Full-wave Finite Difference Research Articles

Alignment-free coupling to arrays of diamond microdisk cavities with fabrication tolerant spin-photon interfaces.

We propose a design for an efficient spin-photon interface to a color center in a diamond microdisk. The design consists of a silicon oxynitride triangular lattice overlaid on a diamond microdisk without any aligmnent between the layers. This enables vertical emission from the microdisk into low-numerical aperture modes, with quantum efficiencies as high as 46% for a tin vacancy (SnV) center. Our design is robust to manufacturing errors, potentially enabling large scale fabrication of quantum emitters coupled to optical collection modes. We also introduce a novel approach for optimizing the free space performance of our device using a dipole model, achieving comparable results to full-wave finite difference time domain simulations with 7 · 106 reduction in computational time.

Quasi-bound states in the continuum in a metal nanograting metasurface for a high figure-of-merit refractive index sensor.

In this work, we investigate the bound states in the continuum (BICs) in a gold nanograting metal-insulator-metal metasurface structure at oblique angles of incidence. The nanograting metasurface consists of a gold nanograting patterned on a silicon dioxide dielectric film deposited on a thick gold film supported by a substrate. With rigorous full-wave finite difference time domain simulations, two bound states in the continuum are revealed upon transverse magnetic wave angular incidence. One BIC is formed by the interference between the surface plasmon polariton mode of the gold nanograting and the FP cavity mode. Another BIC mode is formed by the interference between the metal-dielectric hybrid structure guided mode resonance mode and the FP cavity mode. While true BIC modes cannot be observed, quasi-BIC modes are investigated at angles of incidence slightly off from the corresponding true BIC angles. It is shown that quasi-BIC modes can suppress radiation loss, resulting in narrow resonance spectral linewidths and high quality-factors. The quasi-BIC mode associated with the surface plasmon polariton mode is investigated for refractive index sensing. As a result, a high sensitivity refractive index sensor with a large figure-of-merit of 364 has been obtained.

3D seismic characterization of fractures using elastic P-to-S double beams

Knowledge of the spatial distribution of subsurface fractures is critical for improving the production of geothermal and oil/gas. The double-beam (DB) method using acoustic waves is capable of characterizing the irregularly distributed fractures, including multiple coexisting fracture sets. We have extended the DB method to elastic waves, in particular for P-to-S waves scattered by fractures, using 3C seismic data. This elastic double-beam (EDB) method can add additional information to cross-validate the DB images of P-P waves and increase our confidence in the final fracture characterization results. To test the EDB method, we use a 3D layered reservoir model with multiple nonorthogonal coexisting fracture sets, which captures a wide range of geologic scenarios. Based on this model, we model 3C data using an elastic full-wave finite-difference method. For each subsurface target, the EDB method outputs DB images of P-P, P-Sin, and P-Santi waves, where Sin and Santi represent the in-plane (polarized perpendicular to the fracture plane strike) and antiplane (polarized parallel to the fracture plane strike) scattered vector S-waves, respectively. Numerical results indicate that EDB is a reliable tool to detect fracture distribution using the self-verification feature. In other words, P-P, P-Sin, and P-Santi should all give the same fracture parameters for truly existing fractures. Using EDB, the existence and orientation of fractures are more reliably estimated than fracture compliance for irregular fracture sets. EDB is not sensitive to random noise in data up to high noise levels. EDB can work accurately when velocity models have mild deviations from the true models. The EDB amplitude is large where fractures are dense or the compliance value is large, or both; these can be important in the interpretation of the fluid transport properties of a reservoir.

Assessment of measurement performance for a low field side IDTT plasma position reflectometry system

The Italian Divertor Test Tokamak (IDTT) facility will study advanced exhaust solutions applicable to DEMO. This new machine also opens the possibility to test and validate relevant non-magnetic control diagnostics in support of a DEMO design implementation. Reflectometry, compatible with a full grade reactor implementation, has been proposed as a source of real-time (RT) plasma position and shape measurements for control purposes, in replacement or complement of standard magnetic measurements, to ensure reliability and safety on the machine. Additionally, such a system can be useful to obtain the characteristics of edge turbulence, useful for Neoclassical Tearing Mode (NTM) control or to improve the efficiency of other diagnostics as Collective Thomson Scattering (CTS). This new control technique based on multiple, poloidally distributed, non-magnetic measurements, must be tested, in all of its aspects, before it can be fully implemented in future fusion reactors. IDTT will be one of the best candidates to implement and build a knowledge database of nonstandard reflectometry (away from the equatorial plane) that will be needed on DEMO and is currently unavailable. The performance of three Ordinary mode (O-mode) Plasma Position Reflectometers (PPR) at the Lower Field Side (LFS) on IDTT is assessed using the two-dimensional (2D) full-wave Finite-Difference Time-Domain (FDTD) code, REFMULF, in two of the foreseen IDTT plasma scenarios: 5MA Single Null (SN) and Double Null (DN) configurations.

Modeling Ultrasound Propagation in the Moving Brain: Applications to Shear Shock Waves and Traumatic Brain Injury.

Traumatic brain injury (TBI) studies on the living human brain are experimentally infeasible due to ethical reasons and the elastic properties of the brain degrade rapidly postmortem. We present a simulation approach that models ultrasound propagation in the human brain, while it is moving due to the complex shear shock wave deformation from a traumatic impact. Finite difference simulations can model ultrasound propagation in complex media such as human tissue. Recently, we have shown that the fullwave finite difference approach can also be used to represent displacements that are much smaller than the grid size, such as the motion encountered in shear wave propagation from ultrasound elastography. However, this subresolution displacement model, called impedance flow, was only implemented and validated for acoustical media composed of randomly distributed scatterers. Herein, we propose a generalization of the impedance flow method that describes the continuous subresolution motion of structured acoustical maps, and in particular of acoustical maps of the human brain. It is shown that the average error in simulating subresolution displacements using impedance flow is small when compared to the acoustical wavelength ( λ /1702). The method is then applied to acoustical maps of the human brain with a motion that is imposed by the propagation of a shear shock wave. This motion is determined numerically with a custom piecewise parabolic method that is calibrated to ex vivo observations of shear shocks in the porcine brain. Then the fullwave simulation tool is used to model transmit-receive imaging sequences based on an L7-4 imaging transducer. The simulated radio frequency data are beamformed using a conventional delay-and-sum method and a normalized cross-correlation method designed for shock wave tracking is used to determine the tissue motion. This overall process is an in silico reproduction of the experiments that were previously performed to observe shear shock waves in fresh porcine brain. It is shown that the proposed generalized impedance flow method accurately captures the shear wave motion in terms of the wave profile, shock front characteristics, odd harmonic spectrum generation, and acceleration at the shear shock front. We expect that this approach will lead to improvements in image sequence design that takes into account the aberration and multiple reflections from the brain and in the design of tracking algorithms that can more accurately capture the complex brain motion that occurs during a traumatic impact. These methods of modeling ultrasound propagation in moving media can also be applied to other displacements, such as those generated by shear wave elastography or blood flow.

On the Accuracy of Ray-Theory Methods to Determine the Altitudes of Intracloud Electric Discharges and Ionospheric Reflections: Application to Narrow Bipolar Events.

Narrow bipolar events (NBEs) (also called narrow bipolar pulses [NBPs] or compact intracloud discharges [CIDs]) are energetic intracloud discharges characterized by narrow bipolar electromagnetic waveforms identified from ground‐based very low frequency (VLF)/low‐frequency (LF) observations. The simplified ray‐theory method proposed by Smith et al. (1999, https://doi.org/10.1029/1998JD200045; 2004, https://doi.org/10.1029/2002RS002790) is widely used to infer the altitude of intracloud lightning and the effective (or virtual) reflection height of the ionosphere from VLF/LF signals. However, due to the large amount of high‐frequency components in NBEs, the propagation effect of the electromagnetic fields for NBEs at large distance depends nontrivially on the geometry and the effective conductivity of the Earth‐ionosphere waveguide (EIWG). In this study, we investigate the propagation of NBEs by using a full‐wave Finite‐Difference Time‐Domain (FDTD) approach. The simulated results are compared with ground‐based measurements at different distances in Southern China, and we assess the accuracy of the simplified ray‐theory method in estimating the altitude of the NBE source and the effective reflection height of the ionosphere. It is noted that the evaluated NBE altitudes have a slight difference of about ±1 km when compared with the full‐wave FDTD results, while the evaluated ionospheric reflection heights are found to be bigger than those obtained from FDTD model by about 5 km.

Surface plasmon induced enhancement in selective laser melting processes

PurposeSelective laser melting (SLM) is an advanced rapid prototyping or additive manufacturing technology that uses high power density laser to fabricate metal/alloy components with minimal geometric constraints. The SLM process is multi-physics in nature and its study requires development of complex simulation tools. The purpose of this paper is to study – for the first time, to the best of the authors’ knowledge – the electromagnetic wave interactions and thermal processes in SLM based dense powder beds under the full-wave formalism and identify prospective metal powder bed particle distributions that can substantially improve the absorption rate, SLM volumetric deposition rate and thereby the overall build time.Design/methodology/approachWe present a self-consistent thermo-optical model of the laser-matter interactions pertaining to SLM. The complex electromagnetic interactions and thermal effects in the dense metal powder beds are investigated by means of full-wave finite difference simulations. The model allows for accurate simulations of the excitation of gap, bulk and surface electromagnetic resonance modes, the energy transport across the particles, time dependent local permittivity variations under the incident laser intensity, and the thermal effects (joule heating) due to electromagnetic energy dissipation.FindingsLocalized gap and surface plasmon polariton resonance effects are identified as possible mechanisms toward improved absorption in small and medium size titanium powder beds. Furthermore, the observed near homogeneous temperature distributions across the metal powders indicates fast thermalization processes and allows for development of simple analytical models to describe the dynamics of the SLM process.Originality/valueTo the best of the authors’ knowledge, for the first time the electromagnetic interactions and thermal processes with dense powder beds pertaining to SLM processes are investigated under full-wave formalism. Explicit description is provided for important SLM process parameters such as critical laser power density, saturation temperature and time to melt. Specific guidelines are presented for improved energy efficiency and optimization of the SLM process deposition rates.

Multi-Objective Optimization of a Wireless Body Area Network for Varying Body Positions.

The purpose of this research was to improve the performance of a wireless body area sensor network, operating on a person in the seated and standing positions. Optimization-focused on both the on-body transmission channel and off-body link performance. The system consists of three nodes. One node (on the user’s head) is fixed, while the positions of the other two (one on the user’s trunk and the other on one leg) with respect to the body (local coordinates) are design variables. The objective function used in the design process is characterized by two components: the first controls the wireless channel for on-body data transmission between the three sensor nodes, while the second controls the off-body transmission between the nodes and a remote transceiver. The optimal design procedure exploits a low-cost Estra, which is an evolutionary strategy optimization algorithm linked with Remcom XFdtd, a full-wave Finite-Difference Time-Domain (FDTD) electromagnetic field analysis package. The Pareto-like approach applied in this study searches for a non-dominated solution that gives the best compromise between on-body and off-body performance.

3D seismic characterization of fractures with random spacing using the double-beam method

Obtaining information on the spatial distribution of subsurface natural and induced fractures is critical in the production of geothermal or hydrocarbon fluids. Traditional seismic characterization methods for subsurface fractures are based on the assumption of effective anisotropy medium theory, which may not be true in reality when the fracture distribution is random. We have tested the recently proposed double-beam method to characterize nonuniformly distributed fractures. We built a 3D layered reservoir model; the reservoir layer was geometrically irregular, and it contained a set of randomly spaced fractures with spatially varying fracture compliances. We used an elastic full-wave finite-difference method to model the wavefield, where we treat the fractures as linear-slip boundaries and the data include all elastic multiple scattering. Taking the surface seismic data as input, the double-beam method forms a focusing source beam and a focusing receiver beam toward the fracture target. The fracture information is derived from the interference pattern of these two beams, which includes fracture orientation, fracture spacing, and fracture compliance as a function of spatial location. The fracture orientation parameter is the most readily determined parameter even for multiple nonorthogonal coexistent fracture sets. The beam-interference amplitude depends on the fracture spacing and compliance in a local average sense for random fractures. The beam-interference amplitude is large when there are many fractures or the compliance value is large, which is important in the interpretation of the fluid-transport properties of a reservoir.

Scale-Dependent Light Scattering Analysis of Textured Structures on LED Light Extraction Enhancement Using Hybrid Full-Wave Finite-Difference Time-Domain and Ray-Tracing Methods

A multiscale model that enables quantitative understanding and prediction of the size effect on the scattering properties of micro- and nanostructures is crucial for the design of light-emitting diode (LED) surface textures optimized for high light extraction efficiency (LEE). In this paper, a hybrid process for combining full-wave finite-difference time-domain simulation and a ray-tracing technique based on a bidirectional scattering distribution function model is proposed. We apply this method to study the influence of different pattern sizes of a patterned sapphire substrate on GaN-based LED light extraction from the micro-scale to the nano-scale. The results show that near-wavelength–scale patterns with strong diffraction are not expected to enhance the LEE. By contrast, micro-scale patterns with optical diffusion behavior have the highest LEE at a specific aspect ratio, and subwavelength-scale patterns that have antireflection properties show a marked enhancement of the LEE for a wide range of aspect ratios.

A compressive sensing approach for enhancing breast cancer detection using a hybrid DBT/NRI configuration

This work presents a novel breast cancer imaging approach that uses compressive sensing in a hybrid digital breast tomosynthesis (DBT)/nearfield radar imaging (NRI) system configuration. The non-homogeneous tissue distribution of the breast, described in terms of dielectric constant and conductivity, is extracted from the DBT image, and it is used by a full-wave finite difference in the frequency domain method to build a linearized model of the non-linear NRI imaging problem. The inversion of the linear problem is solved using compressive sensing imaging techniques, which lead to a reduction on the required number of sensing antennas and operational bandwidth without loss of performance.

Stoichiometry and structure driven optical properties of carbon incorporated titanium oxide thin films

This study shows that the optical properties of titanium oxide films depend on their chemical composition as well as structure properties. The chemical composition as well as the structure of the sputter-deposited titanium oxide films on fused quartz glass substrates was modulated by annealing them in Ar/H2 atmosphere, experimentally investigated by various spectroscopic methods including XRD, Raman, SIMS, and UV-Vis spectroscopy and simulated by Bruggeman effective medium theory (BR-EMT) and full wave finite difference time domain (FDTD) approach. The as-deposited films are amorphous titanium oxide films with oxygen deficiency, become closer to titanium dioxide in stoichiometry, and are converted to crystalline anatase TiO2 nano-particles with particle size of ~21 nm by increasing the annealing temperature from 100 °C to 450 °C. The annealing process also allows the titanium oxide films to be spontaneously doped with environmental carbon species or coated with nano-crystalline graphitic carbon. Further the FDTD simulation results indicate that the annealed titanium oxide films become porous. The chemical composition modulation, anatase crystalline formation, carbon doping and graphitic carbon coating are simultaneously achieved by the simply annealing the sputter-deposited amorphous titanium oxide thin films on fused quartz substrates at relatively low temperatures.

Low-Profile Antenna With Elevated Toroid-Shaped Radiation for On-Road Reader of RFID-Enabled Vehicle Registration Plate

A low-profile antenna installed on the road surface for RFID-enabled vehicle communication is presented. The antenna in this application operates as on-road sensor for vehicles identification. To that end, the antenna is required to have an elevated toroid-shaped radiation, so that the reader can easily interrogate an RFID-enabled vehicle license. The antenna is designed using a capacitive-loaded inverted-F radiator, which has an omnidirectional radiation in the azimuth plane. To adjust the radiation pattern to the required elevation above the road surface, and isolate the radiation from the effects of variable road conditions, a corrugated ground plane with perturbed surface impedance is used underneath the radiator. The antenna’s configuration is verified by full-wave finite difference time domain simulations with various road surface conditions. A prototype antenna is then developed and tested in realistic field scenarios as on-road sensor for vehicles identification. The results show that the antenna possess more than 15 dB return loss, more than 1.5 dBi gain, vertical polarization, and desirable radiation pattern from 880 to 960 MHz, which is the standard band licensed for RFID-enabled plates. The antenna also successfully communicates with an RFID-enabled vehicle registration plate.

A theoretical investigation on the parametric instability excited by X‐mode polarized electromagnetic wave at Tromsø

AbstractRecent ionospheric modification experiments performed at Tromsø, Norway, have indicated that X‐mode pump wave is capable of stimulating high‐frequency enhanced plasma lines, which manifests the excitation of parametric instability. This paper investigates theoretically how the observation can be explained by the excitation of parametric instability driven by X‐mode pump wave. The threshold of the parametric instability has been calculated for several recent experimental observations at Tromsø, illustrating that our derived equations for the excitation of parametric instability for X‐mode heating can explain the experimental observations. According to our theoretical calculation, a minimum fraction of pump wave electric field needs to be directed along the geomagnetic field direction in order for the parametric instability threshold to be met. A full‐wave finite difference time domain simulation has been performed to demonstrate that a small parallel component of pump wave electric field can be achieved during X‐mode heating in the presence of inhomogeneous plasma.

Finite-difference time-domain study of modulated and disordered coupled resonator optical waveguide rotation sensors.

We present a full-wave finite difference time domain (FDTD) study of a coupled resonator optical waveguide (CROW) rotation sensor consisting of 8 doubly degenerate ring resonators. First we demonstrate the formation of rotation-induced gap in the spectral pass-band of the CROW and show the existence of a dead-zone at low rotation rates which is mainly due to its finite size and partly because of the individual cavities losses. In order to overcome this deficiency, we modulate periodically the refractive indices of the resonators to effectively move CROW's operating point away from this dead-zone. Finally, we analyze the performance of a structurally disordered CROW to model the unavoidable fabrication errors and inaccuracies. We show that in some cases structural disorder can increase the sensitivity to rotation by breaking the degeneracy of the resonators, thus making such CROW even more sensitive to rotation than its unperturbed ideal counterpart.

Compact-range accuracy improvement: ground-reflection-error mitigation using surface patterning [AMTA corner

Compact-range measurement facilities have been used successfully for many years to characterize antenna performance as well as radar signatures. This paper investigates strategies for improving compact-range-measurement accuracy by mitigating errors associated with ground reflections inherent in most range designs. A methodology is developed for strategically modifying, or patterning, the surface between the range's source antenna and the reflector to reduce error terms, thereby increasing measurement accuracy. Candidate patterns were evaluated using a full-wave computational Finite-Difference Time-Domain (FDTD) model at VHF/UHF frequencies to determine baseline performance, and to develop trade rules for more advanced designs. Physical Optics (PO) models were used to analyze the final design at the frequencies of interest.

3D Nanopillar optical antenna photodetectors

We demonstrate 3D surface plasmon photoresponse in nanopillar arrays resulting in enhanced responsivity due to both Localized Surface Plasmon Resonances (LSPRs) and Surface Plasmon Polariton Bloch Waves (SPP-BWs). The LSPRs are excited due to a partial gold shell coating the nanopillar which acts as a 3D Nanopillar Optical Antenna (NOA) in focusing light into the nanopillar. Angular photoresponse measurements show that SPP-BWs can be spectrally coincident with LSPRs to result in a x2 enhancement in responsivity at 1180 nm. Full-wave Finite Difference Time Domain (FDTD) simulations substantiate both the spatial and spectral coupling of the SPP-BW / LSPR for enhanced absorption and the nature of the LSPR. Geometrical control of the 3D NOA and the self-aligned metal hole lattice allows the hybridization of both localized and propagating surface plasmon modes for enhanced absorption. Hybridized plasmonic modes opens up new avenues in optical antenna design in nanoscale photodetectors.

Characterization of large array of plasmonic nanoparticles on layered substrate: dipole mode analysis integrated with complex image method

In this paper, an efficient analytical method for characterizing large array of plasmonic nanoparticles located over planarly layered substrate is introduced. The model is called dipole mode complex image (DMCI) method since the main idea lies in modeling a subwavelength spherical nanoparticle at its electric scattering resonance with an induced electric dipole and representing the electromagnetic (EM) fields of this electric dipole over the layered substrate in terms of finite complex images. The major advantages of the proposed method are its accuracy and rapid calculation in characterizing various kinds of large periodic and aperiodic arrays of nanoparticles on layered substrates. The computational time can be reduced significantly in compared to the traditional methods. The accuracy of the theoretical model is validated through comparison with numerical integration of Sommerfeld integrals. Moreover, the analytical results are compared well with those determined by full-wave finite difference time domain (FDTD) method. To demonstrate the capability of our technique, the performances of large arrays of nanoparticles on layered silicon substrates for efficient sunlight energy incoupling are studied.

Plasmonic array nanoantennas on layered substrates: modeling and radiation characteristics

In this paper, we theoretically characterize the performance of array of plasmonic core-shell nano-radiators located over layered substrates. Engineered substrates are investigated to manipulate the radiation performance of nanoantennas. A rigorous analytical approach for the problem in hand is developed by applying Green's function analysis of dipoles located above layered materials. It is illustrated that around the electric scattering resonances of the subwavelength spherical particles, each particle can be viewed as an induced electric dipole which is related to the total electric field upon that particle by a polarizability factor. Utilizing this, we can effectively study the physical performance of such structures. The accuracy of our theoretical model is validated through using a full-wave finite difference time domain (FDTD) numerical technique. It is established that by novel arraying of nano-particl and tailoring their multilayer substrates, one can successfully engineer the radiation patterns and beam angles. Several optical nanoantennas designed on layered substrates are explored. Using the FDTD the effect of finite size substrate is also explored.

Rotman lens design and simulation in software [Application Notes

Since its invention in the early 1960s (Rotman and Turner, 1963), the Rotman Lens has proven itself to be a useful beamformer for designers of electronically scanned arrays. Inherent in its design is a true time delay phase shift capability that is independent of frequency and removes the need for costly phase shifters to steer a beam over wide angles. The Rotman Lens has a long history in military radar, but it has also been used in communication systems. This article uses the developed software to design and analyze a microstrip Rotman Lens for the Ku band. The initial lens design will come from a tool based on geometrical optics (GO). A second stage of analysis will be performed using a full wave finite difference time domain (FDTD) solver. The results between the first-cut design tool and the comprehensive FDTD solver will be compared, and some of the design trades will be explored to gauge their impact on the performance of the lens.