Time-domain Calculations Research Articles

Hybrid modes in gold nanoslit arrays on Bragg nanostructures and their application for sensitive biosensors

In this work, we present high-performance surface plasmonic sensors using gold nanostructures and Bragg photonic structures. The gold film on the Bragg structure provides Tamm plasmon states (TPs). The Fano coupling between higher order TPs and Bloch-wave surface plasmon polariton (BW-SPP) on the gold nanoslit array results in a new hybrid Tamm-plasmon mode. Using finite-difference time-domain calculations, we demonstrate that the hybrid mode has the advantages of high surface sensitivity of BW-SPP mode and high resonant quality of Tamm state. The calculated plasmonic field distribution shows that the hybrid mode has a similar evanescent distribution with BW-SPP mode on gold surface and TPs field in the Bragg structure. The experimental results verify that the hybrid mode has one hundred times higher wavelength sensitivity than the Tamm state. The figure of merit of the hybrid mode is five times better than the BW-SPP mode in conventional nanoslit arrays. The real-time sensorgram further confirms that the hybrid mode has a much higher sensitivity and better signal to noise ratios in the biomolecular interaction measurement.

Deep-ultraviolet plasmon resonance of Ni nanoparticles embedded in BaTiO3 matrix

A tunable nanocomposite structure, consisting of Ni nanoparticles (NPs) with various morphologies and controllable BaTiO3 (BTO) films, was designed using a pulse laser deposition method. The morphological transformation of NPs and the crystal structure of BTO could be modulated by the Ni NP concentration. The experimental demonstration of absorption and photoluminescence enhancement using surface plasmon resonance (SPR) excitation in the deep-ultraviolet (DUV) region was investigated. The splitting of the Ni SPR peak was found in the DUV region, accompanied by decreased band gaps as the Ni NP concentration increased. By photoluminescence analysis, more Ni NPs embedding could provide more electron-hole recombination possibilities, enhancing the optical emission intensity and widening the emission range from red to ultraviolet luminescence. The DUV optical response of Ni NPs with various morphologies was further verified by finite-difference time-domain calculations. Compared with the reported metal NPs using the DUV SPR effect, the smaller Ni NPs provide a new non-noble metal NPs alternative with a high performance SPR excitation in the DUV region beyond the previous Al-based materials, giving new opportunities for manipulating high-energy photons.

An efficient optimisation tool for floating offshore wind support structures

Floating offshore wind turbines (FOWTs) are a rapidly developing technology, but still, need to gain economic competitiveness to obtain market acceptance. The reduction of the structural cost of the floating platform represents one of the main targets to reach an optimal design in the initial design process. This paper proposes an efficient optimisation tool that could be applied to any floating wind turbine to optimise the main geometry design parameters. In this study, the OC3 spar buoy concept is considered as an example, where the stability and dynamic performance of the device are verified using hydrostatic and time-domain computations. The numerical simulations are solved using an in-house MATLAB code that uses Salome-Meca to create the mesh and obtain hydrostatic and dynamic properties of the platform, Nemoh to calculate the hydrodynamic coefficients and an aero-hydro-servo Simscape model for the time-domain simulation. The main geometry design parameters of the buoy are optimised using a genetic algorithm in combination with the Kriging surrogate model to minimise the buoy’s structural cost while ensuring the spar’s performance. The optimal design reduces the cost by two to three times the worst configuration of the design space and there is also a 25% cost reduction between the most and the least restrictive optimal cases. The optimal design results demonstrate the hydrostatic constraints’ significance over the dynamic ones as they mostly influence the feasible design area. However, the results of this work could only be specific to the design case considered, and careful consideration should be given to other types of geometries. The optimisation approach presented is particularly important as it is flexible and can be adapted to other platforms.

Finite time stable inversion in discrete frequency domain: Accuracy analysis, improvement and application to wafer stage

Stable inversion represents a classic approach to achieve the exact inverse input for non-minimum-phase (NMP) systems. Solutions deduced based on state–space equations and transfer functions have been frequently proposed, however they are basically developed in the infinite-time horizon and are inherently time-domain computation methods. Considering the practical finite-time tracking tasks, this paper investigates the finite-time stable inversion problem. In particular, the discrete-frequency-domain solution which enables frequency-domain computation is studied. As the main contribution of the paper, the accuracy issues of the discrete-frequency-domain solution are revealed and an easy-to-use procedure is provided to improve the inversion accuracy by utilizing pre-actuation and post-actuation methods. Simulation and experiment both verify the effectiveness of the developed discrete-frequency-domain stable inversion technique.

Raman modes and mapping of graphene nanoparticles on Si and photonic crystal substrates

Mapping of photonic crystal (PC) surface infiltrated by graphene nanoparticles (GNP) has been done for D, G, and 2D mode and compared with those for GNP deposited on Si substrate. We've collected numerous spectra (from about 1000 points) and the changes in D, G, and 2D modes intensity ratio, as well as minor changes in frequency position for both GNP samples, have been detected. The samples were extremely heterogeneous in spectroscopic features. A suppression of luminescence intensity from the opal-based PC after the infiltration of GNP into interglobular cavities of the PC was registered together with up to 3–10 times enhancement of the Raman signal from the GNP which is important for the development of enhanced spectroscopic techniques. Thus, graphene showed a property of suppression of luminescence from the PC similar to a metallic substrate and could be applied as a luminescence quencher and light absorber. These data are confirmed by finite-difference time-domain (FDTD) calculations.

Light management in ultra-thin photonic power converters for 1310 nm laser illumination.

We designed and optimized ultra-thin single junction InAlGaAs photonic power converters (PPC) with integrated back reflectors (BR) for operation at the telecommunications wavelength of 1310 nm and numerically studied the light trapping capability of three BR types: planar, cubic nano-textured, and pyramidal nano-textured. The PPC and BR geometries were optimized to absorb a fixed percentage of the incident light at the target wavelength by coupling finite difference time-domain (FDTD) calculations with a particle swarm optimization. With 90% absorptance, opto-electrical simulations revealed that ultra-thin PPCs with 5.6- to 8.4-fold thinner absorber layers can have open circuit voltages (Voc) that are 9-12% larger and power conversion efficiencies (PCE) that are 9-10% (relative) larger than conventional thick PPCs. Compared to a thick PPC with 98% absorptance, these ultra-thin designs reduce the absorber layer thickness by 9.5-14.2 times while improving the Voc by 12-14% and resulting in a relative PCE enhancement of 3-4%. Of the studied BR designs, pyramidal BRs exhibit the highest performance for ultra-thin designs, reaching an efficiency of 43.2% with 90% absorptance, demonstrating the superior light trapping capability relative to planar and cubic nano-textured BRs.

DNA sequencing by Förster resonant energy transfer.

We propose a new DNA sequencing concept based on nonradiative Förster resonant energy transfer (FRET) from a donor quantum dot (QD) to an acceptor molecule. The FRET mechanism combined with the nanopore-based DNA translocation is suggested as a novel concept for sequencing DNA molecules. A recently-developed hybrid quantum/classical method is employed, which uses time-dependent density functional theory and quasistatic finite difference time domain calculations. Due to the significant absorbance of DNA bases for photon energies higher than 4 eV, biocompatibility, and stability, we use Zinc-Oxide (ZnO) QD as a donor in the FRET mechanism. The most sensitivity for the proposed method to DNA is achieved for the Hoechst fluorescent-dye acceptor and 1 nm ZnO-QD. Results show that the insertion of each type of DNA nucleobases between the donor and acceptor changes the frequency of the emitted light from the acceptor molecule between 0.25 to 1.6 eV. The noise analysis shows that the method can determine any unknown DNA nucleobases if the signal-to-noise ratio is larger than 5 dB. The proposed concept and excellent results shed light on a new promising class of DNA sequencers.

A comparison study of the born-kuhn model and the finite-difference-time-domain calculations on stacked plasmonic nanorods

A detailed analysis of the Born-Kuhn (BK) model for two coupled oscillators and comparison to the chiral optical properties of the corner-stacked plasmonic nanorods (CSPNs) have been investigated. Based on the normalized coupling constant c (= coupling frequency /resonance frequency ) and the damping constant g (= dissipation γ/), the chiral optical response of the BK model could be significantly different, and the c-g space can be divided into two phase regions, a weak coupling and a strong coupling region. Once the c and g parameters are extracted from CSPN structures based on finite-difference time domain (FDTD) calculations, the BK model can be directly applied to predict the chiral plasmonic response of the CSPNs. In particularly, at , the BK model suggests the strongest optical rotation dispersion while stronger circular dichroism occurs with a small g and a large c. These predictions have been confirmed by the FDTD calculations of CSPN structures. The detailed analysis of the BK model and the use of the model to direct fit the optical property of CSPNs demonstrate that the BK model is a very versatile class model to predict the chiral response of a chiral plasmonic structure or help to design and optimize a specific chiral plasmonic structure for a specific application.

In situ observation of photo-induced shortening of single Au nanorod for plasmon-enhanced formic acid dehydrogenation

Photo-induced selective shortening strategy was developed to synthesize Au nanorods (NRs) with different aspect ratios, and in situ observation of photo-induced shortening of single Au nanorod was realized, which is helpful for understanding the relationship between SPR decay and geometric nanostructure. The as-synthesized plasmonic Pd–Au NRs exhibit efficient formic acid dehydrogenation. Very impressively, the interfacial interaction between plasmonic bimetallic nanostructures and adsorbed molecules (HCOOH) was explored in situ at the single-particle level. Significant photoluminescence (PL) quenching of Pd–Au NRs was observed when HCOOH contacted the catalyst, confirming the charge transfer between Pd–Au NRs and HCOOH molecules. Finally, we shed light on the catalytic mechanism of plasmon-induced HCOOH dehydrogenation by coupling single-particle PL measurement with finite difference time domain (FDTD) and density functional theory (DFT) calculations.

Tunable narrow transparency windows induced by the coupled quasi-guided modes in borophene plasmonic nanostructure

A grating-assisted triple-layer continuous borophene sheets structure is presented to investigate the phase-coupled plasmon-induced transparency (PIT) effect at the near-infrared communication band. In the proposed structure, the diffraction grating is utilized to couple the normal incident waves, which excites the quasi-guided mode in triple-layer continuous borophene sheets. Compared to the patterned structure, the requirement of complicated processing techniques and extra doping configurations for tuning carrier density can be avoided by using the continuous borophene sheet. Additionally, the narrower transparency windows are shown in the spectral responses via the phase coupling between two detuned quasi-guided modes with high quality factors. The transparency windows can be efficiently tuned by adjusting the carrier densities in borophene sheets or the separation distances between the borophene sheets. The analytical results based on temporal coupled mode theory and transfer matrix method are consistent with the numerical simulations via finite-difference time-domain calculations. The proposed scheme provides the promising design of a tunable PIT component that is compatible with optical communication technology.

Three-level nanogrooves by vibration-assisted fly-cutting for diffraction regulation and array output.

Integrating geometric and diffractive optics functions is urgently needed to develop compact equipment for integrating diffraction manipulation and arrayed outputs. In this Letter, a superimposed three-level-grooved surface is proposed to manipulate the diffraction of visible light and provide an array output. Structure design, vibration-assisted fly-cutting, finite-difference time-domain calculations, and diffraction tests are conducted to fabricate the three-level grooves and explore the diffraction mechanism. Nanogrooves with a period close to the middle wavelength of the spectrum primarily enhances the diffraction at low diffraction orders and angles because of resonance. Optical tests prove that these superimposed three-level nanogrooves have a large bandwidth when providing the array output and serving to control and transmit diffracted light. They also show stronger performance for manipulating low diffraction orders.

Enhanced Light Absorption and Efficient Carrier Collection in MoS2 Monolayers on Au Nanopillars.

We fabricated hybrid nanostructures consisting of MoS2 monolayers and Au nanopillar (Au-NP) arrays. The surface morphology and Raman spectra showed that the MoS2 flakes transferred onto the Au-NPs were very flat and nonstrained. The Raman and photoluminescence intensities of MoS2/Au-NP were 3- and 20-fold larger than those of MoS2 flakes on a flat Au thin film, respectively. The finite-difference time-domain calculations showed that the Au-NPs significantly concentrated the incident light near their surfaces, leading to broadband absorption enhancement in the MoS2 flakes. Compared with a flat Au thin film, the Au-NPs enabled a 6-fold increase in the absorption in the MoS2 monolayer at a wavelength of 615 nm. The contact potential difference mapping showed that the electric potential at the MoS2/Au contact region was higher than that of the suspended MoS2 region by 85 mV. Such potential modulation enabled the Au-NPs to efficiently collect photogenerated electrons from the MoS2 flakes, as revealed by the uniform positive surface photovoltage signals throughout the MoS2 surface.

Frequency-domain models in the SP[formula omitted] approximation for neutron noise calculations

Simulations of the neutron flux fluctuations, known as neutron noise, can be performed by means of the spherical harmonics equations (SPN) approximation with higher accuracy than with the diffusion equation. In this sense, one can solve these equations in the time-domain or in the frequency-domain. This last approach permits solving the neutron noise without performing complete time-dependent simulations for monochromatic perturbations. This work presents two formulations of the SPN equations in the frequency domain, that are obtained by using different treatments of the time derivatives of the field moments. The methodology is verified with several neutron noise problems where the numerical results are compared with the time-domain computations of FEMFFUSION code. The C5G7 noise benchmark compares both SPN formulations, showing the applicability of the diffusive SPN approximation.

Electric field calculations for real-time space weather alerting systems

SUMMARY Space weather alerting systems for power systems require real-time calculations of the electric fields that drive geomagnetically induced currents. In this paper, we present a new method for calculating the Earth impulse response that can be convolved with real-time magnetic field data to give the required electric fields. We start with the Earth transfer function which can be expressed in two ways: as a relationship between the electric field, E, and the geomagnetic field, B, which has characteristics equivalent to that of a high-pass filter, and between E and the time derivative of the magnetic field, dB/dt, which has characteristics of a low-pass filter. An inverse Fourier transform of these transfer functions should then give the corresponding Earth impulse responses in the time domain. This works well for a uniform conductivity model for which the inverse Fourier transform of the transfer function has an analytic solution. However, the inverse Fourier transform of the transfer function for a non-uniform conductivity model requires numerical calculations and produces an acausal impulse response with oscillations because of the Gibbs phenomenon. To investigate the origin of the Gibbs oscillations, the real and imaginary parts of the transfer functions are transformed separately. This shows that the Gibbs oscillations arise from the inverse transform of the real and imaginary parts of the high-pass transfer function and the imaginary part of the low-pass transfer function. A new method is introduced that just transforms the real part of the low-pass transfer function and uses the requirement of causality to construct the full low-pass impulse response. From this, the derivative theorem of convolution is used to obtain the high-pass impulse response. Electric fields can then be calculated by convolution of the low-pass impulse response with the rate of change of the magnetic field, dB/dt, or by convolution of the high-pass impulse response with the magnetic field, B. Tests of the new method by comparison with analytic solutions for specified earth models and synthetic magnetic field data gave very high correlation coefficients, slopes near 1.0 and intercepts near zero, showing the accuracy of the new method. The method can be used with any Earth transfer function whether obtained from magnetotelluric measurements or from 1-D, 2-D or 3-D conductivity models. Thus, it provides a versatile technique that avoids the Gibbs phenomenon and produces a causal impulse response suitable for time domain calculations of electric fields.

SERS molecular-ruler based DNA aptamer single-molecule and its application to multi-level optical storage

Real-time analysis of DNA aptamer conformational changes is essential to understanding the function of DNA aptamers at the single-molecular level. Herein, we design core-satellite nanostructures as surface-enhanced Raman scattering (SERS) molecular-rulers to monitor the change process of single Hg2+ aptamer under dark-field microscopy (DFM-) correlated Raman spectroscopy. Applying the SERS molecular-ruler, the individual Hg2+ aptamer conformational change events triggered by Hg2+ that leads to the stepwise closing of the satellite to the core is continuously monitored by changes in SERS intensity. Moreover, individual Hg2+ aptamer conformational change events are confirmed by in-situ scanning electron microscopy (SEM) and finite difference time-domain (FDTD) calculation. Furthermore, the SERS molecular-ruler is a valid method for studying aptamer intermediate states, the binding sites in aptamer and the kinetics of Hg2+ aptamer configuration change at the single-molecule level. Such findings are promising for exploring the mechanism of DNA aptamer configuration change and providing the fundamental understanding for designing various DNA aptamer based biological applications. For future applications, SERS molecular-ruler has the potential to realize multi-level optical storage and presents a new practical and effective method for designing multi-level storage system.

Hydrodynamic characteristics of the crane vessel-three-bucket jacket foundation coupling hoisting system during the process of lowering

In the present study, a novel three-bucket jacket foundation with air-cushion launching technology is proposed. Through the theoretical analysis and commercial hydrodynamic software Moses, a crane vessel-three-bucket jacket foundation coupling hoisting system is established, and time-domain calculations are performed accordingly. Moreover, based on statistical information of the actual sea conditions in a certain sea area throughout the year, the corresponding permissible sea conditions for the construction of the sea are calculated to provide technical guidance for practical engineering. Furthermore, the hydrodynamic characteristics of the crane vessel, the motion response of the coupling hoisting system and variations of the cable tension under different configurations and wave directions are obtained. It is found that the vibration frequencies of the crane vessel and the foundation are the same, and the phase is maintained at 180°. It is considered that when the foundation is located at the stern of the vessel and encounters transverse waves, resonance occurs between the crane vessel and the foundation, which should be avoided in actual construction.

Sensitive Oligonucleotide Detection Using Resonant Coupling between Fano Resonance and Image Dipoles of Gold Nanoparticles.

The surface plasmon resonance (SPR)-based sensor has been widely used for biodetection. One of the attractive roles is the gold nanostructure with Fano resonance. Its sharp resonant profile takes advantage of the high figure of merit (FoM) in high-sensitivity detection. However, it is still difficult to detect small molecules at low concentrations due to the extremely low refractive index changes on the metallic surface. We propose using the coupling of image dipoles of gold nanoparticles (AuNPs) and Fano resonance of periodic capped gold nanoslits (CGNs) for sensitive small-molecule detections. The coupling mechanism was verified by three-dimensional finite-difference time-domain calculations and experiments. AuNPs on CGN form image dimer assemblies and induce image dipole with resonance wavelengths ranging from 730 to 550 nm. The surface plasmon polaritons (SPPs) interact with the image dipole of the AuNP on the CGNs and then scatter out through the periodic gold caps. The experimental results show that the peak intensity of grating resonance is decreased by the effect of image dipole and exhibits the maximum intensity change when the Fano resonance matches the resonance of image dipole. The 50 nm AuNPs can be detected with a surface density of less than one particle/μm2 by using the intensity change as the signal. With the resonant coupling between Fano resonance and image dipole extinction, the oligonucleotide with a molecular weight of 5.5 kDa can be detected at a concentration of 100 fM. The resonant coupling dramatically pushes the sensitivity boundary, and we report the limit of detection (LOD) to be 3 orders of magnitude lower than that of the prism-based SPR. This study provides a promising and efficient method for detecting low concentrations of small molecules such as aptamers, miRNA, mRNA, and peptides.

Time-domain continuous power flow calculation of electricity–gas integrated energy system considering the dynamic process of gas network

The power flow calculation is a significant basis for the optimized operation of integrated energy systems. For two-way coupled electricity–gas integrated energy systems, the dynamic characteristics of the slow time scale of the gas subsystem make the steady-state energy flow calculation unable to accurately obtain its real-time operating status. In this paper, for the electricity–gas integrated energy system, propose a time-domain continuous power flow calculation method considering the dynamic process of the gas subsystem, which can obtain continuous change of the state variables. The Taylor series expansion method is applied to the power subsystem, the unified energy path model and matching calculation method is used in the natural gas subsystem. The solution is realized by alternating the input and output variables of the coupling element. At the same time, the input of the power subsystem derivative information and the natural gas subsystems frequency domain information makes this method obtain continuous solutions in the time domain, which can meet the time scale requirements in different scenarios. Finally, a calculation example is carried out, and the results show that compared with traditional discrete method, this method can greatly reduce the calculation time under the condition of ensuring a certain degree of accuracy, which verifies the effectiveness of the proposed method.

Numerical Prediction of Duality Principle with Bloch-Floquet Periodic Boundary Condition in Fully Anisotropic FDTD

In this paper, in accordance with fully anisotropic electromagnetic materials, the duality principle is successfully validated by the fully anisotropic finite-difference time-domain (FDTD) with Bloch-Floquet periodic boundary condition (BPBC), which in theory is first effectively applied to the verification of time-domain electromagnetic computation. Starting from the conventional duality principle of isotropy, those conditions can be given without any loss term. Without loss of generality, the electromagnetic duality rules involving dielectric and magnetic lossy tensors could be obtained by combining complex extension from original real parameters. In our further research, we introduce the duality principle into the BPBC cases, then execute and validate three different fully anisotropic models by means of the FDTD method under either TE or TM modes. From highly accurate numerical point of view, we apply ourselves to a more effective verification which can forecast the reflection and transmission coefficients and detect the subsurface echoes through the duality principle.

Experiment and Electro-Thermo-Chemical Modeling on Rapid Resistive Discharge of Large-Capacity Lithium Ion Battery

Heat generation and temperature of a battery is usually presented by an equation of current. This means that we need to adopt time domain calculation to obtain thermal characteristics of the battery. To avoid the complicated calculations using time domain, ‘state of charge (SOC)’ can be used as an independent variable. A SOC based calculation method is elucidated through the comparison between the calculated results and experimental results together. Experiments are carried for rapid resistive discharge of a large-capacitive lithium secondary battery to evaluate variations of cell potential, current and temperature. Calculations are performed based on open-circuit cell potential (SOC, T), internal resistance (SOC, T) and entropy (SOC) with specific heat capacity.