Time-domain Calculations Research Articles

Spectrally Selective Thermal Emission from Graphene Decorated with Metallic Nanoparticles

We showed in past work that nanopatterned monolayer graphene (NPG) enables spectrally selective thermal emission in the mid-infrared (mid-IR) from 3 to 12 μm. In that case, the spectral selection is realized by means of the localized surface plasmon (LSP) resonances inside graphene. Here, we show that graphene decorated with metallic nanoparticles, such as Ag nanocubes or nanospheres, also realizes spectrally selective thermal emission but, in this case, by means of acoustic graphene plasmons (AGPs) localized between graphene and the Ag nanoparticle inside a dielectric material. Our finite-difference time-domain (FDTD) calculations show that the spectrally selective thermal radiation emission can be tuned by means of a gate voltage into two different wavelength regimes, namely, the atmospherically opaque regime between λ = 5 and 8 μm or the atmospherically transparent regime between λ = 8 and 12 μm. This allows for electrical switching between a radiative heat trapping mode for the former regime and a radiative cooling mode for the latter regime. Our theoretical results can be used to develop graphene-based thermal management systems for smart fabrics.

A Time-Domain Piecewise Analytical AC Harmonic Current Calculation Method Suitable for Capacitor Commutated Converter

The Capacitor Commutated Converter (CCC) is constructed as a conventional Line Commutated Converter (LCC) modified with a series capacitor between the converter transformer and thyristor valves in each phase. This paper proposes a time-domain piecewise analytical AC harmonic current calculation method for the CCC system. Compared to the existing calculation methods, the proposed method can calculate both characteristic and non-characteristic harmonics simultaneously when various asymmetric factors are considered. Firstly, the overlap angles are calculated considering the asymmetric factors after partitioning the piecewise intervals in a fundamental period. Secondly, the AC harmonic currents are calculated considering the DC side harmonic currents based on the Thevenin equivalent model of the DC circuit. Then, the effectiveness of the proposed calculation method is validated by simulation results in PSCAD/EMTDC. The comparison result shows that the proposed method is high-efficiency and high-accuracy for the AC harmonic current calculation of the CCC system and can meet the requirement of CCC filter design in practical projects. Finally, the influence of the asymmetric factors on the overlap angle calculation and the influence of the commutation capacitor variation on the commutation parameters are studied.

Boosting light absorption of a therapeutic microcapsule by means of auxiliary solid nanoparticles

Multilayer microparticles with a liquid core and a polycomposite light-absorbing shell are important components of modern bio- and medical technologies, in particular, serving as transport microcontainers in the targeted delivery of therapeutic nanodoses to the desired region of the biological tissues. For reliably opening the microcapsule shell by an optical radiation and releasing the payload, it is necessary to dramatically increase the light absorption of such a microcontainer. To this end, we propose surrounding the microcapsule with specially added auxiliary nanoparticles, which can accumulate optical energy near its surface and direct a concentrated photonic flux to the target microcapsule thus leading to its booster heating. Using numerical finite-difference time-domain (FDTD) calculations, we simulate and examine the absorption dynamics of the near-infrared optical radiation in a spherical microcapsule surrounded by solid nanoparticles of different optical properties (metal, biocompatible dielectric). We show that due to light scattering on nanoparticles, the optical field superlocalization in the ”hot regions” on the microcapsule surface occurs. Up to the three-fold light absorption enhancement can be achieved due to the addition of nanoparticles.

Au nanocakes as a SERS sensor for on site and ultrafast detection of dioxins

Dioxin is a highly toxic and carcinogenic pollutant created during industrial production and waste incineration. Pollutant monitoring has made extensive use of surface-enhanced Raman spectroscopy (SERS) as a quick, non-destructive, sensitive, and affordable analytical method. However, the fabrication of a SERS substrate with ultrahigh sensitivity is a challenging task, the detection cost is expensive. Currently, these are the challenges faced by SERS technique for the detection of dioxin pollutants. In this paper, we present Au nanocakes (Au NCs) as an easy-to-build SERS substrate for sensitive SERS detection of dioxins in real samples. Based on three-dimensional finite-difference time-domain (3D-FDTD) calculations, the enhancement factor of the substrate following aggregation is approximately 1010. Using a portable Raman spectrometer, the limit of detection for dioxins such as 1-chloro-dibenzo-p-dioxin (1-CDD), 2,8-dichloro-dibenzo-p-dioxin (2,8-DCDD), and 2,3,7-trichloro-dibenzo-p-dioxin (2,3,7-TrCDD) in clean water reached as low as 5, 5, and 10 ng/mL, respectively. As a real-world application, the same toxic pollutants were detected in sewage water samples. Our findings could lead to the development of novel SERS-based sensors for the rapid detection of dioxins in real-world scenarios. Moreover, a portable Raman spectrometer is used to detect the pollutants, which is easy to use and inexpensive.

A Comparison Study of Time-Domain Computation Methods for Piecewise Smooth Fractional-Order Circuit Systems

The role of fractional calculus in circuit systems has received increased attention in recent years. In order to evaluate the effectiveness of time-domain calculation methods in the analysis of fractional-order piecewise smooth circuit systems, an experimental prototype is developed, and the effects of three typical calculation methods in different test scenarios are compared and studied in this paper. It is proved that Oustaloup’s rational approximation method usually overestimates the peak-to-peak current and brings in the pulse–voltage phenomenon in piecewise smooth test scenarios, while the results of the two iterative recurrence-form numerical methods are in good agreement with the experimental results. The study results are dedicated to provide a reference for efficiently deploying calculation methods in fractional-order piecewise smooth circuit systems. Some quantitative analysis results are concluded in this paper.

Deep learning accelerated discovery of photonic power dividers

Abstract This article applies deep learning-accelerated inverse design algorithms and discovers a spectrum of photonic power dividers with exceptional performance metrics despite the simplicity in the design geometry. The deep learning models exhibit high precisions on the order of 10−6 to 10−8 for both TE and TM polarizations of light. These models enable ultrafast search for an empirically describable subspace that simultaneously satisfy compact footprints, ultralow losses, ultrawide bandwidth, and exceptional robustness against fabrication randomness. We demonstrate a spectrum of devices for silicon photonics with programmable power splitting ratios, excess losses as small as 0.14 dB, to the best of our knowledge, the smallest footprints on the scale of sub-λ 2, and low loss bandwidths covering the whole telecommunication spectrum of O, S, E, C, L and U-bands. The robustness of the devices is statistically checked against the fabrication randomness and are numerically verified using the full three-dimensional finite difference time domain calculation.

Biological cell trapping and manipulation of a photonic nanojet by a specific microcone-shaped optical fiber tip.

Trapping and manipulating mesoscopic biological cells with high precision and flexibility are very important for numerous biomedical applications. In particular, a photonic nanojet based on a non-resonance focusing phenomenon can serve as a powerful tool for manipulating red blood cells and tumor cells in blood. In this study, we demonstrate an approach to trap and drive cells using a high-quality photonic nanojet which is produced by a specific microcone-shaped optical-fiber tip. The dynamic chemical etching method is used to fabricate optical-fiber probes with a microcone-shaped tip. Optical forces and potentials exerted on a red blood cell by a microcone-shaped fiber tips are analyzed based on finite-difference time-domain calculations. Optical trapping and driving experiments are done using breast cancer cells and red blood cells. Furthermore, a cell chain is formed by adjusting the magnitude of the optical force. The real-time backscattering intensities of multiple cells are detected, and highly sensitive trapping is achieved. This microcone-shaped optical fiber probe is potentially a powerful device for dynamic cell assembly, optical sorting, and the precise diagnosis of vascular diseases.

Determination of Shielding Effectiveness of a Subnanosecond High-Power EM Interference by an Enclosure with Aperture Using Time Domain Approach

Determination of Shielding Effectiveness of a Subnanosecond High-Power EM Interference by an Enclosure with Aperture Using Time Domain Approach

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.

Focusing of surface plasmon polaritons propagating at the SiO2/Ag interface with 2-level and 4-level Fresnel phase zone pad structures.

We propose and demonstrate dielectric Fresnel phase zone pad (FPZP) structures for focusing surface plasmon polaritons (SPPs) propagating at the SiO2/Ag interfaces. We exploited up-conversion fluorescence microscopy to characterize the SPP focusing. We first report on the SPP focusing with 2-level FPZP structures that introduced a π-phase shift in the SPP wavefront between adjacent zones. We optimized the SPP focusing by fine-tuning the longitudinal width of the FPZP structure. This led to the enhancement of the peak intensity of the SPP focal spot and the reduction of the focal spot size in both the longitudinal and transverse directions. Such focusing was also demonstrated with different focal lengths. To further improve the SPP focusing, we developed a 4-level FPZP structure, which introduced a π/2-phase shift in the SPP wavefront between adjacent zones. With the optimized 4-level FPZP structure, the SPP focal spot peak intensity is further improved, and the spot size is reduced. To assist the design of the FPZP structures, we carried out theoretical analysis and numerical calculations to determine the SPP wavelengths at various oxide/Ag interfaces. We also carried out finite difference time domain (FDTD) calculations to simulate the SPP focusing with the FPZP structures. The results of the FDTD simulation agree with the experimental results qualitatively.

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.

Gold Nanoparticle Enabled Localized Surface Plasmon Resonance on Unique Gold Nanomushroom Structures for On-Chip CRISPR-Cas13a Sensing.

A novel localized surface plasmon resonance (LSPR) system based on the coupling of gold nanomushrooms (AuNMs) and gold nanoparticles (AuNPs) is developed to enable a significant plasmonic resonant shift. The AuNP size, surface chemistry, and concentration are characterized to maximize the LSPR effect. A 31 nm redshift is achieved when the AuNMs are saturated by the AuNPs. This giant redshift also increases the full width of the spectrum and is explained by the 3D finite-difference time-domain (FDTD) calculation. In addition, this LSPR substrate is packaged in a microfluidic cell and integrated with a CRISPR-Cas13a RNA detection assay for the detection of the SARS-CoV-2 RNA targets. Once activated by the target, the AuNPs are cleaved from linker probes and randomly deposited on the AuNM substrate, demonstrating a large redshift. The novel LSPR chip using AuNP as an indicator is simple, specific, isothermal, and label-free; and thus, provides a new opportunity to achieve the next generation multiplexing and sensitive molecular diagnostic system.

Acoustic Shooting and Bounce Ray Method for Calculating Echoes of Complex Underwater Targets

The acoustic scattering characteristics of a target are of great significance to the design of underwater acoustic detection systems. With the improvement in underwater weapon detection ability, the precision and accuracy of underwater target echo characteristic simulations are required to be higher and higher. The traditional underwater target simulation based on bright spot can no longer meet the needs of an underwater acoustic detection system for targeting fine feature recognition. This paper proposes a calculation using the fine complex underwater target echo bounce beam line method, in addition to the establishment of a bandwidth signal time domain calculation model and the integral dimension reduction to accelerate the algorithm on the basis of thorough target subspace division, combined with the geometrical acoustics method (GA) and physical acoustics (PA) for the pipeline space beam propagation tracking and aperture sound field to solve and realize the acoustic scattering characteristic calculation of a complex underwater target. Finally, the proposed method was verified by the Benchmark standard submarine. The accuracy and computational efficiency of the PA acceleration method are given, and the integrity of the target information calculated by the method is verified by acoustic ISAR imaging analysis. The simulation results show that the pipeline method of bouncing an acoustic beam is suitable for the simulation of complex underwater, target-wide band fine echoes.

High-Throughput Imaging and Spectral Analysis of Gold Nanoparticles Through Waveguide Excitation

A waveguide (WG) excitation-based hyperspectral imaging (HSI) system equipped with a white-light source and Fourier transform spectrometer is presented. Both the microscopic images and spectra of spatially resolved gold nanoparticles (AuNPs) with sizes from 60 to 100 nm are taken at selected pixels. This system is also compared with the conventional one based on dark-field (DF) excitation using the same microscopic imaging setup. The spectral peaks and linewidths of imaged AuNPs are verified with finite-difference time-domain (FDTD) calculations. The high-throughput capability of our system is demonstrated with the bulk and surface refractive index (RI) sensing of AuNPs. The measured bulk wavelength sensitivity depends on the size of AuNPs and ranges from 120 to 262 nm/RIU for sizes between 60 and 100 nm. The surface RI sensing of AuNPs shows an index change of about 0.01 RIU for a three-layer stack of bovine serum albumin (BSA) adsorbed on 100-nm AuNPs, which agrees with our estimation.

Ultrasensitive determination of lipid soluble antioxidants in food products using silver nano-tripod SERS substrates

Lipid soluble antioxidants can effectively prevent the oxidative deterioration of oils and fat-containing foods. However, long-term consumption of foods containing antioxidants may damage human health. Herein, a highly sensitive surface-enhanced Raman scattering (SERS) method was explored for the detection of antioxidants by constructing multi-hot silver nano-tripod (AgNT) substrates. Finite difference time domain (FDTD) calculation demonstrated that the “hot spots” of AgNT structures are widely spread, which endows the substrate with excellent detection ability. The enhancement factors (EFs) for trans-1,2-bis(4-pyridyl)ethylene (BPE) and rhodamine 6G (R6G) were 1.08 × 109 and 4.46 × 108, respectively. Combined with density functional theory, AgNT substrates achieved rapid detection of four common antioxidants such as butylated hydroxyanisole (BHA), tertiary butylhydroquinone (TBHQ), butylated hydroxytoluene (BHT) and propyl gallate (PG). The limit of detection (LOD) of BHA, TBHQ and PG was 1 mg/L, and the LOD of BHT was 2 mg/L. Moreover, the substrate combined with principal component analysis can effectively identify antioxidants in fried potato chips. This substrate-based SERS method does not require complex sample pretreatments, and can identify and analyze types and contents of antioxidants added in food in a short time (2 min), providing a portable means for food safety control.

Dual-parameter optical sensor with cascaded ring resonators for simultaneous refractive index and temperature sensing

The optical sensors with the capabilities of eliminating external interference (EEI) and high sensitivity (S) are attractive properties in the next-generation photonic integration application. A cascaded period structure grating (PSG) ring resonator (PSGRR) with the properties of EEI and high S is proposed, in which two ring resonators are multiplexed serially and coupled to the PSG bus waveguide. In the structure, the light is optimally coupled into the two ring resonators by adjusting the width and the silicon duty cycle of the PSG pillar. By three-dimensional finite-difference time-domain (3D-FDTD) calculation, combined with the radiation and absorption loss, an optimal mode volume overlap factor of about 0.37 is achieved when the width and the silicon duty cycle of the PSG pillar are 600 nm and 0.65, respectively. The simulation result shows that the refractive index (RI) and temperature (T) sensitivities of the Ring_1 resonator covered by SU-8 are 0 and −67.3 pm/K, respectively. Simultaneously, the RI and T sensitivities of the Ring_2 resonator covered by analyte are 361.4 nm/RIU and 18.3 pm/K, respectively. Moreover, the maximum deviation values are about 0.149% and 0.230% with the change of T and RI for the Ring_1 and Ring_2 resonators, respectively. As a result, in the process of sensing without temperature controller, the Ring_1 resonator is considered as a temperature sensor, and also as a reference ring to resist the interference of T when the Ring_2 is used to detect the change of the RI. Therefore, the results indicate that the proposed structure is a promising dual-parameter and anti-interference sensor with high sensitivity and small detection deviation.

Analysis of coupled flutter mechanism of air rudder with acoustic black hole

Flutter refers to the self-excited vibration of an elastic body with increasing amplitude under the joint action of aerodynamic force, inertial force and elastic force. When the structure is composed of multiple components, the flutter of the structure is often accompanied by the coupling of different modes, and the vibration response form of the structure will also have great uncertainty, which makes it difficult to effectively control the flutter of the structure. In this paper, the flutter mechanism of an air rudder coupled with an acoustic black hole is studied. Firstly, the bi-directional fluid-structure coupling method is used to solve the flutter of air rudder with EABH in time domain, and the vibration displacement response of the structure under different inflow velocity is obtained. Then, the theoretical calculation results of piston are compared with the time domain calculation results. It can conclude that the flutter process goes through two continuous different stages due to the mode participation of the EABH. When flutter starts to occur, it can be suppressed by increasing the appropriate flow velocity, but flutter will occur sharply when velocity increases to a certain extent.

Reciprocal polariton-induced transparency in alpha-molybdenum trioxide-graphene heteronanostructures

An ultrathin heterogeneous nanostructure consisting of periodic orthorhombic-phase molybdenum trioxide (α-MoO3), graphene nanoribbons, and a dielectric spacer layer has been proposed. Coupling between a hyperbolic phonon polariton from α-MoO3 and a surface plasmon polariton from graphene leads to the presence of reciprocal polariton-induced transparency (RPoIT). The near-field coupling is further evaluated by combining the current distribution values of the multipole decomposition. The RPoIT can be controlled by changing the structural parameters and tuning the chemical potential of graphene. Effects of the incidence angle and polarization angle have been investigated thoroughly from the finite-difference time-domain calculations. The potential applications in the refractive index sensor and mid-infrared (MIR) absorbers have also been investigated. This research provides a clear understanding and practical guidance for the realization of tunable RPoIT, which is desirable from the development of nano-devices and multifunctional hybrid polarization devices in the MIR region.

Study on Characteristics and Optimal Layout of Components in Shallow Water Mooring System of Floating Wind Turbine

Offshore wind energy resources far exceed those on land. However, the increase in water depth of the continental shelf in China’s sea area slows down as the distance from the shore increases. In view of the characteristics of China’s sea, how to optimize the design of mooring systems to adapt to the water depth conditions in China’s waters and to resist the harsh sea conditions is one of the major problems encountered in the development of floating wind turbines. In this paper, the 5MW-OC4 semi-submersible floating wind turbine is taken as the research object, and the frequency domain and time domain calculation of the floating wind turbine are carried out by using SESAM software under the water depth of 40 m in the Bohai Sea. By comparing the motion response and tension of catenary and tension mooring floating wind turbine. The mooring system is optimized by combining the buoy and clump weight. The results show that the catenary mooring mode is more suitable for the floating wind turbine under shallow water conditions. Mooring accessories used in combination have the same effect on the overall response of the floating wind turbine as changing the same mooring parameters when used alone, but the effect of changing the mooring parameters on the performance optimization of the mooring system is more obvious when used in combination, and a reasonable combination of accessories can significantly change the overall characteristics of the floating system and affect the safety and cost of the system.

Theoretical manual of ‘YTU DEEP’ SHIP motion program

The theoretical background for an in-house seakeeping code ‘YTU DEEP’ that calculates heave, roll and pitch motions of displacement ships is established in this paper. The code consists of two main parts: frequency and time-domain calculations. Two-dimensional added mass and damping coefficients are calculated by using Ursell's multipole expansion theory. Lewis conformal mapping technique is used together with Tasai's method to compute the sectional hydrodynamic coefficients. The strip theory of Salvesen et al. is used to obtain the global hydrodynamic coefficients. The damping values for the roll motion are calculated by using Ikeda's method. Excitation terms for pitch and heave motions are computed by using head seas approximation. The excitation term for roll motion is predicted by using low steepness waves approximation. For frequency domain code validation, Response Amplitude Operators for the AME CRC hulls motions are plotted at different forward speeds and in head waves. The results are compared with the experimental results and a good agreement is observed. For time-domain calculations, Cummins' method is utilized which used the sectional hydrodynamic coefficients in the frequency domain. The coupled pitch & heave and uncoupled roll motion equations are solved via the 4th order Runge Kutta Method. The solution of Cummins equations is tested for a container and passenger ship form simulation in bow quartering waves. A user-friendly interface is created for performing both frequency and time-domain simulations. Pre-processing, solution, and post-processing are applied with a MATLAB code. The proposed seakeeping code aims to create a quick and efficient ship motion program.