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This letter introduces an unconditionally stable finite‐difference time‐domain (FDTD) method, based on the locally one‐dimensional (LOD) technique, for the solution of the two‐dimensional scalar wave equation (WE) in homogeneous media. The second spatial derivatives in the WE are discretized by using a three‐point compact (implicit) finite‐difference formula with a free parameter. This formula has second‐order accuracy and becomes fourth‐order by properly selecting the parameter value. Moreover, the resulting algorithm only involves tridiagonal matrices, as when using standard (explicit) second‐order finite differences. Additionally, a stability analysis is performed and the numerical dispersion relation of the method is derived. The proposed compact LOD‐WE‐FDTD technique has been applied to the calculation of resonant frequencies in a metallic ridge cavity. The accuracy of the results obtained has been studied as a function of the parameter value. [ABSTRACT FROM AUTHOR]

Ground-penetrating radar is used to study defects in the form of internal layers in concrete structures. It is found that modulus of deflection coefficient of GPR signals changes as function of the layer thickness and electromagnetic properties of the material filling the layer (sand, wet sand or air). The experimental and numerical research used the method of peak-to-peak amplitude ratio, which enabled determining the Fresnel coefficients for the upper and lower boundaries of a layer. The minimal layer thickness recorded by GPR was 2 mm. [ABSTRACT FROM AUTHOR]

In this paper, plasmonic band-pass and band-stop filters employing ring resonators within the framework of the Wilkinson power divider structure are presented. It is shown that the resonance wavelengths can be effectively manipulated by adjusting the inner and outer radii of the ring resonators. Furthermore, by precisely controlling the gap size between the ring resonators and the waveguides, we have successfully designed efficient band-pass and band-stop filters. This study encompasses a thorough investigation into the propagating modes of surface plasmon polaritons (SPPs) within these filters. The transmission characteristics of the proposed filters are meticulously examined and analyzed using the finite-difference time-domain (FDTD) method. The proposed plasmonic filters exhibit a compact size, thus holding promising potential for integrated optics applications. Moreover, they possess the inherent capability to eliminate higher-order modes, enhancing their utility in practical implementations which require a single mode response. The proposed design has the possibility of manipulating the filtering wavelength by adjusting the stubs and ring parameters. This capability opens up exciting avenues for tuning the filter's response and tailoring it to the desired application requirements. [ABSTRACT FROM AUTHOR]

Plasmonic coloration derived from metal nanoparticles and arrays has attracted great attention in the fields of color filters, sensors, and various photonic/optoelectronic devices. However, there are cases in which localized surface plasmon resonance (LSPR) coloration becomes an obstacle, and transparent plasmons are needed. In this study, we report an extremely transparent network structure film at an air–water interface composed of patchy spherical gold nanoparticles with two different ligands (oleylamine and mPEG-thiol). These films exhibit high surface pressure as a liquid film at an extremely low areal density of ∼10% while maintaining extremely transparent plasmonic colors even at high areal densities, reaching 40–60%. Such states of materials are regarded as "empty liquids" (liquid states with a vanishing density), which were predicted in theory but found in only a few cases experimentally in the past. The extremely transparent coloration of two-dimensional empty liquid films obtained in this study is reasonably interpreted by finite-difference time-domain (FDTD) model simulations of network assembled structures with shorter gap distances. These findings will be crucial for the design of high-density transparent LSPR metamaterials for certain applications, such as plasmonic solar cells and smart windows, in the future. [ABSTRACT FROM AUTHOR]

To improve the design of the 3D-printed W-band reflective Fresnel lens antenna based on acrylonitrile butadiene styrene (ABS) plastic, we have examined the parameter sensitivity related to the dielectric material constant. Although we have developed a highgain millimeter-wave reflective Fresnel lens antenna, the material constant of the ABS filament used in 3D printing needs further investigation to optimize antenna performance. First, a 150-mm-diameter W-band reflector Fresnel antenna is designed and analyzed using finitedifference time-domain (FDTD) analysis. The analyzed and measured maximum antenna gains are 33.3 and 32.4 dBi, respectively. Subsequent sensitivity analysis focused on the impact of the loss tangent, relative dielectric constant, and folding length of the lens, based on both FDTD analysis and measurements. [ABSTRACT FROM AUTHOR]

Optical nanoantennas demonstrate the ability to confine and enhance electromagnetic fields. This ability makes nanoantennas essential tools for highresolution microscopy. The nanoantenna resonance and response can be tuned by changing their size, shape, and material as well as adjusting the probing conditions, e.g. excitation wavelength. In this paper we simulated the propagation and interaction of visible light with computer generated models of butterfly nanoantenna arrays using the finite-difference time-domain (FDTD) method. The simulations were used to understand and predict the experimental results obtained with scanning nearfield microscopy (SNOM) on commercially available samples. Simulation parameters are chosen carefully to reflect the measurement conditions. [ABSTRACT FROM AUTHOR]

In this study, a hybrid Chebyshev pseudo-spectral finite-difference time-domain (CPS-FDTD) algorithm is proposed for simulating 2D acoustic wave propagation in heterogeneous media, which is different from the other traditional numerical schemes such as finite element and finite difference. This proposed hybrid method integrates the efficiency of the FDTD approach in the time domain and the high accuracy of the CPS technique in the spatial domain. We present the calculation formulas of this novel approach and conduct simulation experiments to test it. The biconjugate gradient is solved by combining the large symmetric sparse systems stabilized algorithm with an incomplete LU factorization. Three numerical experiments are further presented to illustrate the accuracy, efficiency, and flexibility of the hybrid CPS-FDTD algorithm. [ABSTRACT FROM AUTHOR]

To calculate the optical parameters, the finite difference method in the time domain (FDTD) was used, which can be applied to solve Maxwell's equations. A large number of combinations of a planar structure with metal nanoparticles and a structure with nanowires and metal nanoparticles (NPs) were calculated. The height of nanowires h varied from 50 to 3000 nm, the period of the structure P was 100-600 nm, and the diameter of metal nanoparticles d was 50-400 nm. The reduction of light reflection was determined by the anti-reflection effect of the Si-NWs array itself and the direct scattering effect of metal nanoparticles. It was shown that all structures gave significantly lower reflection coefficients compared to that of a solid silicon plate. [ABSTRACT FROM AUTHOR]

Industrial robots experience posture-dependent inertial variation and interference forces between joints, which in turn generates undesired vibrations due to the inherent elasticity of the reduction gears, making it difficult to control each axis independently. Conventionally, dynamic compensation is used to compensate the robot dynamics and decouple each axis, while state feedback control based on a two-inertia model is used for dealing with the vibrations caused by the elasticity of the reduction gears. However, the control design for this approach usually assumes a fixed moment of inertia, so its performance is compromised when large and fast changes in the robot posture occurs. In this paper, a posture-variant approach that takes into account the posture-dependent inertial variation in real time is proposed to achieve exact dynamic compensation and independent control of each axis regardless of the robot posture. The state equations of the posture-variant two-inertia system model for each robot axis are discretized using the finite-difference time-domain (FDTD) method and adapted on both the dynamic compensation and feedback control parts, allowing to easily redesign the whole control system considering the inertial variation at each control cycle. The effectiveness of the method is confirmed by experimental verification on a 6-axis industrial robot. [ABSTRACT FROM AUTHOR]

The conventional finite‐difference time‐domain (FDTD) method has been formulated in the Cartesian coordinate system. In this case, the staircase approximation should be used even for the analysis of arbitrarily shaped dielectric media. As a result, small meshes are required to accurately model these media. In this letter, we extend the contour‐path effective‐permittivity (CP‐EP) technique to three‐dimensional (3D) problems in which a reasonably accurate solution can be obtained with larger meshes for arbitrarily shaped dielectric media. The formulation is performed with the position of the dielectric medium being considered in each cell, leading to its effective permittivity. We compare the result from the present method with the Mie solution and that from the conventional staircase FDTD method to confirm the effectiveness of the 3D CP‐EP technique. The relative error of the scattering cross‐section is reduced to about half that of the staircase FDTD method. [ABSTRACT FROM AUTHOR]

ObjectivesElectromagnetic pulse can seriously affect and even destroy naval shipborne electronic information systems, weapons, equipment and so on. Due to the complexity of the antenna port load circuit, it is necessary to formulate a numerical method which can easily obtain the antenna load current.MethodsThe induced current of the terminal is obtained on the basis of the finite-difference time-domain (FDTD) and Holland model. According to the Thévenin equivalent circuit, the conductor is equivalent to the voltage source to realize decoupling between the conductor and load end, and the load circuit is solved using Simulink software.ResultsCompared with the traditional terminal model, this method is simple and feasible for high-order circuit processing. The effectiveness of the model is verified using non-linear elements and parallel capacitors and resistors as the load respectively. The results show that the calculation results of this method are consistent with those of the traditional methods. Finally, the antenna of the end-connected protector is calculated. The results show that the lead inductance caused by the installation of the protector is an important factor affecting the protection performance.ConclusionsThe method proposed in this paper can greatly reduce the simulation complexity of the coupling current of an antenna with a complex load, giving it useful reference value for the field of ship electromagnetic pulse protection design.

Numerical codes for electromagnetic wave propagation in magnetized plasmas are mainly based on frequency-domain asymptotic methods, which provide a fast solution and are thus valuable for experiment design and control applications. However, in several cases of practical interest (e.g. mode conversion), these tools run close to their limits of validity and should be compared to full-wave solutions. The code RFFW solves Maxwell's equations with the finite-difference time-domain method in 3D geometry, for scenarios involving high-frequency waves with arbitrary electric field spectrum in plasmas with axisymmetric equilibrium. In fusion-related problems, the code may conduct investigations of wave propagation and absorption relevant to auxiliary plasma heating and current drive, reflectometry and instability control. The code has been parallelized with a hybrid OpenMP-MPI scheme, which has allowed exploiting the much larger processing power and memory of current-day supercomputers. In this work, we present the main aspects of the physics implemented in the code, and also refer shortly to the parallelization scheme. Furthermore, we show results that exhibit the strong scaling performance of the code, and examine cases of electron-cyclotron heating application in medium-sized tokamaks. [ABSTRACT FROM AUTHOR]

With the development of the aerospace industry, the close exploration of the Sun has become a human demand. However, close-range exploration means that the detection satellite needs to accept the test of high temperatures above 1400 °C, so a thermal protective coating is a necessary part of the detection satellite to isolate heat and reflected light. Al2O3 coating has the characteristics of high emissivity and low absorptivity, and it is the best choice for thermal protection coating. However, the coating is subjected to thermal cycles, including heating and cooling, as the satellite rotates around the Sun, which could result in a change in the structure and properties of the coating. Therefore, thermal shock experiments were carried out, and the influence of microstructure on the absorption rate of the Al2O3 coating was investigated. In this study, an Al2O3 coating was prepared by atmospheric plasma spraying (APS). The coating was subjected to a thermal shock (TS) test at 1400 °C using a self-made flame shock device, and coating samples under different thermal shock degrees were obtained. A scanning electron microscope (SEM) was used to characterize the coating pores, and the effects of the coating pore size, aspect ratio (A/R) and cracks in the coating on the optical properties of the coating under different thermal shock degrees were analyzed. In order to clarify the influence of coating microstructure changes on the optical properties of the coating under different thermal shock degrees, not only relevant experiments were carried out, but also the solar reflectivity of Al2O3 coatings with different pore structures was analyzed by the finite element method using finite-difference time-domain (FDTD). The results show that increasing the porosity and aspect ratio of the pores can improve the partial solar absorption of the coating. It was also found that the transverse crack propagation improves the solar reflectance of the coating. [ABSTRACT FROM AUTHOR]

In this work, we propose an efficient finite-difference time-domain (FDTD) simulation technique for the electromagnetic (EM) wave analysis of the Faraday rotation angle in the ionosphere. For this purpose, we first model the physical ionosphere as a scaled-down FDTD computational domain by a space-compression factor. Next, the Faraday rotation angle calculated from the FDTD simulation is calibrated by multiplying the space-compression factor. Numerical examples demonstrate that this novel space-compression-and-calibration technique can lead to a computationally efficient FDTD simulation for the EM analysis of the Faraday rotation angle without accuracy degradation.

Reverberation time (RT) is an important indicator of room acoustics, however, most studies focus on the mid-high frequency RT, and less on the low-frequency RT. In this paper, a hybrid approach based on geometric and wave methods was proposed to build a more accurate and wide frequency-band room acoustic impulse response. This hybrid method utilized the finite-difference time-domain (FDTD) method modeling at low frequencies and the Odeon simulation at mid-high frequencies, which was investigated in a university classroom. The influence of the low-frequency RT on speech intelligibility was explored. For the low-frequency part, different impedance boundary conditions were employed and the effectiveness of the hybrid method has also been verified. From the results of objective acoustical parameters and subjective listening experiments, the smaller the low-frequency RT was, the higher the Chinese speech intelligibility score was. The syllables, consonants, vowels, and the syllable order also had significant effects on the intelligibility score.

Plasmonic optical nanolithography using extraordinary optical transmission through a metallic nanohole mask has been actively applied to the high-resolution fabrication of nanostructures over a large area. Although there have been studies on improving the nanostructure fabrication performance in optical nanolithography, such as on adjustable external gap spacing, additional performance enhancement is required for practical applications and commercialization of large-area and high-resolution nanostructure array fabrication techniques. In this study, we design and apply a plasmonic bull’s eye nanostructured meta-mask to enhance the performance of optical nanolithography. Through simulation results and experimental verification, it is confirmed that advanced optical nanolithography using the bull’s eye nanostructured meta-mask has several merits compared to conventional Talbot lithography using nanoholes: (1) Optical nanolithography using the bull’s eye nanostructured meta-mask effectively fabricates nanopillar arrays even at a shorter exposure time than conventional optical lithography using nanoholes. (2) It is possible to create a large-area nanopillar array with various nanopillar diameters by exposure time control in optical nanolithography using the bull’s eye meta-mask. (3) Using water or objective immersion oil to increase the refractive index of the contact medium, light can be focused on smaller sizes, and large-area nanopillar arrays with smaller nanopillar diameters are established. With the upgradation of hardware for large-area fabrication, application of immersion media supplying techniques, and additional studies to establish complex nanostructures, optical nanolithography using the bull’s eye nanostructured meta-mask is an efficient modality to produce various nanostructure-based devices.

Compared with the traditional finite difference time-domain (FDTD) method, the discontinuous Galerkin time-domain (DGTD) method may face the issue of intense computation. In this paper, a novel 3-D DGTD-FDTD hybrid method is proposed to dramatically reduce the unknowns of the DGTD method. Instead of the common mass-lumped elements, this virtual layer of the Yee grid is implemented on the intersecting boundary, which simplifies the mesh generation and reduces the number of unknowns. To validate the proposed method, two examples of sphere scattering and horn antenna are considered. The simulation results demonstrate the effectiveness of the proposed method. [ABSTRACT FROM AUTHOR]

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Lead-free Cs2AgBiBr6 perovskites have emerged as a promising, non-toxic, and eco-friendly photovoltaic material with high structural stability and a long lifetime of carrier recombination. However, the poor-light harvesting capability of lead-free Cs2AgBiBr6 perovskites due to the large indirect band gap is a critical factor restricting the improvement of its power conversion efficiency, and little information is available about it. Therefore, this study focused on the plasmonic approach, embedded metallic nanospheres in Cs2AgBiBr6 perovskite solar cells, and quantitatively investigated their light-harvesting capability via finite-difference time-domain method. Gold and palladium were selected as metallic nanospheres and embedded in a 600 nm thick-Cs2AgBiBr6 perovskite layer-based solar cell. Performances, including short-circuit current density, were calculated by tuning the radius of metallic nanospheres. Compared to the reference devices with a short-circuit current density of 14.23 mA/cm2, when a gold metallic nanosphere with a radius of 140 nm was embedded, the maximum current density was improved by about 1.6 times to 22.8 mA/cm2. On the other hand, when a palladium metallic nanosphere with the same radius was embedded, the maximum current density was improved by about 1.8 times to 25.8 mA/cm2. [ABSTRACT FROM AUTHOR]

This paper presents a novel design of a high-speed optical 2-to-4 decoder based on the photonic crystal (PhC) composed of silicon rods. The proposed structure consists of three inputs (one for enable and two for decoder inputs), three PhC X-shaped resonators, several waveguides, and four outputs. Each resonator includes silica rods covered by graphene shells. The plane-wave expansion (PWE) method is used to calculate the band structure of the fundamental PhC. The finite-difference time-domain (FDTD) method is applied to compute optical light's transmission efficiency and electric field distribution inside the designed decoder. We demonstrate that the proposed decoder can operate in the third communication window, ranging from 1530 to 1565 nm. The numerical results reveal that the normalized transmission values less than 30% and greater than 50% are supposed to be logics 0 and 1, respectively. The decoder's maximum delay and total footprint are 2.5 ps and 690 µm2, respectively. Thus, due to the decoder's relatively simple and low-cost fabrication and its applications in photonics-based systems, the proposed device can be used in optical communications and networking. [ABSTRACT FROM AUTHOR]

In the authors' previous research, a new method for partial discharge detection in the switchgear based on near-field detection was proposed. The content of this paper is the continuation of the authors' previous research. In order to realize the reasonable layout of the near-field magnetic field probe for partial discharge detection in the switchgear, this paper simulates and analyzes the influence of the internal structure of the switchgear on the near-field propagation characteristics of the electromagnetic wave signal generated by partial discharge, and determines the installation position of the near-field probe in the switchgear. Firstly, the propagation characteristics of electromagnetic wave signals in the different media of the switchgear are analyzed, and the switchgear model is established. Then, based on the finite difference time domain method, the influence of different devices in the switchgear on the near-field propagation of the partial discharge electromagnetic wave signal is simulated. The simulation results show that the current transformer, insulator, busbar and cabinet all obviously attenuate the amplitude of the near-field electromagnetic wave signals generated by partial discharge, and the insulator causes obvious signal distortion. Finally, it is determined that the near-field probe can be installed on the inner wall or the right wall near the bottom plate of the switchgear. [ABSTRACT FROM AUTHOR]

This study explores the enhancement of emission from a single photon emitter in a quantum communication network by coupling the source with an optical nanofiber and leveraging gold nanoparticles for Purcell enhancement. Large Purcell enhancements of more than 50 times were recently reported experimentally, but the understanding of important issues, including the maximum Purcell factor and limits to quantum efficiency due to ohmic losses, is still lacking. Our findings reveal that the reported experimental results are reasonable, and confirm that such composite devices provide a promising route for high-efficiency single photon sources coupled to an optical fiber-based quantum communication network.

In this study, a hybrid Chebyshev pseudo-spectral finite-difference time-domain (CPS-FDTD) algorithm is proposed for simulating 2D acoustic wave propagation in heterogeneous media, which is different from the other traditional numerical schemes such as finite element and finite difference. This proposed hybrid method integrates the efficiency of the FDTD approach in the time domain and the high accuracy of the CPS technique in the spatial domain. We present the calculation formulas of this novel approach and conduct simulation experiments to test it. The biconjugate gradient is solved by combining the large symmetric sparse systems stabilized algorithm with an incomplete LU factorization. Three numerical experiments are further presented to illustrate the accuracy, efficiency, and flexibility of the hybrid CPS-FDTD algorithm.

Plasmon-induced transparency (PIT) in the transparent window provides new insights into the design of optical devices such as optical sensors. Therefore, in this paper, four novel structures based on the PIT phenomenon are proposed to design plasmonic refractive index sensors (RISs). The designed structures consist of metal–insulator-metal (MIM) waveguides, stub resonators (SR), and nano-disk resonators (NDRs) containing metal strips (MSs). By using an MIM waveguide, an SR, and an NDR containing MSs, the first RIS (main RIS) is designed and simulated using the finite difference time domain (FDTD) method. To verify FDTD simulations, the stub-coupled MIM waveguide system which is used to design the main RIS is analyzed using the transmission line method (TLM). The maximum sensitivity and FOM of the main RIS obtain 725.1 nm/RIU and 91.78 RIU−1, respectively. By coupling two SRs, two NDRs containing MSs, and two SRs and NDRs containing MSs simultaneously, the other three RIS structures are designed. Increasing Q factors of the designed RISs results in higher FOM values for these new structures. The maximum FOM values for RIS I, RIS II, and RIS III are achieved at 120.18, 144.27, and 113.07 RIU−1, respectively. [ABSTRACT FROM AUTHOR]

The optical spectra of structures with Ag nanoparticles between rough and cylindrical nanowires are calculated. The simulation was carried out using the finite-difference time-domain method (FDTD). As a source of radiation, a plane wave with the range of wavelengths 300–1000 nm is used. It is shown that with an increase in the root mean square (RMS) roughness of rough nanowires, the absorption coefficient decreases in the range of 500–750 nm due to an increase in the reflection effect. When silver nanoparticles are added, peaks appear at the wavelength of 840 nm (for cylindrical nanowires) and 900 nm (for rough nanowires), which indicates the manifestation of the surface plasmon resonance effect. It is shown that the electric field strength in a system with rough nanowires is higher than in a system with cylindrical nanowires. [ABSTRACT FROM AUTHOR]

• In the study of special fiber optic tweezers with hollow holes, in order to preserve the hollow holes inside the fiber optic probe, grinding method is mostly used to make fiber optic probes. In this article, the melt taper method is chosen, which can preserve the patency of the hollow holes inside the probe during the production process, providing a foundation for using hollow holes to achieve particle manipulation and sterile transportation. The production method is simple and has high repeatability. • Single hole dual core suspended optical fiber has one hollow hole and two asymmetric cores, which leads to an asymmetric and complex distribution of the outgoing light field. Based on the designed single hole dual core suspended optical fiber probe structure, simulations were conducted to analyze the asymmetric light field distribution, and further analysis was conducted on the optical trap force distribution in the asymmetric and complex optical field in both the horizontal and axial directions. Preliminary conclusions were obtained. • Based on the characteristics of single hole dual core suspension fiber optic cables and combined with melt taper technology, two types of single hole dual core suspension optical simulation calculation fiber probes were designed and manufactured, namely parabolic type and flat head oblique cone type. Through simulation analysis and some experiments, it is known that the flat head oblique cone type single hole dual core suspension fiber optic probe can achieve particle capture, and the parabolic type single hole dual core suspension fiber optic probe can achieve particle ejection or capture for particles of different diameters, providing a new idea for single fiber optic tweezers to achieve particle sorting with different particle diameters. In order to achieve the manipulation performance of particles and improve the integration and miniaturization of optical fiber tweezers systems, this paper proposes a flat-head taper single-hole dual-core suspended-core optical fiber tweezers structure based on the characteristics of single-hole dual-core suspended-core optical fibers. By establishing a model of the optical fiber probe's force on micrometer-sized particles according to the principle of momentum conservation and using the finite-difference time-domain method, the distribution of optical trapping forces along each axial direction and the possible influence of particle diameter on the optical trapping forces are simulated and analyzed. Additionally, the capability of the flat-head taper single-hole suspended-core optical fiber to trap particles is verified through experiments. Through analysis and validation, the flat-head taper single-hole dual-core suspended-core fiber optical tweezers with 2 μm diameter hollow hole can achieve diverse manipulation of 2 μm, 5 μm, and 10 μm polystyrene particles. This research provides new insights into the application of single-hole suspended-core optical fibers in optical fiber tweezers. [ABSTRACT FROM AUTHOR]

Fast and compact optoelectronic devices are highly sought after for applications in high-speed signal processing in optical communication networks. One approach to realizing such devices is through all-optical digital logic circuits. One of the main building blocks of such circuits is a decoder. In this work, we present a novel design for a tunable optoelectronic 2-to-4 binary decoder. The presented structure is realized by utilizing three photonic crystal (PhC) ring resonators. Each PhC ring resonator is formed by silicon rods encircled by silica (SiO2) rods coated with graphene nanoshells (GNSs). By adjusting the chemical potential of GNS with a proper gate voltage, we can tune the desired PhC resonant mode. The fundamental PhC microstructure's photonic band structure is analyzed by using the plane wave expansion method. Furthermore, the finite-difference time-domain technique is used to solve Maxwell's equations and analyze the light propagation within the structure. Our numerical results reveal that 0.8 ps and 0.3 ps are the maximum rise and fall times for the final structure, respectively and the total size of this device is 850 µm2. Due to the short rise and fall times and its size which are among very important features in high-speed systems, the proposed design could be utilized for high-speed signal processing systems in miniaturized optical communication network devices. [ABSTRACT FROM AUTHOR]

Due to their extraordinary abilities to manipulate light propagation at the nanoscale, dielectric resonators that generate electric and magnetic Mie resonances for minimal optical loss have recently attracted great interest. Based on an all-dielectric metasurface, made of H-type silicon nanoarrays, this study proposed and constructed a visible-wavelength-range color filter, with high-quality Mie resonance and the ability to synthesize new colors. Using the finite-difference time-domain (FDTD) approach, we can create a larger color gamut by modifying the H-type array's structural properties. The all-dielectric color filter suggested has a high color saturation and narrow bandwidth. The Mie resonance can be adjusted by manipulating the structural characteristics. By translating the reflectance spectrum into color coordinates and using the CIE1931 chromaticity diagram, a wide range of colors can be generated. This color filter offers a larger color range and saturation than other color filters. We produced color passband filters that span the visible spectrum using Mie resonator arrays, based on an H-type nanoresonator. This technology could have many applications, including high-resolution color printing, color-tunable switches, and sensing systems. [ABSTRACT FROM AUTHOR]

The recent multilevel Monte Carlo method is here proposed for uncertainty quantification in electromagnetic problems solved by the finite-difference time-domain (FDTD) method, when material parameters are modeled as random variables. It improves the estimations of the mean and variance of the quantities of interest computed on a FDTD spatial grid by sampling at coarser levels of discretization. The proposed approach can amply reduce the computational cost of the standard Monte Carlo FDTD, at the price of a small reduction of its accuracy. It is advantageous with respect to polynomial chaos FDTD, when the latter fails or becomes prohibitive for computational requirements. It also appears to widely outperform stochastic FDTD in terms of accuracy. [ABSTRACT FROM AUTHOR]

The interaction between high-power microwave and dielectric surface discharge in vacuum and low-pressure gas is investigated by using an electromagnetic particle-in-cellâ€"Monte Carlo collision model. Maxwell equations are solved by the finite-difference time-domain method combined with the boundary condition between the total and scattered field. The simulation results show that the transmission power loss is small and mainly attributed to the absorption of surface discharge, when the secondary electron multipactor reaches a steady state in vacuum. The simulated value of transmission power loss in vacuum is in good agreement with the experimental data. At a low pressure, the multipactor is the main source of electrons in the initial stage of discharge. After the multipactor reaches a steady state, the ionization leads to a significant increase in the number density of plasma near the dielectric surface. The absorbed power of plasma is greater than the reflected power in the initial stage of discharge, but with the increase of time, the latter becomes larger and even close to the power of incident wave. As the pressure increases, the transmission power decays faster due to the increase of ionization rate. When the microwave field near the dielectric surface decays significantly at a low pressure, the steady state of multipactor disappears, and the peak of plasma number density is near the surface, but not closest to the surface. [ABSTRACT FROM AUTHOR]

Lead-free Cs2AgBiBr6 perovskites have emerged as a promising, non-toxic, and eco-friendly photovoltaic material with high structural stability and a long lifetime of carrier recombination. However, the poor-light harvesting capability of lead-free Cs2AgBiBr6 perovskites due to the large indirect band gap is a critical factor restricting the improvement of its power conversion efficiency, and little information is available about it. Therefore, this study focused on the plasmonic approach, embedded metallic nanospheres in Cs2AgBiBr6 perovskite solar cells, and quantitatively investigated their light-harvesting capability via finite-difference time-domain method. Gold and palladium were selected as metallic nanospheres and embedded in a 600 nm thick-Cs2AgBiBr6 perovskite layer-based solar cell. Performances, including short-circuit current density, were calculated by tuning the radius of metallic nanospheres. Compared to the reference devices with a short-circuit current density of 14.23 mA/cm2, when a gold metallic nanosphere with a radius of 140 nm was embedded, the maximum current density was improved by about 1.6 times to 22.8 mA/cm2. On the other hand, when a palladium metallic nanosphere with the same radius was embedded, the maximum current density was improved by about 1.8 times to 25.8 mA/cm2.

Soil conductivity and permittivity have a substantial influence on lightning-electromagnetic transients in power system. In this paper, lightning-induced voltages are computed on a multi-conductor overhead line due to nearby first and subsequent return strokes. Besides, this study considers the longitudinal complex inductance of the line due to the penetration depth of electromagnetic fields through finitely conducting ground as well as the frequency dependent soil parameters using the finite-difference time-domain method. Various case studies in light of variations of return stroke velocity, line height, and soil conductivity are explored for thoughtful investigations. The obtained results reveal that the influence of this complex inductance on peak values of lightning induced voltages changes with varying the return stroke velocity. Also, the frequency dependence of soil parameters markedly impacts the computed lightning-induced voltages particularly for poor soil conductivity. [ABSTRACT FROM AUTHOR]

This is a tutorial paper on the basics and applications of the finite-difference time-domain (FDTD) method. Two types of discretization of the linear governing equations, the scalar-type FDTD method and the vector-type one, are first discussed. Then the basic concept of the compact explicit-FDTD (CE-FDTD) method is described. By considering the relationship between the cutoff frequency and the computer resources, it is shown that the interpolated wide band scheme requires the least computer resources among the derivative schemes of the CE-FDTD method. The discretization of the arbitrary shaped sound field by voxels and its boundary conditions, and the implementation of the density variation are also described. The sound field rendering and its real time renderer "Silicon concert hall" are introduced. [ABSTRACT FROM AUTHOR]

A variety of microscopes have been developed to clarify microstructures. The observation principle of photoacoustic microscope is based on generation and propagation of thermal and elastic waves by heating the sample surface which allows us to observe the detailed microstructure. The resolution of this microscope depends on the characteristic properties of the waves, such as the amplitude and decay rate of the thermal and elastic waves propagating through the sample, and thus requires an accurate understanding how temperature and stress field are changed with time in the medium. However, the classical Fourier heat conduction equation cannot reproduce the propagation of such thermal waves. In this study, we attempt to analyze numerically a two-dimensional dynamic thermoelastic wave problem using the time-domain finite difference method based on the coupled non-Fourier heat conduction equation and the dynamic thermoelastic equation, in which the delay time effect and the thermoelastic effect are considered in classical Fourier's law. In this study, we investigate how thermal and elastic waves propagate in a square plate problem without defect. The results showed that thermal and elastic waves were generated and propagated in concentric circles around the temperature input point, and reflection phenomena were observed at the boundary surface. It was also found that the delay time had a stronger effect on the temperature and stress fields than the thermoelastic coupling effect.

In this study, we consider a photonic crystal slab with a triangular lattice, which consists of air holes with a circular shape in a tellurium background on top of a Teflon substrate. In this structure, by enlarging the size of an air hole and infiltrating it with nonlinear polystyrene material we introduce a nonlinear cavity in the mentioned structures. Then, we optimize the geometrical parameters, using the finite-difference time-domain method, to obtain optimal parameters. The results reveal that the triangular lattice represents a nonlinear cavity with a large quality factor (). The mentioned value is much greater than the reported values in similarly designed structures. Then, the designed high-quality cavity is placed between two waveguides symmetrically, and thus a coupled cavity-waveguide structure is created. These waveguides are used to couple light in and out of the cavity. Our investigation shows that by changing the structural parameters such as distance between the cavity and waveguides, the strong coupling between the cavity and waveguides is obtained. In the end, the optical bistability diagram of the structure corresponding to optimum parameters is presented. It is observed that the threshold power is significantly low in the designed structure. In the optical switching phenomenon, the threshold intensity and the response time of the nonlinear materials are very important. The response time of polymers is significantly shorter than that of semiconductors and due to the use of polymers instead of semiconductors in the current study, the obtained results represent some advantages compared with the previously published results.

In the authors’ previous research, a new method for partial discharge detection in the switchgear based on near-field detection was proposed. The content of this paper is the continuation of the authors’ previous research. In order to realize the reasonable layout of the near-field magnetic field probe for partial discharge detection in the switchgear, this paper simulates and analyzes the influence of the internal structure of the switchgear on the near-field propagation characteristics of the electromagnetic wave signal generated by partial discharge, and determines the installation position of the near-field probe in the switchgear. Firstly, the propagation characteristics of electromagnetic wave signals in the different media of the switchgear are analyzed, and the switchgear model is established. Then, based on the finite difference time domain method, the influence of different devices in the switchgear on the near-field propagation of the partial discharge electromagnetic wave signal is simulated. The simulation results show that the current transformer, insulator, busbar and cabinet all obviously attenuate the amplitude of the near-field electromagnetic wave signals generated by partial discharge, and the insulator causes obvious signal distortion. Finally, it is determined that the near-field probe can be installed on the inner wall or the right wall near the bottom plate of the switchgear.

A dissipation theory is proposed for the potentials-based finite-difference time-domain (FDTD) algorithm for the case of inhomogeneous lossless media. We show that under the Courant–Friedrichs–Lewy limit, the equations describing the time evolution of scalar and vector potentials can be seen as a lossless system. The developed theory provides insights into how electromagnetic energy and power flow are approximated in FDTD schemes. It can also be used to create new algorithms with guaranteed stability. [ABSTRACT FROM AUTHOR]

In this paper, an unconditionally stable time-domain method is proposed to investigate the external electromagnetic (EM) field illumination on the nonlinearly loaded transmission lines. In the proposed algorithm, the field to line coupling equations and linear/nonlinear boundary conditions are incorporated and expressed in a matrix form. The derived differential matrix equations are then discretized and solved using the proposed finite difference time domain (FDTD) method approach. The discretized nonlinear matrix equations are also solved by applying a globally convergent iterative method to ensure the stability of the method. Moreover, the experimental investigations are conducted using a transverse EM (TEM) cell for a single microstrip line and a power detector circuit as a nonlinearly loaded transmission line to confirm the accuracy and stability of the proposed method. With the merit of satisfactory accuracy and unconditional stability for the entire range of time steps, the presented approach would be applicable for analyzing the microwave linear/nonlinear circuits subjected to the external incident EM wave. [ABSTRACT FROM AUTHOR]

Fibre optic microlenses are small optical elements formed on the end-faces of optical fibres. Their dimensions range from a few tens to hundreds of micrometres. In the article, four optical fibre microlenses are modelled and analysed. Microlenses are used for light beam manipulation and quantitative metrics are needed to evaluate the results, for example, the size of focusing spot or intensity distribution. All four lenses tested are made of rods of the same refractive index; they were welded to a single-mode fibre. Two modelling methods were used to analyse the lenses: ray-tracing and finite-difference time-domain. The ray-tracing algorithm moves rays from one plane to another and refracts them on the surfaces. Finite-difference time-domain consists of calculating Maxwell's equations by replacing spatial and temporal derivatives by quotients of finite differences. In this paper, the results of the microlenses analyses obtained from ray-tracing and finite-difference timedomain methods were compared. Both methods of analysis showed the presence of undesirable side lobes related to lens design, namely rods too long for lens fabrication. The test results were compared with the measurements made with the knife-edge method. The use of a single tool to determine parameters of an optical fibre lens does not allow for precise determination of its properties. It is necessary to use different tools and programs. This allows a complete analysis of the beam parameters, letting us find the causes of technical issues that limit the performance of the lenses. [ABSTRACT FROM AUTHOR]

Many failures of wind turbine blades and electrical equipment due to powerful lightning strikes have occurred. The lightning-induced wind turbine failures that have occurred in recent years can be classified into two main categories. The first is a mechanical component failure. The second is a failure or malfunction of electric or electronic equipment inside or around the wind turbine due to an increase in the potential of the wind turbine grounding system or the electromagnetic induction caused by the lightning strike. In the latter case, the equipment often fails owing to the increase in the potential of the grounding system, the method of suppressing the potential rise by laying a grounding wire vertically or horizontally to the wind turbine foundation is adopted. We propose an empirical expression for the effective length of counterpoises connected to a large grounding electrode such as a wind turbine foundation, and verify the expression using finite-difference time-domain (FDTD) method. [ABSTRACT FROM AUTHOR]

In view of the large scientific and technical interest in the MEMS accelerometer sensor and the limitations of capacitive, resistive piezo, and piezoelectric methods, we focus on the measurement of the seismic mass displacement using a novel design of the all-optical sensor (AOS). The proposed AOS consists of two waveguides and a ring resonator in a two-dimensional rod-based photonic crystal (PhC) microstructure, and a holder which connects the central rod of a nanocavity to a proof mass. The photonic band structure of the AOS is calculated with the plane-wave expansion approach for TE and TM polarization modes, and the light wave propagation inside the sensor is analyzed by solving Maxwell's equations using the finite-difference time-domain method. The results of our simulations demonstrate that the fundamental PhC has a free spectral range of about 730 nm covering the optical communication wavelength-bands. Simulations also show that the AOS has the resonant peak of 0.8 at 1.644µm, quality factor of 3288, full width at half maximum of 0.5nm, and figure of merit of 0.97. Furthermore, for the maximum 200nm nanocavity displacements in the x- or y-direction, the resonant wavelengths shift to 1.618µm and 1.547µm, respectively. We also calculate all characteristics of the nanocavity displacement in positive and negative directions of the x-axis and y-axis. The small area of 104.35 µm2 and short propagation time of the AOS make it an interesting sensor for various applications, especially in the vehicle navigation systems and aviation safety tools. [ABSTRACT FROM AUTHOR]

In this study, we have considered a two-dimensional triangular lattice photonic crystal composed of liquid crystal infiltrated air holes in Si background. Then, we investigate the band structure, equifrequency contours, and the field intensity distribution for different values of structural parameter; using plane wave expansion and finite-difference time-domain methods. In the following, it is found that for the optimum values of geometrical parameters the structure represents a similar behavior with a system with a negative refractive index for a relatively wide frequency range. The negative refractive index causes that in a specified frequency width, the image of a light source appears perfectly on another side of the designed photonic crystal. In the end, the effect of an externally applied voltage on liquid crystals is studied. Our simulations reveal that applying the external electric filed changes the refractive index of structure and can be used for tuning the negative refractive effect.

The proposed all-optical circuit utilizes the deep sub-wavelength confinement properties of plasmonic metal–insulator–metal waveguides. All-optical priority encoder accepts 2 n inputs and produces n outputs. The priority encoder is designed using the plasmonics waveguide-based Mach–Zehnder interferometer (P-MZI). The P-MZI works as an all-optical switch. The switching of the signal depends on the power of the applied optical input signal. This waveguide has a nonlinear material inside. The proposed work is analyzed using the finite-difference time-domain method, and a mathematical description is also given. Parameters such as insertion loss and extinction ratio are reported as 15.82 dB and 0.57 dB, respectively. [ABSTRACT FROM AUTHOR]

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Heat transfer in media is generally characterized by the empirical heat conduction law which was proposed by Fourier. However, the application of this empirical law to the problem of fast transient heat transfer such as a pulsed laser heat is not available, which is an important engineering problem. A heat conduction equation that considers about the time delay in the heat flux has been proposed to overcome such problem. In addition, Load and Shulman attempted to extend the heat conduction equation coupled with thermoelastic deformation. In this study, a nonlinear partial differential equation for the coupled problem of heat waves and thermoelastic waves with time delay was derived. Furthermore, temperature and thermal stress analysis was performed on the one-dimensional bar problem based on the finite-difference time-domain method. As results obtained in this study, thermal wave split into two peaks and propagated along the bar by considering the coupled thermoelasticity effect in the heat conduction equation. The elastic wave caused by the thermoelasticity effect also propagated. However, it was found that the peak stress in elastic wave was relaxed as the coupling effect being stronger.

Due to their extraordinary abilities to manipulate light propagation at the nanoscale, dielectric resonators that generate electric and magnetic Mie resonances for minimal optical loss have recently attracted great interest. Based on an all-dielectric metasurface, made of H-type silicon nanoarrays, this study proposed and constructed a visible-wavelength-range color filter, with high-quality Mie resonance and the ability to synthesize new colors. Using the finite-difference time-domain (FDTD) approach, we can create a larger color gamut by modifying the H-type array’s structural properties. The all-dielectric color filter suggested has a high color saturation and narrow bandwidth. The Mie resonance can be adjusted by manipulating the structural characteristics. By translating the reflectance spectrum into color coordinates and using the CIE1931 chromaticity diagram, a wide range of colors can be generated. This color filter offers a larger color range and saturation than other color filters. We produced color passband filters that span the visible spectrum using Mie resonator arrays, based on an H-type nanoresonator. This technology could have many applications, including high-resolution color printing, color-tunable switches, and sensing systems.