Finite-difference Time-domain Computations Research Articles

Detection of individual metallic nanoparticles via plasmon-mediated optomechanical coupling.

Characterization of individual nanoparticles is a challenge due to the diffraction limit. To overcome this constraint we investigate the transfer of their optical properties to a mechanical degree of freedom of a larger object. From finite-difference time-domain computations, we estimate the mechanical frequency shift caused by metallic nanoparticles traveling through a microfluidic channel. Due to plasmonic effects we find relative shifts on the order of 1% for a 1 mW incident optical power for particles with radius ranging from 25 to 150 nm. The extreme sensitivity of this detection scheme enables real-time and in situ observation of optical dynamics at nanoscale.

An Uncertainty Analysis on Finite Difference Time-Domain Computations With Artificial Neural Networks: Improving accuracy while maintaining low computational costs

Artificial neural networks (ANNs) have appeared as a potential alternative for uncertainty quantification (UQ) in the finite difference time-domain (FDTD) computation. They are applied to build a surrogate model for the computation-intensive FDTD simulation and to bypass the numerous simulations required for UQ. However, when the surrogate model utilizes an ANN, a considerable number of data are generally required for high accuracy, and generating such large quantities of data becomes computationally prohibitive. To address this drawback, a number of adaptations for ANNs are proposed, which additionally improves the accuracy of ANNs in UQ for the FDTD computation while maintaining a low computational cost. The proposed algorithm is tested for application in bioelectromagnetics, and considerable speed up, as well as the improved accuracy of UQ, is observed compared to traditional methods such as the nonintrusive polynomial chaos (NIPC) method.

Rational Fabrication of Ag Nanocone Arrays Embedded with Ag NPs and Their Sensing Applications.

Colloidal lithography is used to design and construct a high-performance plasmonic sensor based on Ag nanocone arrays embedded with Ag NPs. The surface plasmon polariton (SPP) of the Ag nanocone array and the localized surface plasmon resonance (LSPR) of Ag NPs inside the nanocones can both couple incident photons. Sharp reflectance troughs are considerably enhanced by coupling the SPPs and LSPR, which is made possible by carefully tuning the nanocone sizes. To maximize the line shape and sensitivity, other geometric factors, such as the thickness of the silver layer and the size of the Ag NPs, are modified. Finite-difference time-domain computations confirm these hypotheses and experimental findings. We use well-researched solvents with various refractive indices as a model system to demonstrate good sensing performance as a proof of concept. The crystal used in this investigation has the ideal refractive index sensitivity, having 500 nm lattice constant, 350 nm nanocone height, and 350 nm base diameter (aspect ratio = 1). The Ag nanocone array embedded with Ag NPs is a good contender for a sensing platform due to its compact structure and efficient read-out apparatus.

Morphing for faster computations with finite difference time domain algorithms

In the framework of wave propagation, finite difference time domain (FDTD) algorithms, yield high computational time. We propose to use morphing algorithms to deduce some approximate wave pictures of their interactions with fluid-solid structures of various shapes and different sizes deduced from FDTD computations of scattering by solids of three given shapes: triangular, circular and elliptic ones. The error in theL2norm between the FDTD solution and approximate solution deducedviamorphing from the source and destination images are typically less than 1% if control points are judiciously chosen. We thus propose to use a morphing algorithm to deduce approximate wave pictures: at intermediate time steps from the FDTD computation of wave pictures at a time step before and after this event, and at the same time step, but for an average frequency signal between FDTD computation of wave pictures with two different signal frequencies. We stress that our approach might greatly accelerate FDTD computations as discretizations in space and time are inherently linkedviathe Courant–Friedrichs–Lewy stability condition. Our approach requires some human intervention since the accuracy of morphing highly depends upon control points, but compared to the direct computational method our approach is much faster and requires fewer resources. We also compared our approach to some neural style transfer (NST) algorithm, which is an image transformation method based on a neural network. Our approach outperforms NST in terms of theL2norm, Mean Structural SIMilarity, expected signal to error ratio.

Tunable optical behavior of infrared filters with phase change material

We propose an asymmetric to quasisymmetric behavioral tunability of a structured filter operating in the mid-infrared (IR) spectral region, where the atmospheric transparency is high. The structure is designed through electromagnetic wave analysis using finite-difference time-domain computations. The behavioral tunability provides dynamic control of the optical behavior of the IR filter without geometric and structural changes and opens up a field of research area in the tunable IR devices. It is shown that optical characteristic can be switched from an optical diode (unidirectional isolator or asymmetric light transmission) to the bidirectional isolator (quasisymmetric behavior) based on the phase transition of vanadium dioxide in the entire mid-IR spectrum. The proposed structure can be fabricated with the current nano and microfabrication techniques and can be utilized in smart front IR windows for protecting delicate sensors in the IR imaging systems and IR missile seekers under strong IR laser radiation.

Broadband and polarization-independent asymmetric transmission of visible light through a three-dimensional trapezoidal metallic metasurface

In modern optical applications, it has become an important need to flow light unidirectionally. An optical diode realizes this task as an electrical counterpart manipulates the flow of electrons in semiconductor materials. In this study, we show a broadband and polarization-independent optical diode-like mechanism in a metasurface configuration in the visible spectrum. The approach is passive such that the operating principle does not depend on any type of external biasing scheme. The constituted metasurface composed of a periodic type three-dimensional nanoarray of trapezoidal-shaped aluminum metal on a sapphire substrate is designed to produce the desired optical response for opposite directions of illumination. The optical transmission properties were systematically investigated using finite-difference time-domain computations. The asymmetric transmission frequency interval of the designed metasurface is associated with the Wood–Rayleigh anomaly, and the physical principle lies in the generation of the different number of higher order modes upon oppositely incident light. Our design has forward transmission of greater than 50%, backward transmission of less than 28%, and contrast ratio of greater than 3 dB in the entire visible spectrum. Specifically, a maximum forward transmission of 88% at 550 nm wavelength and a very high contrast ratio (∼23 dB) at a wavelength of 461 nm were obtained. It is numerically shown that the asymmetric transmission has been directly related to the appearance of high-order diffractions for only one direction excitation case. This study provides a path toward the realization of optical diodes for applications, such as optical communications and laser systems.

Development and analysis of anatomically real breast phantoms using different dispersion models

An effective method to develop anatomically real numerical breast phantoms for T1-weighted MR images of different tissue densities is presented. The dielectric properties for breast tissues are calculated and analyzed using different dispersion models (i.e., one- and two-pole cole–cole and Debye models). The method presented in this paper propounds significant improvements in comparison with existing MRI-based numerical phantoms in terms of denoising of images, tissue segmentation, nonlinear mapping of dielectric properties with realistic shapes using all the dispersion models and densitywise classification of phantoms. This method is a multistep approach in which each MRI voxel is mapped with the appropriate dielectric properties according to different dispersion models. The MRI data was collected and interpolated according to the size of the uniform grid for finite difference time domain computations followed by the preprocessing of MR images to enhance them. Thereafter, the voxel intensities were segregated into two groups as adipose and fibro glandular tissues. These tissue intensities were assigned the corresponding dielectric properties. Three-dimensional (3-D) numerical phantoms were created according to all the dispersion models. After the comparison among the models, it has been found that, along with frequency, the dielectric properties vary according to the variation of the dispersion model parameters. It was also observed that the dielectric properties calculated from one-pole cole–cole and two-pole Debye models are more close to the real properties of breast tissues than other models. A generalized method has been defined for developing the 3-D phantoms for all classes of breast according to the inhomogeneity of fibroglandular tissues using the dispersion models. The frequency-dependent and dispersion model parameters-dependent dielectric properties have been assigned to the phantoms. These real-like phantoms after 3-D printing would help researchers working in the field of breast cancer detection studies.

Electromagnetic disturbances in gas-insulated substations and VFT calculations

This paper is focused on the oscillating frequencies of very-fast transients (VFTs) generated by disconnector and circuit breaker operations in gas-insulated substations (GISs) related to electromagnetic disturbances (EMDs). Test results of the VFT amplitudes and oscillating frequencies are summarized, and the measured results have shown that there is no significant difference of the frequencies in the high-voltage main circuits and the low-voltage control circuits in the GISs. Modeling of GIS elements for VFT simulations by EMT-type software are explained, and simulation examples are demonstrated in comparison with test results. Also, FDTD (finite-difference time-domain) computations are performed, and the calculated results are compared with EMTP simulation results. If proper modeling is adopted, EMTP and FDTD results show a reasonable agreement.

FDTD Modeling of LEMP Propagation in the Earth‐Ionosphere Waveguide With Emphasis on Realistic Representation of Lightning Source

AbstractThe finite difference time domain (FDTD) method in the 2‐D cylindrical coordinate system was used to compute the nearly full‐frequency‐bandwidth vertical electric field and azimuthal magnetic field waveforms produced on the ground surface by lightning return strokes. The lightning source was represented by the modified transmission‐line model with linear current decay with height, which was implemented in the FDTD computations as an appropriate vertical phased‐current‐source array. The conductivity of atmosphere was assumed to increase exponentially with height, with different conductivity profiles being used for daytime and nighttime conditions. The fields were computed at distances ranging from 50 to 500 km. Sky waves (reflections from the ionosphere) were identified in computed waveforms and used for estimation of apparent ionospheric reflection heights. It was found that our model reproduces reasonably well the daytime electric field waveforms measured at different distances and simulated (using a more sophisticated propagation model) by Qin et al. (2017). Sensitivity of model predictions to changes in the parameters of atmospheric conductivity profile, as well as influences of the lightning source characteristics (current waveshape parameters, return‐stroke speed, and channel length) and ground conductivity were examined.

Cancer Detection in Highly Dense Breasts Using Coherently Focused Time-Reversal Microwave Imaging

Detection of tumors in highly dense breasts is a critical but challenging issue for early-stage breast cancer detection. We present the application of coherent focusing for time-reversal (TR) microwave imaging in beamspace for the detection and localization of multiple tumors in highly dense 3-D breast phantoms. We propose a novel coherent beamspace time reversal maximum likelihood (C-B-TR-ML) technique to obtain accurate tumor locations with reduced computational burden. To compare the performance, the coherent beamspace processing is also extended for conventional decomposition of the TR operator (DORT) and TR-MUSIC algorithms. A novel hybrid technique involving time of arrival and entropy is also proposed for early-time artifact removal as well as for estimating the Green's function of an equivalent virtual medium required for the TR operation. Finite-difference time-domain computations on anatomically realistic 3-D numerical breast phantoms are used to obtain the backscattered data. The results demonstrate the superior capabilities of the proposed C-B-TR-ML microwave-imaging technique in detecting and localizing multiple tumors embedded inside highly dense breast phantoms.

Laser-induced plasmonic colours on metals

Plasmonic resonances in metallic nanoparticles have been used since antiquity to colour glasses. The use of metal nanostructures for surface colourization has attracted considerable interest following recent developments in plasmonics. However, current top-down colourization methods are not ideally suited to large-scale industrial applications. Here we use a bottom-up approach where picosecond laser pulses can produce a full palette of non-iridescent colours on silver, gold, copper and aluminium. We demonstrate the process on silver coins weighing up to 5 kg and bearing large topographic variations (∼1.5 cm). We find that colours are related to a single parameter, the total accumulated fluence, making the process suitable for high-throughput industrial applications. Statistical image analyses of laser-irradiated surfaces reveal various nanoparticle size distributions. Large-scale finite-difference time-domain computations based on these nanoparticle distributions reproduce trends seen in reflectance measurements, and demonstrate the key role of plasmonic resonances in colour formation.

Two-color field enhancement at an STM junction for spatiotemporally resolved photoemission.

We report measurements and numerical simulations of ultrafast laser-excited carrier flow across a scanning tunneling microscope (STM) junction. The current from a nanoscopic tungsten tip across a ∼1 nm vacuum gap to a silver surface is driven by a two-color excitation scheme that uses an optical delay-modulation technique to extract the two-color signal from background contributions. The role of optical field enhancements in driving the current is investigated using density functional theory and full three-dimensional finite-difference time-domain computations. We find that simulated field-enhanced two-photon photoemission (2PPE) currents are in excellent agreement with the observed exponential decay of the two-color photoexcited current with increasing tip-surface separation, as well as its optical-delay dependence. The results suggest an approach to 2PPE with simultaneous subpicosecond temporal and nanometer spatial resolution.

Intensity correlations in metal films with periodic-on-average random nanohole arrays

We report detailed numerical studies based on three-dimensional finite-difference time domain computations of the intensity–intensity correlations in deliberately randomized, periodic-on-average systems. Correlation analyses are carried out in plasmonic thin films with nanohole arrays as a function of strength of disorder. We find that the intensity at certain uncharacteristic wavelengths remains strongly correlated with that in the periodic system, and these wavelengths do not match the global maxima of the periodic transmission spectrum. The study indicates that the strength of correlations is related to the pinning of the intensity to the holes. Since the intensity pinning is special characteristic of metals, the effect is only applicable in plasmonic systems.

A second order dispersive FDTD algorithm for transverse electric Maxwell’s equations with complex interfaces

This work overcomes the difficulty of the finite-difference time-domain (FDTD) algorithm in solving the transverse electric (TE) Maxwell’s equations with inhomogeneous dispersive media. For such TE problems, the electric fields are discontinuous across the dispersive interfaces. Moreover, such discontinuities are time variant. A novel matched interface and boundary time-domain (MIBTD) method is proposed to solve such problems through new developments in both mathematical formulations and numerical algorithms. Mathematically, instead of handling all zeroth and first order jump conditions in a local coordinate, we directly construct the TE jump conditions which are needed in the FDTD computations in the Cartesian coordinate. Such Cartesian direction conditions depend on the time, as well as tangential and normal components of the electric flux. Driven by the jump condition modeling, we adopt the standard Maxwell’s equations in coupling with the Debye auxiliary differential equations for the electric flux as the governing equations. Computationally, the leapfrog scheme is employed for integrating the Maxwell system and time dependent jump conditions. Sophisticated interface treatments are developed in both producing the TE jump conditions and enforcing them in the FDTD algorithm, based on a staggered Yee lattice. The numerical accuracy, stability, and efficiency of the proposed scheme are investigated by considering dispersive interfaces of various shapes. The MIBTD method achieves a spatially second order of convergence in all tests. To the best of our knowledge, the present MIBTD scheme is the first FDTD method in the literature that can restore second order accuracy in treating curved dispersive interfaces for the TE Maxwell system.

Increasing the Enhancement Factor in Plasmon-Enhanced Fluorescence with Shell-Isolated Nanoparticles

Three central experimental variables are looked at to increase the observed enhancement factor in plasmon-enhanced fluorescence: nanoparticle size, nanoparticle aggregation, and the sample temperature. The rise in emission intensity is demonstrated here in carefully chosen experimental conditions. First, it is shown that shell-isolated nanoparticles of gold (Au-SHINs) with a core of 100 nm produce a higher enhancement factor than smaller Au-SHINs (with a core of 40 nm) in solution and on Langmuir–Blodgett (LB) films. The enhancement factor is further increased by aggregation of the Au-SHINs in solution, and the findings are supported by finite-difference time-domain (FDTD) computations. Second, the enhancement factor (110-fold) of plasmon-enhanced fluorescence achieved with large Au-SHINs on an LB film of eosin decyl ester (EOSDEC) increases dramatically at low temperatures (170 fold). High enhancement factors are underpinned by a close matching of the plasmon extinction and fluorescence emission.

Optical field emission from resonant gold nanorods driven by femtosecond mid-infrared pulses

We demonstrate strong-field photoelectron emission from gold nanorods driven by femtosecond mid-infrared optical pulses. The maximum photoelectron yield is reached at the localized surface plasmon resonance, indicating that the photoemission is governed by the resonantly-enhanced optical near-field. The wavelength- and field-dependent photoemission yield allows for a noninvasive determination of local field enhancements, and we obtain intensity enhancement factors close to 1300, in good agreement with finite-difference time domain computations.

FDTD simulation of direct lightning strike to a phase conductor: Influence of corona on transient voltages at the tower

In this paper, a simplified model of corona discharge for the finite-difference time-domain (FDTD) computations has been applied to analysis of transient voltages at the tower of a transmission line caused by direct lightning strikes to an upper phase conductor. In the simulations, three 40-m towers, separated by 300m, with one overhead ground wire and three phase conductors are employed. Corona is assumed to occur only on the upper phase conductor struck by lightning. The progression of corona streamers from the conductor is represented as the radial expansion of cylindrical conducting region around the conductor. The reduction of transient-voltage peak due to corona is not very significant, so that the voltage at the nearest tower exceeds the insulation level of 66/77kV power line considered in this paper. For a 10-kA peak current, the upper-phase-conductor voltage peaks are reduced by 26% and 21% for a positive stroke with 1-μs- and 3-μs-risetime currents, respectively, and those for negative-stroke case are reduced by 18% and 13%, respectively. For a 20-kA peak current, the corresponding upper-phase-voltage peaks are reduced by 32% and 25% for the positive-stroke case, and those for the negative-stroke case are reduced by 22% and 15%.

Shape Optimization of Wideband Antennas for Microwave Energy Harvesters Using FDTD

This paper presents parameter and topology optimizations of wideband antennas for microwave energy harvesters based on finite difference time-domain computations. The antenna shapes are optimized to reduce return losses in a specific frequency band using micro-genetic algorithm. The shape parameters of a two-arm planar spiral antenna are optimized. In addition, the topology optimization is performed using normalized Gaussian network, where spatial symmetries are assumed. It is shown that the return losses of the optimized antennas are <−10 dB from 1 to 2 GHz in which they have isotropic directivity.

Nanostructured thin film–based near-infrared tunable perfect absorber using phase-change material

Nanostructured thin film absorbers embedded with phase-change thermochromic material can provide a large level of absorption tunability in the near-infrared region. Vanadium dioxide was employed as the phase-change material in the designed structures. The optical absorption properties of the designed structures with respect to the geometric and material parameters were systematically investigated using finite-difference time-domain computations. Absorption level of the resonance wavelength in the near-IR region was tuned from the perfect absorption level to a low level (17%) with a high positive dynamic range of near-infrared absorption intensity tunability (83%). Due to the phase transition of vanadium dioxide, the resonance at the near-infrared region is being turned on and turned off actively and reversibly under the thermal bias, thereby rendering these nanostructures suitable for infrared camouflage, emitters, and sensors.

Enhanced broadband transmission through structured plasmonic thin films for transparent electrodes

Periodic arrays of subwavelength holes in plasmonic thin films can provide for large broadband transmission in the near-infrared spectrum. The optical transmission properties of these thin films with respect to the film thickness, size, and shape of the holes and the period of the arrays was systematically investigated using finite difference time domain computations. Structured gold and silver films of 20-nm thickness can provide for up to 90% transmittance at near-infrared wavelength, thereby rendering them suitable as transparent conducting electrodes for near-infrared emitters or sensors and photovoltaic applications.