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A refractive index sensor composed of two straight metal-insulator-metal waveguides and a ring resonator is presented. One end of each straight waveguide is sealed and the other end acts as port. The transmission spectrum and magnetic field distribution of this sensor structure are simulated using finite-difference time-domain method (FDTD). The results show that an asymmetric line shape is observed in the transmission spectrum, and that the transmission spectrum shows a filter-like behavior. The quality factor and sensitivity are taken to characterize its sensing performance and filter properties. How structural parameters affect the sensing performance and filter properties is also studied.

Abstract In this paper, an advanced numerical technique is proposed for a low-frequency electromagnetic field simulation. The finite difference time domain (FDTD) method is one of the best methods for analyzing electromagnetic field problems, which require high levels of precision, such as a heterogeneous whole-body voxel human model. However, directly using the standard FDTD method in the quasi-static frequency band requires an extra-large number of iterations. We overcome this drawback using the hybrid of surface equivalence theorem and quasi-static FDTD (QS-FDTD) method. The surface equivalence theorem is used to consider arbitrary shape of source and the QS-FDTD is applied to obtain electromagnetic response quickly in the low-frequency region. The results of the proposed method are in good agreement with those from a commercial electromagnetic simulator. The proposed method is valid where the quasi-static approximation is satisfied. [ABSTRACT FROM AUTHOR]

A refractive index sensor based on plasmonics metal–insulator–metal (MIM) waveguide coupled with an analogy T shaped resonator is proposed. The analogy T shaped resonator consists of two perpendicular rectangle cavities. At visible light range, the transmission characteristics of the sensor are investigated by using the finite-difference time-domain (FDTD) method. The transmission spectrum exhibits a Fano resonance shape, which is caused by the interaction between the broadband and narrowband resonances of the two rectangle cavities. The refractive index sensing property is systematically studied with varied structural parameters and refractive indices. The result shows the maximum sensitivity of the sensor is 680 nm/RIU. The sensor may be potential applied in integrated of optical circuits. [ABSTRACT FROM AUTHOR]

In this paper, we develop a fully implicit scheme on staggered grids to solve the Maxwell's equations when Drude metamaterial is involved. Unconditional stability and optimal error estimate of the scheme are proved. Numerical results are provided to support the theoretical analysis, and used to demonstrate the applicability of the scheme to simulate the complicated backward wave propagation phenomenon occurring in metamaterials. [ABSTRACT FROM AUTHOR]

Quality defects, such as noncompactness, cavity, and water leakage, typically appear behind tunnel linings given the lack of standardization and loose management in tunnel construction. Thus, a method for acquiring characteristic information from tunnel lining cavity fillings using frequency band energy of wavelet packet transform (FBEWPT) was proposed in this study to master the reflection characteristics of ground-penetrating radar (GPR) signal. A forward simulation of tunnel lining cavity fillings, including air, wet clay, silt, and water, was explored through finite-difference time-domain method. The spectrum characteristics of tunnel cavity fillings were analyzed from time, frequency, and time-frequency domains. Results demonstrate that amplitude reflection clearly occurs at the inhomogeneous interface when GPR electromagnetic waves collide with the tunnel lining cavity fillings. However, the degree of attenuations is different considering the various dielectric constants of objects. These relative dielectric constants reveal the reflection interface position and other information between lining and fillings. With the increase in the difference in relative dielectric constants for lining cavity fillings, the attenuation of FBEWPT for GPR signals increasingly strengthens, and the variance and standard deviation of the FBEWPT for GPR signals constantly decrease, thereby reflecting the strong reflection and attenuation of GPR signals that appear in the propagation process. This study provides a favourable reference for obtaining GPR signal spectrum interpretation for tunnel lining cavity fillings in actual projects. [ABSTRACT FROM AUTHOR]

In this study, a compact plasmonic structure for realising plasmon‐induced transparency is proposed and numerically investigated by using the finite‐difference time‐domain method. The suggested device consists of double side‐coupled square ring resonators based on a metal–insulator–metal platform. The coupled mode theory is introduced to describe the spectral response of the proposed structure, which is verified by numerical simulations. By changing the geometrical and material parameters such as outer side length and width of square ring resonators and metal collision frequency, the transmission characteristics can be easily tuned. The simulation results show that the proposed plasmonic system is used as a sensor by filling the insulator region with different refractive index. The sensitivity and figure of merit of this sensor are calculated, 806 nm/RIU and 66 RIU–1, respectively. It is found that the slow light effect can appear in the proposed structure. By design optimisation, the maximum value of the group index is achieved close to 65. Meanwhile, the group index profile is obtained for different values of geometrical parameters like coupling aperture widths. The proposed structure provides a new way to utilise in nanosensor devices and slow light systems. [ABSTRACT FROM AUTHOR]

We demonstrate high-performance InGaN/GaN blue light emitting diodes (LEDs) embedded with an air-void layer produced by a dry-etch of nano-pillars in an n-GaN layer grown on patterned sapphire substrate (PSS), filling the space between nano-pillars with SiO2 nano-particles (NPs) and subsequent epitaxial overgrowth. The structure exhibits enhanced output power compared to similarly grown reference conventional LED without the air-void layer. This change in growth procedure contributes to the increase of internal quantum efficiency (IQE) and light extraction efficiency (LEE) resulting in a 13.5% increase of light output. LEE is 2 times more affected than IQE in the modified structure. Simulation demonstrates that the main effect causing the LEE changes is due to the emitted light being confined within the upper space above the air-void layer and thus enhancing the light scattering by the SiO2 NPs and preferential light via front surface. [ABSTRACT FROM AUTHOR]

In optical coherence tomography (OCT) systems, to precisely obtain the scattering properties of samples is an essential issue in diagnostic applications. Especially with a higher density turbid medium, the light interferes among the adjacent scatters. Combining an OCT experiment with the finite-difference time-domain simulation method, the multiple scattering effect is shown to affect the scattering properties of medium depending on the interparticle spacing. The far-field scattering phase function of scatters with various diameters was simulated to further analyze the corresponding anisotropy factors, which can be introduced into the extended Huygens-Fresnel theory to find the scattering coefficient of measured samples. [ABSTRACT FROM AUTHOR]

The protective effect of a shield wire against direct lightning flashes to a buried power cable has been clarified in this paper. The finite-difference time-domain (FDTD) method is used in the investigation. The FDTD model considers a 1-km-long insulated cable protected by a shield wire, and a lightning strike right above them. The model has been validated by comparing its results with experimental results available in the literature. The computed results show that the presence of the shield wire can effectively prevent the cable from being directly struck by the lightning flash. The voltage across the cable insulation at the striking point depends on the ground resistivity (the higher the resistivity, the higher the voltage), but this dependence is not linear. It is also shown that the soil surface ionization is relevant to the current delivered to the shield wire and to the voltage developed across the cable insulation. On the other hand, the soil ionization around the shield wire has negligible effects. The intrinsic resistance of the shield wire does not influence significantly the peak voltage across the cable insulation, but it influences its duration (the higher the intrinsic resistance, the longer the voltage duration). [ABSTRACT FROM PUBLISHER]

We explore an integrated approach to sound generation that supports a wide variety of physics-based simulation models and computer-animated phenomena. Targeting high-quality offline sound synthesis, we seek to resolve animation-driven sound radiation with near-field scattering and diffraction effects. The core of our approach is a sharp-interface finite-difference time-domain (FDTD) wavesolver, with a series of supporting algorithms to handle rapidly deforming and vibrating embedded interfaces arising in physics-based animation sound. Once the solver rasterizes these interfaces, it must evaluate acceleration boundary conditions (BCs) that involve model-and phenomena-specific computations. We introduce acoustic shaders as a mechanism to abstract away these complexities, and describe a variety of implementations for computer animation: near-rigid objects with ringing and acceleration noise, deformable (finite element) models such as thin shells, bubble-based water, and virtual characters. Since time-domain wave synthesis is expensive, we only simulate pressure waves in a small region about each sound source, then estimate a far-field pressure signal. To further improve scalability beyond multi-threading, we propose a fully time-parallel sound synthesis method that is demonstrated on commodity cloud computing resources. In addition to presenting results for multiple animation phenomena (water, rigid, shells, kinematic deformers, etc.) we also propose 3D automatic dialogue replacement (3DADR) for virtual characters so that pre-recorded dialogue can include character movement, and near-field shadowing and scattering sound effects. [ABSTRACT FROM AUTHOR]

In this paper, we propose and numerically investigate a surface plasmon resonance-based nanophotonic biosensor structure providing an extremely high sensitivity and transmission efficiency with a high figure-of-merit (FOM). The sensor structure consists of a 1-D metalic grating, rectangular nanoslabs, and single elliptical nanorod (NR) in a metal–insulator–metal configuration. The spectral response of the proposed sensor is evaluated numerically by using the finite-difference-time-domain computation method for its operation in the near-infrared region of the spectrum. The results show strong enhancement and very high spectral sensitivity of the local electromagnetic fields near the tip of the NR. The performance parameters of the nanosensor are then optimized as a function of the shape, size, cavity length, and other parameters of the nanostructures. The optimized sensitivity of this device is found to be 1960 nm/RIU with a transmission efficiency of 82.30% and an FOM of 42.26, which are very promising as compared to those of the contemporary nanosensors. As a specific biosensing application, the proposed sensor device is numerically simulated with different concentrations of satellite tobacco mosaic virus (STMV) solutions and found to be capable of sensing the virus with extraordinary optical sensitivity up to 0.245 nm/(g/l) making the nanosensor sensitive even to the smallest of changes in concentrations of the STMV solutions. This highly sensitive nanosensor may have aspiring applications in biomedical and environmental sensing mechanisms and nanophotonic integrated circuits. [ABSTRACT FROM AUTHOR]

In this paper, we simulated transient voltages across insulators and back-flashover at the transmission line tower struck by lightning in the presence of corona on the ground wires. The FDTD method and simplified (engineering) model of corona discharge were used. Three two-circuit 60-m high towers, separated by 400 m, with two overhead ground wires, and six phase conductors (two three-phase circuits) were considered. Back-flashover was assumed to occur at the upper, middle, or lower phase. Operating voltages of phase conductors were modeled by using a voltage source inserted between each phase conductor and ground. It is observed that ground-wire corona serves to reduce voltage peaks and delay the occurrence of flashover. For the upper-phase back-flashover due to a positive 50-kA stroke, the peak voltages across the upper-, middle-, and lower-phase insulators are reduced by 11%, 12%, and 12% by the ground-wire corona, and for a negative 50-kA stroke they are reduced by 8%, 9%, and 9%. Influences of ground-wire corona on transient voltages were also studied for the back-flashover at the middle or lower phase. Presence of operating voltages can lead to either increase or decrease of the peak of transient voltage, with the change being less than 20%. To the best of our knowledge, this is the first FDTD simulation of back-flashover at a transmission-line tower considering both ground-wire corona and operating voltages. [ABSTRACT FROM AUTHOR]

An all-optical plasmonic diode, comprising a metal–insulator–metal waveguide coupled with a stub cavity, is proposed based on a nonlinear Fano structure. The key technique used is to break structural spatial symmetry by a simple reflector layer in the waveguide. The spatial asymmetry of the structure gives rise to the nonreciprocity of coupling efficiencies between the Fano cavity and waveguides on both sides of the reflector layer, leading to a nonreciprocal nonlinear response. Transmission properties and dynamic responses are numerically simulated and investigated by the nonlinear finite-difference time-domain method. In the proposed structure, high-efficiency nonreciprocal transmission can be achieved with a low power threshold and an ultrafast response time (subpicosecond level). A high maximum transmittance of 89.3% and an ultra-high transmission contrast ratio of 99.6% can also be obtained. The device can be flexibly adjusted for working wavebands by altering the stub cavity length. [ABSTRACT FROM AUTHOR]

We report on the influence of silver-silica core-shell nanoparticles (Ag@SiO 2 NPs) and bare silver nanoparticles (Ag NPs) on the photoelectric current of plasmonic inverted organic solar cells. The synthesized Ag@SiO 2 NPs are deposited on a zinc oxide layer. The optimized plasmonic devices concern the Ag@SiO 2 NPs with 5 nm shell thickness and present the highest short-circuit current density of 13.44 mA/cm 2 , enhanced by 12% compared to the reference device. Such enhancements can mainly be attributed to the localized surface plasmon resonance and to the light scattering effect originating from Ag NPs and Ag@SiO 2 NPs. Finite-difference time-domain simulation reveals that a thin shell thickness facilitates the extension of a strong localized enhanced electromagnetic field in the active layer, as confirmed by measured and simulated optical absorption and photoluminescence measurements. Furthermore, AFM images present a well-distributed Ag@SiO 2 NPs array on the ZnO films, which contributes to such an improvement. [ABSTRACT FROM AUTHOR]

The numerical dispersion relation for the finite-difference time-domain method in three-dimensional spherical coordinates is derived. Derivation is based on matching spherical wave functions and results in a relation that is equally valid in the entire spherical space. It also matches the sensitivity of the underlying method to singular solutions behavior near the origin and the z-axis. Derived relation is confirmed to converge in the far field to the corresponding relation for Cartesian space. It also converges to the appropriate continuous space limit when all discrete steps approach zero. Detailed sensitivity analysis to mesh parameters and absolute position within spherical space is also presented. [ABSTRACT FROM AUTHOR]

Observation of sea ice thickness is one of key issues to understand regional effect of global warming. One of approaches to monitor sea ice in large area is microwave remote sensing data analysis. However, ground truth must be necessary to discuss the effectivity of this kind of approach. The conventional method to acquire ground truth of ice thickness is drilling ice layer and directly measuring the thickness by a ruler. However, this method is destructive, time-consuming and limited spatial resolution. Although there are several methods to acquire ice thickness in non-destructive way, ground penetrating radar (GPR) can be effective solution because it can discriminate snow-ice and ice-sea water interface. In this paper, we carried out GPR measurement in Lake Saroma for relatively large area (200m by 300m, approximately) aiming to obtain grand truth for remote sensing data.. GPR survey was conducted at 5 locations in the area. The direct measurement was also conducted simultaneously in order to calibrate GPR data for thickness estimation and to validate the result. Although GPR Bscan image obtained from 600MHz contains the reflection which may come from a structure under snow, the origin of the reflection is not obvious. Therefore, further analysis and interpretation of the GPR image, such as numerical simulation, additional signal processing and use of 200MHz antenna, are required to move on thickness estimation. [ABSTRACT FROM AUTHOR]

Stochastic media problems present a severe impact on the properties of the electromagnetic fields that usually deterministic methods tend to overlook. In this paper, we present a finite-difference time-domain (FDTD) technique, which is free from time step restrictions and manages to efficiently calculate the statistics of the involved fields in one realization. The proposed algorithm is a variation of the Stochastic FDTD method and exhibits satisfactory outcomes in high density grids. Numerical results verify the validity of our scheme compared to other methods, such as the Stochastic FDTD and the Monte Carlo techniques. [ABSTRACT FROM AUTHOR]

Abstract: The aim of this paper is to investigate the impact of the ground‐material uncertainty on the properties of electromagnetic fields generated by lightning strikes. Although several analytical as well as numerical approaches have been proposed throughout previous years for the prediction of lightning‐induced fields, these are mostly applied within a fully deterministic context, thus overlooking the degree of randomness that the corresponding problems may exhibit. Considering the rotational independence of setups with vertical lightning channels and flat terrains, we propose a stochastic finite‐difference time‐domain algorithm that takes advantage of the generalized polynomial‐chaos theory. The latter enables the representation of random processes via expansions with respect to specific orthogonal polynomials and facilitates capturing the fundamental statistical properties of the produced fields. The resulting scheme preserves the structure of the original Yee algorithm and, although computationally heavier than the latter, is clearly more efficient than conducting brute‐force Monte Carlo studies. The potential of the proposed stochastic method is assessed via various numerical results, calculation of statistical metrics, and comparisons with reference solutions. [ABSTRACT FROM AUTHOR]

Summary: In a system of transmission lines with regularly spaced resonant‐tunneling diodes (RTDs), where several straight RTD lines are connected halfway to a closed RTD line, a pulse‐shaped rotary traveling wave develops on the closed line by mutual synchronization of the oscillatory edge developed in each straight RTD line. The oscillating edge on each straight line is synchronized with the traveling pulse, such that the system has the potential to generate multiphase oscillatory signals in millimeter‐wave frequencies. To examine the dynamics of traveling pulses at such high frequencies, the system is modeled in the framework of the finite‐difference time‐domain method. It is found that a traveling pulse develops in the closed RTD line synchronized with the oscillatory edges moving in the straight lines, assuming a microstrip structure for each RTD line. We then compare the results of the finite‐difference time‐domain calculation with those predicted by the transmission line theory with parameter values obtained by the quasi‐transverse electromagnetic estimation. In addition, the RTD line that compactly confines the electromagnetic fields is shown to have the potential to generate multiphase oscillatory signals at submillimeter‐wave frequencies. [ABSTRACT FROM AUTHOR]

The extraordinary optical transmission and Faraday effects of the bilayer heterostructure consisting of a metallic film perforated with subwavelength hole arrays and a uniform dielectric film magnetized perpendicular to its plane were systematically studied by three-dimensional finite-difference time-domain method. Results of the calculation found that for the magneto-plasmonic crystals under polarized incident light with transverse magnetic mode, the resonant transmittance reached 36.9%, the Faraday rotation acquired 1.216°, and the ellipticity got a positive value of 0.840. The value of Faraday rotation and ellipticity is respectively 15.2 and 93.3 times enhancement of the 0.08° and −0.009 of the bare BIG film at the wavelength. In the transverse electric mode, the Faraday effects of the systems also had a large enhancement in contrast to the bare magnetic film. The magneto-optical effects of the systems could be manipulated by polarization mode of incident light, geometry of perforated subwavelength hole arrays, and thickness of metallic and magnetic films. Evolution of the magneto-optical properties on the structural parameters was also analyzed. Possible mechanisms underlying the extraordinary phenomena were profoundly discussed. All these results indicated that the systems could find potential applications in magneto-optical devices such as data storages, sensors, and telecommunications. [ABSTRACT FROM AUTHOR]

Environmental vibrations in cities are transmitted to buildings and propagate through the buildings via complex paths composed of the structural elements in the building, such as concrete slabs, beams, and columns. In this study, the transmission characteristics of such structure-borne sound in building structures composed of concrete were experimentally and numerically investigated. The vibration and radiated sound characteristics of a five-storey concrete structure obtained experimentally through an excitation test using the hammering method and numerically through wave-based numerical calculations are presented and compared. In this study, the finite-difference time-domain (FDTD) method, which treats the target structure as a composition of two-dimensional plate and one-dimensional beam elements to enable a low-cost calculation, is applied as a wave-based scheme. The propagation characteristics of the vibration and sound within the same floor and across different floors were investigated by considering various combinations of receiver and source points, and the structure-borne sound transmission characteristics of a concrete structure with frame elements are discussed. [ABSTRACT FROM AUTHOR]

Numerical investigation shows that an optimised leaky-mode resonant diffraction gratings fabricated in $${\mathrm{{Ta}}}_2{\mathrm{{O}}}_5$$ layer deposited on the facet of an optical fibre can have a modal reflectance peak of more than 95%, 3 dB spectral bandwidth of about 50 nm and polarization extinction ratio of almost 18 dB. The grating is intended to be employed as a high-reflectance, wavelength- and polarization-dependent mirror in a fibre laser. We compare results of 2D and 3D modelling, investigate influence of grating parameters on spectral shape of modal reflectance and discuss fabrication tolerances. [ABSTRACT FROM AUTHOR]

We proposed an optonumerical method that supports the optimization of the fabrication process of polymer microtips manufactured at the ends of optical fibers. The optimization was aimed at obtaining the functional parameter of microtips--the output beam distribution in the far-field diffraction region. This parameter depends on refractive index distribution within the microtip and its geometrical properties, which are determined by the optical power distribution of the actinic light and the exposition time during the photopolymerization process. The proposed method constitutes a convenient feedback loop for modification of the fabrication parameters. A single cycle of the proposed scheme includes numerical simulations and measurements of the functional parameter, tomographic measurements, and modifications of the fabrication process. We proposed utilization of the measured values of three-dimensional refractive index distribution of microtips as input data for the finitedifference time-domain simulations. It was proven that the iterative process leads to controlled modification of the technology parameters and finally to obtaining the desired functional parameter of fabricated microtips. [ABSTRACT FROM AUTHOR]

The effects of subsurface inhomogeneities on the detection of buried metal pipes in ground-penetrating radar(GPR) signals are investigated numerically. To model the electrical properties of the subsurface inhomogeneities, the continuous random media(CRM) generation technique is introduced. For the electromagnetic simulation of GPR signals, the finite-difference time-domain(FDTD) method is implemented. As a function of the standard deviation and the correlation length of the relative permittivity distribution for a randomly inhomogeneous ground, the GPR signals of the buried metal pipes are compared using numerical simulations. As the subsurface inhomogeneities increase, the GPR signals of the buried pipes are distorted because of the effect of the subsurface clutter. [ABSTRACT FROM AUTHOR]

The optical properties and the local electromagnetic field enhancement of a compound structure with rectangular nanohole arrays in Au-interlayer-Au multilayer films are numerically studied using finite-difference time domain method. We investigated some of structure parameters that influence the optical properties of the compound nanostructure. Adjustment of the length of rectangular nanoholes (L), the polarization direction (θ), the thickness of interlayer (SiO, H), and the materials in the interlayer can change the absorption intensity and the resonance peaks. An interlayer is located between the double Au films (Au-interlayer-Au multilayer films) with the function of confining local electromagnetic field. The simulation of the electromagnetic field distribution shows that the location of the local electromagnetic field enhancement can specify the different resonance patterns. The proposed compound nanostructure will be promising for application in portable nanoplasmonic multisensing and imaging. [ABSTRACT FROM AUTHOR]