties are sensitive to the lengths of the graphene-ribbon resonators, and each stopband of dual-band BSF can be individually designed and tuned without affecting the other stopband. Moreover, the center frequencies of the two stopbands can be actively tuned independently by respective chemical potential of graphene-ribbon resonators. The proposed filter structure is promising to be utilized in future photonic integrated circuit applications as an ultracompact dual-band BSF.

A compact triplexer based on E-stub-loaded composite right-/left-handed (ESL-CRLH) resonators with quasi-lumped impedance matching network is presented in this paper. The equivalent circuit model of the ESL-CRLH resonator is presented first and its left-/right-handed capacitance/inductance elements are fully derived. Then, a quasi-lumped impedance matching circuit is designed to connect the three ESL-CRLH resonator based filter channels for the triplexer construction. Finally, the designed triplexer obtains high isolations among the ports and low in-band insertion losses of the three filter channels centered at 1.86, 2.41, and 3.25 GHz, of which a miniaturized layout has been realized. Good agreement between the simulated and measured results can be observed to validate the design idea.

Owing to its advantage in fast calculation, ray-tracing technique has been widely applied in solving electromagnetic scattering of electrically large target. In order to save computational time in ray tracing, KD-tree structure is introduced to reduce the number of ray-scene intersection tests. Nevertheless, KD-tree construction usually becomes the most time-consuming step in the implementation of ray tracing. In this paper, a point-based KD-tree construction method (PKCM) is proposed to accelerate the construction of KD-tree. The basic idea is to substitute point numbers for facet numbers to accelerate the process of probing optimal splitting plane with surface area heuristic. Owing to this replacement, the searching area of PKCM is dramatically decreased compared with that of KD-tree structure with facets, thus giving rise to significant saving in computational time but without degrading performance. Simulations of the scattering of electrically large complex objects demonstrate the effectiveness of the proposed PKCM.

Two novel microstrip patch antennas with multiple parasitic patches and shorting vias have been presented for the bandwidth enhancement. Based on the conventional triangular patch antenna, two more resonances can be obtained with the introduction of multiple parasitic patches, and consequently, the antenna bandwidth can be broadened. Parametric analysis of the patches has been studied for the verification of bandwidth enhancement. An example of the proposed antenna with multiple parasitic patches is designed, fabricated, and tested. The measured bandwidth with $\vert S_{11}\vert < -10$ dB ranges from 5.46 to 6.27 GHz (13.8%), and good far-field radiation patterns can be obtained within the frequency band. In addition, two shorting vias are inserted into the above proposed antenna to decrease the input impedance, resulting in further bandwidth enhancement of the antenna. This antenna is fabricated and tested as well, which achieves a measured 10-dB impedance bandwidth of 17.4% from 5.5 to 6.55 GHz.

This paper focuses on the range migration (RM) and Doppler frequency migration corrections of the long-time coherent integration, and a fast algorithm is proposed for maneuvering target detection and motion parameters estimation under a low signal-to-noise ratio (SNR). By utilizing the autocorrelation function with respect to the slow time, the newly defined extended keystone transform, scaled Fourier transform, and fast Fourier transform (FFT), the proposed algorithm coherently integrates the target energy into a peak in a three-dimensional (3-D) parameter space. Thereafter, the target’s radial velocity and acceleration are estimated by the peak’s coordinate. After compensating off the RM and DFM via the estimated motion parameters, the inverse FFT along the range frequency and azimuth FFT are used to realize the coherent integration for the target detection. The cross term characteristic is also analyzed and shows the applicability of the proposed algorithm in the scenario of multiple targets. Comparisons with representative algorithms in the computational complexity, motion parameters estimation, and target detection are presented in this paper, which leads us to conclude that the proposed algorithm can greatly reduce the computational complexity with an acceptable integration SNR gain loss. Finally, the experiments with the simulated and real measured radar data are conducted to verify the proposed algorithm.

A wideband antenna with quad-polarization reconfigurable operation is proposed. It consists of a crossed tri-port feed linearly polarized (LP) radiation structure and a switchable feed network. A dual-port LP element with wideband characteristic and good port isolation is first introduced. The inherent 180° phase difference (PD) between two feed ports vastly simplifies the complexity of the design and improves the efficiency of the feed network. By assembling two grooved LP elements, the crossed tri-port feed radiation structure is formed. Meanwhile, a wideband 90° phase shifter and an unequal Wilkinson power divider are applied in designing the switchable feed network. Through controlling the PIN diodes implanted in the feed line, four working modes with equal amplitude and reconfigurable PD (0° or 90°) are achieved. Based on this, the desired quad-polarization reconfiguration, including two orthogonal linear polarization (LP) states and two orthogonal circular polarization (CP) states, is realized. Measured results show that the proposed antenna has a wide overlapped operation impedance bandwidth of 84.5%, ranging from 1.36 ~ 3.35 GHz, for all polarization states within a compact volume. The measured 3-dB AR bandwidth is 100%, covering the operation impedance frequency range. Radiation patterns exhibit unidirectional and stable quality from the point view of beam slant and split. Besides, a peak efficiency of 75% and a peak gain of 7.8 dBi are achieved.

Double dipoles on a single-layer substrate are utilized to construct a triple-mode printed quasi-Yagi antenna for the multi-band and broadband antenna applications. A stub-loaded dipole generating two resonant modes (i.e., lower dual-mode dipole) is allocated on the underside of a simple dipole (i.e., upper single-mode dipole) introducing the third resonant mode. Using these three resonant modes, three compact printed quasi-Yagi antennas, i.e., tri-band, dual-band, and broadband printed quasi-Yagi antennas, are designed with the same antenna prototype but different parameter values. Seen from the measured results, all of these three antennas have good unidirectional radiations, high radiation efficiencies, and low cross-polarization levels at the operating frequencies within the impedance bandwidths.

The multivariate range function of the high-speed maneuvering target induces modulations on both the envelop and phase, i.e., the range cell migration (RCM) and Doppler frequency migration (DFM) which degrade the long-time coherent integration used for detection and localization. To solve this problem, many long-time coherent integration methods have been proposed. Based on mechanisms of typical methods, this paper names two signal processing modes, i.e., processing unification (PU) mode and processing separation (PS) mode, and presents their general forms. Thereafter, based on the principle of the PS mode, a novel long-time coherent integration method, known as the generalized dechirp-keystone transform (GDKT), is proposed for radar high-speed maneuvering target detection and localization. The computational cost, energy integration, peak-to-sidelobe level (PSL), resolution, and anti-noise performance of the GDKT are analyzed and compared with those of the maximum likelihood estimation (MLE) method and keystone transform-dechirp (KTD) method. With mathematical analyses and numerical simulations, we validate two main superiorities of the GDKT, including (1) the statistically optimal anti-noise performance, and (2) the low computational cost. The real radar data is also used to validate the GDKT. It is worthwhile noting that, based on closed analytical formulae of the MLE method, KTD method, and GDKT, several doubts in radar high-speed maneuvering target detection and localization are mathematically interpreted, such as the blind speed sidelobe (BSSL) and the relationship between the PU and PS modes.

Ultrawideband simulation of negative refraction in bicrystals is important for the design optimization of devices involving such anisotropic media, but it is a nontrivial task, especially when low-order methods are utilized. This work proposes an improved discontinuous Galerkin time domain (DGTD) method for simulating time-dependent electromagnetic fields for inhomogeneous media with full anisotropic constitutive parameters (full anisotropic media). It employs the electric field intensity E and magnetic flux density B to solve Maxwell's equations. The EB-scheme-based anisotropic Riemann solver and nonconformal mesh are employed for domain decomposition to allow efficient spatial discretization. An unsplit-field Maxwellian multiaxial perfectly matched layer for full anisotropic media is derived and shown to be effective to absorb outgoing waves and suppress the potential late-time instability found in classical PML. In addition, the total-field/scattered-field technique is further studied to allow a nonconformal mesh, vector basis functions, and half-space situation. This newly improved DGTD method is validated with test cases and applied to the negative reflection in YVO4 bicrystal.

The characteristic mode theory (CMT) can provide physically intuitive guidance for the analysis and design of antenna structures. In CMT applications, the antenna current distribution is decomposed into the superposition of multiple characteristic modes, and the proportion of each current mode is characterized by the modal weighting coefficient (MWC). However, different characteristic currents themselves have different radiation efficiencies reflected by the eigenvalues. Therefore, from the perspective of the contribution to the radiation field, the modal proportion should be more accurately determined by the combination of the modal weighting coefficient and the mode current itself. Since the discrete mode currents calculated using the electromagnetic numerical method are distributed on the whole conductor surface, we can actually use the radiation field to quantify the modal proportion or estimate it using the far field in the maximum radiation direction. The numerical examples provided in the paper demonstrate that this modal proportion can effectively evaluate antenna performance.

A plasmonic sensor based on the nano-cylinder photonic crystal slab is modeled and designed for the refractive index sensing application. Design considerations and sensor characteristics are explored in detail using the higher order accuracy spectral element method. The geometrical parameters of the designed plasmonic sensor are optimized. With the shallowly etched configuration and the small radius of the nano-cylinder, the sensor simultaneously achieves a high sensing performance of figure of merit $\textrm{FoM}=280$ and a clear registration of full-height/full-width at half-maximum $\textrm{FH/FWHM}=0.34\ \textrm{nm}^{-1}$ for the lattice period $P=750\ \textrm{nm}$, the cylinder radius $r=85\ \textrm{nm}$, and the cylinder thickness $t=125\ \textrm{nm}$. The proposed plasmonic sensor can find its great potential for applications relying on detecting refractive index.

In this paper, a wafer-scale uniform silver nanocave array is fabricated by soft ultraviolet nanoimprint lithography. We investigate its plasmonic effects using far-field and near-field experimental approaches and illuminate the physics inside by theoretical analysis and computational simulation. The array shows robust multispectral features for various surrounding media and possesses the sensitivity up to 514.7 nm/RIU in the visible range, which is promising for the mass production of high-performance plasmonic refractive index sensors.

On the basis of a single multimode resonator (MMR), a miniaturised microstrip sext‐band bandpass filter (BPF) with high band‐to‐band isolations has been presented. The proposed MMR is an stepped impedance resonator (SIR) with one short‐circuited stub loaded in the middle and two identical open‐circuited stubs located at two sides. Transmission poles (TPs) and transmission zeros (TZs) can be excited when the electrical lengths of two open stubs increase. The TZs introduced by open stubs separate the TPs into multiple groups; therefore, the design of a sext‐band BPF becomes available. Moreover, two input/output tapped branches and radical stub‐loaded short‐circuited lines are treated as perturbation structures in order to increase design freedoms for improving filter performance. Good agreement between simulated and measured results validates the design method.

In this paper, a compact frequency reconfigurable circular patch antenna with an arc-shaped slot loaded in the ground layer is proposed for multiband wireless communication applications. By controlling the ON/OFF states of the five PIN diodes mounted on the arc-shaped slot, the effective length of the arc-shaped slot and the effective length of antennas current are changed, and accordingly six-frequency band reconfiguration can be achieved. The simulated and measured results show that the antenna can operate from 1.82 GHz to 2.46 GHz, which is located in DCS1800 (1.71–1.88 GHz), UMTS (2.11–2.20 GHz), WiBro (2.3–2.4 GHz), and Bluetooth (2.4–2.48 GHz) frequency bands and so forth. Compared to the common rectangular slot circular patch antenna, the proposed arc-shaped slot circular patch antenna not only has a better rotational symmetry with the circular patch and substrate but also has more compact size. For the given operating frequency at 1.82 GHz, over 55% area reduction is achieved in this design with respect to the common design with rectangular slot. Since the promising frequency reconfiguration, this antenna may have potential applications in modern multiband and multifunctional mobile communication systems.

A tunable demultiplexer with three output channels infiltrated by liquid crystal (LC) is presented, which is based on a metal-insulator-metal (MIM) waveguide. The operating frequencies of the three output channels can be tuned simultaneously at will by changing the external bias electric field applied to the LC. By analyzing the Fabry-Pérot (FP) resonance modes of the finite-length MIM waveguide both theoretically and numerically, the locations of the three channels are delicately determined to achieve the best demultiplexing effects. Terahertz (THz) signals input from the main channel can be demultiplexed by channels 1, 2 and 3 at 0.7135 THz, 1.068 THz and 1.429 THz, respectively. By applying an external electric field to alter the tilt angle of the infiltrating LC material, the operating frequencies of channels 1, 2 and 3 can be relatively shifted up to 12.3%, 9.6% and 9.7%, respectively. The designed demultiplexer can not only provide a flexible means to demultiplex signals but also tune operating bands of output channels at the same time.

We propose and numerically demonstrate two novel terahertz absorbers made up of periodic single- and double-layer decussate graphene ribbon arrays. The simulated results show that the proposed absorbers have narrowband near-unity terahertz absorption with ultra-wide frequency reconfiguration and angular stability. By tuning the Fermi level of graphene ribbons, the over 90% absorbance peak frequency of the absorber with single-layer graphene structure can be flexibly adjusted from 6.85 to 9.85 THz for both the transverse magnetic (TM) and transverse electric (TE) polarizations. This absorber with single-layer graphene demonstrates excellent angular stability with the absorbance peaks of the reconfigurable absorption bands remaining over 99.8% in a wide angle of incidence ranging from 0 to 70°. The tuning frequency can be significantly enhanced by using the absorber with double-layer graphene structure from 5.50 to 11.28 THz and 5.62 to 10.65 THz, approaching two octaves under TM and TE polarizations, respectively. The absorbance peaks of the reconfigurable absorption band of this absorber for both polarizations maintain over 70%, even at a large angle of incidence up to 70°. Furthermore, an analytical fitting model is also proposed to accurately predict the absorbance peak frequencies for this variety of absorbers. Benefitting from these attractive properties, the proposed absorber may have great potential applications in tunable terahertz trapping, detecting, sensing, and various terahertz optoelectronic devices.

The effects of electric field on lipid membrane and cells have been extensively studied in the last decades. The phenomena of electroporation and electrofusion are of particular interest due to their wide use in cell biology and biotechnology. However, numerical studies on the electrofusion of cells (or vesicles) with different deformed shapes are still rare. Vesicle, being of cell size, can be treated as a simple model of cell to investigate the behaviors of cell in electric field. Based on the finite element method, we investigate the effect of vesicle shape on electrofusion of contact vesicles in various medium conditions. The transmembrane voltage (TMV) and pore density induced by a pulsed field are examined to analyze the possibility of vesicle fusion. In two different medium conditions, the prolate shape is observed to have selective electroporation at the contact area of vesicles when the exterior conductivity is smaller than the interior one; selective electroporation is more inclined to be found at the poles of the oblate vesicles when the exterior conductivity is larger than the interior one. Furthermore, we find that when the exterior conductivity is lower than the internal conductivity, the pulse can induce a selective electroporation at the contact area between two vesicles regardless of the vesicle shape. Both of these two findings have important practical applications in guiding electrofusion experiments.

We numerically demonstrate a broadband terahertz (THz) absorber that is based on a hybrid-patterned graphene metasurface with excellent properties of polarization insensitivity, wide-angle, and active tunability. Our design is made up of a single-layer graphene with periodically arranged hybrid square/disk/loop patterns on a multilayer structure. We find that broadband absorption with 90% terahertz absorbance and the fractional bandwidth of 84.5% from 1.38 THz to 3.4 THz can be achieved. Because of the axisymmetric configuration, the absorber demonstrates absolute polarization independence for both transverse electric (TE) and transverse magnetic (TM) polarized terahertz waves under normal incidence. We also show that a bandwidth of 60% absorbance still remains 2.7 THz, ranging from 1.3 THz to 4 THz, for a wide incident angle ranging from 0° to 60°. Finally, we find that by changing the graphene Fermi energy from 0.7 eV to 0 eV, the absorbance of the absorbers can be easily tuned from more than 90% to lower than 20%. The proposed absorber may have promising applications in terahertz sensing, detecting, imaging, and cloaking.

For inverse synthetic aperture radar (ISAR) imaging of a ship target moving with ocean waves, the image constructed with the standard range-Doppler (RD) technique is blurred and the range-instantaneous-Doppler (RID) technique has to be used to improve the image quality. In this paper, azimuth echoes in a range cell of the ship target are modeled as noisy multicomponent cubic phase signals (CPSs) after the motion compensation and a RID ISAR imaging algorithm is proposed based on the integrated cubic phase bilinear autocorrelation function (ICPBAF). The ICPBAF is bilinear and based on the two-dimensionally coherent energy accumulation. Compared to five other estimation algorithms, the ICPBAF can acquire higher cross term suppression and anti-noise performance with a reasonable computational cost. Through simulations and analyses with the synthetic model and real radar data, we verify the effectiveness of the ICPBAF and corresponding RID ISAR imaging algorithm.

Non-Cartesian sampling is widely used for fast magnetic resonance imaging (MRI). Accurate and fast image reconstruction from non-Cartesian k-space data becomes a challenge and gains a lot of attention. Images provided by conventional direct reconstruction methods usually bear ringing, streaking, and other leakage artifacts caused by discontinuous structures. In this paper, we tackle these problems by analyzing the principal point spread function (PSF) of non-Cartesian reconstruction and propose a leakage reduction reconstruction scheme based on discontinuity subtraction. Data fidelity in k-space is enforced during each iteration. Multidimensional nonuniform fast Fourier transform (NUFFT) algorithms are utilized to simulate the k-space samples as well as to reconstruct images. The proposed method is compared to the direct reconstruction method on computer-simulated phantoms and physical scans. Non-Cartesian sampling trajectories including 2D spiral, 2D and 3D radial trajectories are studied. The proposed method is found useful on reducing artifacts due to high image discontinuities. It also improves the quality of images reconstructed from undersampled data.