Time-domain Simulation Method Research Articles

Design of Tunable Far-Infrared Plasmonic Absorber Based on Chalcogenide Phase Change Materials

We propose a wide-angle metamaterial absorber with more than 90% absorption in the far-infrared (F-IR) and terahertz (THz) regimes. Our metal-dielectric metamaterial absorber consists of a phase change layer (Ge2Sb2Te5), a dielectric spacer (MgF2), and a bottom refractory metal layer (TiN). We numerically designed the structure by finite-difference time-domain simulation method and demonstrated a perfect absorption in the spectral range from 10 μm to 50 μm (30 THz to 6 THz). Furthermore, it shows a broad peak with maximum absorption of 93% at the resonant wavelength of 22.5 μm when the phase change layer is in the amorphous (disorder) state. In contrast, the peak resonance is red-shifted to 29.5 μm when the Ge2Sb2Te5 switches to the crystalline (order) state, demonstrating a resonant band tunability of Δλ= 7μm. The proposed structure shown here is a simple planner structure, lithographic-free and easy to fabricate with spectral band tunability, which offers great potential for ultra-cooled detection, imaging, security scanning, gas leakage detection, and remote monitoring applications.

Optofluidic R6G microbubble DBR laser: A miniaturized device for highly sensitive lab-on-a-chip biosensing

In this work, we propose a miniaturized optofluidic bio-sensor based on organic Rhodamine 6G optofluidic microbubble distributed Bragg reflector (R6G OMB DBR) laser and investigate its performance using a 3-D finite difference time domain (FDTD) simulation method. For this aim, lasing characteristics are precisely calculated and then exploited to scrutinize the capability of the proposed bio-laser in bio-sensing. The designed laser is pumped with a 532 nm Nd: YAG laser and operates at a wavelength of 561.1 nm. The threshold pump energy density and quality (Q)-factor of this single-mode microlaser are 3.6 μJ/mm2 and 4.28 × 104, respectively. The results declare that the presence of Polio virus-like-particles (VLPs) leads to a spectral redshift in the laser output wavelength while decreasing its output intensity. The obtained sensitivity and intensity sensitivity of the proposed bio-sensor are 1377 nm/RIU and 0.98 × 104%/RIU, respectively. Therefore, the proposed miniaturized device shows proper lasing characteristics as well as high abilities in real-time lab-on-a-chip (LOC) bio-sensing applications.

Multiple poles resonances coupling with high sensitivity sensing for multiple bulging black phosphorus-based metasurface

Herein, a multiple bulging black phosphorus (BP)-based metasurface is proposed for studying its reflection responses and sensing performances through the finite-difference time-domain simulation method. It is shown that, the reflection dips are caused by the coupling between dipole resonance modes and poly-poles resonance modes. Moreover, the dipoles resonance modes and poly-poles resonance modes can mutually enhance and inhibit each other, and tunable reflection spectra can be realized by symmetrically and asymmetrically adjusting the bulging of the proposed BP-based metasurface. In addition, the reflection spectra as a function of the polarization of incident light are discussed. We can find that a dipole resonance mode on the vertical side at the direction of ZZ for BP is gradually fully excited, resulting in an additional obvious reflection dip as the polarization angle θ increases from 0° to 90°. Especially, the sensing performance with the maximum of sensitivity S = 1.5 μm/RIU can be realized in the proposed BP-based metasurface. The results may provide a way to design micro-nano plasmonic devices.

Effect of the Ag thickness on the Light Extraction Efficiency of the OLED with an ITO/Ag/ITO Anode

The light extraction efficiency of the organic light emitting diodes (OLEDs) with an ITO/Ag/ITO anode was analyzed by using the finite-difference time domain (FDTD) simulation method. For different inserted Ag thickness, the light extraction ratio was reduced as increasing the Ag thickness. The simulation results also showed the light extraction ratio began to be saturated at a critical angle as increasing the simulation monitor num as a result of the extraction loss due to the wave-guide mode by a total internal reflection. From the simulated electric field profiles, the light-outcoupling loss due to the surface plasmon polariton (SPP) mode was more apparently investigated as increasing the Ag thickness. In the fabrication of the OLED with ITO/Ag/ITO anode, the Ag thickness should be optimized with respect to the light extraction efficiency of the OLED as well as the electrical conductivity of the anode.

Designed Growth of AgNP Arrays for Anti-counterfeiting Based on Surface-Enhanced Raman Spectroscopy Signals.

Based on etched PS sphere arrays, the different growths of Ag nanoparticles with tunable LSPR are designed when SiO2-25 nm/Ag-30 nm/SiO2-100 nm sandwich nanocavity structures are annealed at 500 °C, including the hexagonal silver nanoparticle rings, circular silver nanoparticle rings, and aggregated silver nanoparticles. The uniformity of particle size and regularity of position generate enhanced electromagnetic field and good surface-enhanced Raman spectroscopy signals as confirmed by UV-vis observation and finite difference time domain method simulation. The developed nanostructures are effectively used as stable, nonreproducible, and markable anti-counterfeiting signs.

Numerical investigation on hopf bifurcation problem for nonlinear dynamics of a towed vessel in calm water and waves

In this study, the Hopf bifurcation problem with regard to the nonlinear dynamics of a towed vessel was numerically investigated. A time-domain simulation method based on three degrees-of-freedom maneuvering equations of motion was applied to study the nonlinear dynamic responses of a towed vessel in calm water, wind and waves. In the towing operation, it is assumed that a vessel is passively towed by a tug via a towline with a constant towing speed following a straight line. In the case of wind or wave conditions, the steady wind or wave drift forces are considered as additional external force components. The hydrodynamic forces acting on the towed vessel are modeled as a modular-type hull force model, and the towline force is simply modeled as a linear spring. First, for the validation, the motion trajectories of the towed vessel were compared between the experimental data and numerical simulation results under the calm water condition. Discussions are provided on the effect of skegs on the dynamics of the towed vessel and related Hopf bifurcation according to the skeg size. Next, the wind speed was changed, and the resulting limit cycles were studied. It can be observed that the Hopf bifurcation can be delayed according to the wind speed and direction. Finally, the dynamic characteristics of the towed vessel in regular waves were investigated via numerical simulations. The stability of the towed vessel with no skegs increases owing to the mean wave drift forces for both the model test and numerical simulation. The effects of the wave height and period on the Hopf bifurcation were studied based on numerical simulations. It was found that the occurrence of Hopf bifurcation for nonlinear dynamics of a towed vessel is sensitive to the conditions of wave height and period.

Experimental demonstration of inverse-designed silicon integrated photonic power splitters

Abstract The on-chip optical power splitter is a common and important device in photonic integrated circuits (PICs). To achieve a low insertion loss and high uniformity while splitting the guided light, multi-mode interferometer-based structures utilizing a self-imaging principle are widely used mainly in the form of a 1 × 2 configuration. Recently, an inverse design method for nanophotonic devices has emerged to overcome the limited capability of the conventional design methods and make it possible to explore the vast number of design parameters. Because of the non-intuitive shape of inverse-designed structures, they allow us to discover interesting and complex optical responses which are almost impossible to find with conventional design methods. Here, we report two kinds of inverse-designed 1 × 4 optical power splitters composed of silicon bars of different lengths, which are fabricated with a standard CMOS-compatible process. The particle swarm optimization method was used to minimize the insertion loss and divide the power evenly into each output port with finite-difference time-domain method simulation. The first optical power splitter has a compact size of 8.14 × 12 μm2 and the second optical power splitter has an even more compact size of 6.0 × 7.2 μm2. With the inverse designed structures, we fabricated the chip with a CMOS-compatible fabrication process. Experimental verification of the structures is provided and good agreement with the numerical results is obtained. The first 1 × 4 optical power splitter has a low insertion loss of less than 0.76 dB and uniformity of less than 0.84 dB, and the second more compact optical power splitter has a low insertion loss of less than 1.08 dB and uniformity of less than 0.81 dB. As the complexity of on-chip photonic systems has steadily increased, the inverse design of photonic structures holds great potential to be an essential part of advanced design tools.

Convex-optimization-based post-processing for computing room impulse response by frequency-domain FEM

Numerical simulation of impulse responses is an important task for room acoustic design and prediction. Since an impulse response is a concept in the time domain, time-domain simulation methods are popular for this task. At the same time, frequency-domain simulation methods are also used for simulating a room impulse response by computing frequency responses and applying the inverse discrete Fourier transform (IDFT). In this case, well-established frequency-domain solvers, including finite element method (FEM) and boundary element method (BEM), can be used. However, IDFT generates some artifacts that are undesirable for room acoustic evaluation. If such artifacts can be attenuated, then the frequency-domain methods become more attractive for simulating room impulse responses. In this paper, we propose a post-processing method to attenuate the artifacts from the impulse responses computed by the combination of frequency-domain simulation and IDFT. It is formulated as a convex optimization problem, and the alternating direction method of multipliers (ADMM) is applied to obtain the solution. Numerical experiments confirmed that the proposed method can effectively reduce the undesirable artifacts from the reconstructed impulse responses. An example implementation of the proposed method in MATLAB is provided at https://doi.org/g9h5.

Design and research of multiparameter tunable optical filter based on phase-change Bragg grating

On-chip filters are widely used in integrated optical systems and are indispensable for expanding the communication capacity. A silicon waveguide filter is fabricated into a subwavelength Bragg grating structure using the optical phase-change material Ge2Sb2Se4Te1 (GSST). Three-dimensional finite-difference time-domain method simulations showed that this structure performed multichannel filtering in the communications C-band (1530 to 1565 nm), demonstrating a minimum insertion loss of 0.238 dB and a full width at half-maximum of 3.4 nm with an incident TE0 mode. The number and width of channels can be adjusted using structural parameters. Furthermore, the overlapping frequencies of passbands and stopbands are obtained by the phase transition of GSST. This structure could potentially find use as an optical signal modulation device in on-chip integrated systems.

An Accurate Power Flow Method for Microgrids with Conventional Droop Control

With conventional droop control, the droop relationship between the voltage and reactive power is not purely linear because of the filter reactance. This paper focuses on the theoretical analysis to account for this characteristic and presents a precise power flow method for conventional droop control. The proposed method is universal, which can handle not only conventional droop control, but also other control strategies, such as robust droop control, constant power control, and constant voltage–frequency control. It can also handle frequency-dependent active and reactive power loads and is adapted for islanded and grid-connected systems. The proposed method extends the applicability of conventional power flow methods to microgrids so that the framework of the method is generic; any conventional power flow algorithm can be adapted to this framework. Compared with the time-domain simulation method, the proposed method is accurate, simple, and easy to implement for industrial applications.

Enhancing performance of tin-based perovskite solar cells by polyvinyl pyrrolidone doping strategy

The power conversion efficiency (PCE) of tin-based perovskite solar cells (TPSCs) was significantly improved when polyvinyl pyrrolidone (PVP) was doped into the formamidinium tin iodide (FASnI3) layer with an optimized doping mass. After doping, the FASnI3 grain size, film smoothness and average carrier lifetime were largely enhanced, which demonstrate the crystallization quality improvement to FASnI3. The champion PCE of the enhanced TPSC was improved to 7.83%, which is relatively 27% larger than that of the control, 6.15%. After continuous storage for 15 days, the PCE of the enhanced TPSC remained 74.4% of its original value. Using the finite difference time domain (FDTD) simulation method, we demonstrated that the light absorption ratio of the FASnI3:PVP layer was larger than that of the FASnI3 , and the photo-generated carrier density of the enhanced TPSC was approximately 2% ∼ 3% larger than that of the control. The ultimate efficiency of the enhanced TPSC was calculated to be 26.23%. It is potential to dope PVP into FASnI3 to further enhance the device performance of TPSCs.

Research on reliability of mode coupling chatter of orthopedic surgery robot

In this paper, the reliability probability method is introduced into the prediction of mode coupling chatter of the orthopedic surgical robot milling to avoid the influence of the randomness of the input parameters on the chatter prediction results. The reliability of mode coupling chatter is defined to represent the probability of the stability (no mode coupling chatter occurs) of milling system. Probability model (reliability model) of mode coupling chatter is established to predict robotic milling chatter, in which the modal stiffness K x, K y and cutting force coefficient Ks involved in the prediction of chatter vibration during robotic milling are considered as random variables. The fourth moment method is adopted to solve the reliability model of milling system to obtain the reliability level. The mode coupling chatter reliability circle diagram (MCCRCD), which is a contour line with a specified reliability level as a function of the angle between the main stiffness direction and the cutting force and the depth of cut, is presented to identify the mode coupling chatter reliable and unreliable regions. An example studies the influence of random variables on the reliability of milling processing, and the time-domain simulation method and experiment method are used to verify the possibility and effectiveness of using MCCRCD to predict the stability of the robotic milling. It can be concluded that MCCRCD can be effectively used to predict the stability of robotic milling.

Tunable plasmonic optical responses and the sensing application in graphene-based metasurface

It is meaningful that the sensing performance is achieved in a simple graphene structure. In this paper, we proposed a simple graphene-based metasurface structure which contain a graphene ribbon and U-shaped graphene pattern, showing outstanding sensing performance in terahertz band. The transmission spectrum with three dips is excited in our proposed structure calculated by the finite-difference time domain (FDTD) simulation method. The results show that the transmission spectrum with three dips can be effectively tuned by the Fermi energy. Intriguingly, the tri-frequency detection of the sensing performance is focused on the proposed graphene-based metasurface. The maximum of sensitivity and figure of merit(FOM) for the sensing performance are 3.4269 THz/RIU and 21.92, respectively. These research results may pave the way for the development of optical terahertz sensing devices.

Results of an international benchmark study on numerical simulation of flooding and motions of a damaged ropax ship

Survivability of damaged ships, especially ro-ro/passenger (ropax) vessels, is of paramount interest. Nowadays, time-domain simulation of flooding and motions of damaged ships are more frequently performed to obtain a more realistic overview of the actual survivability in case of a flooding accident. An international benchmark study on simulation of flooding and motions of damaged ropax vessels was conducted within the EU Horizon 2020 project FLARE, using new dedicated model tests as a reference. The test cases include transient flooding in both calm water and in irregular beam seas, as well as gradual flooding and capsizing in beam seas. The studied damage case is a two-compartment collision damage, and the studied intact metacentric height values were lower than the statutory requirements to achieve also capsize cases. Numerical results were carefully compared against measurement data from the model tests. In transient flooding cases the capsize conditions were generally detected well by most codes. However, much variation was observed in the internal flooding and capsize mechanisms. For gradual flooding in beam seas, the results for capsize rate and time-to-capsize were characterized by significant variability among the codes. Results indicate that more research is needed to further improve the time-domain flooding simulation methods to correctly capture both transient flooding phenomena and motions of damaged ship in high waves.

Introducing MRAC-PSS-VI to Increase Small-Signal Stability of the Power System after Wind Power Integration

With the gradual increase of wind power penetration in the power system, the impact of wind farm on system stability is becoming more significant. This study designs the power system stabilizer (PSS) based on the model reference adaptive control (MRAC) method and virtual impedance (VI) control strategy. Then, the active power difference of the doubly fed induction generator (DFIG) is selected as input signal of MRAC-PSS-VI. The small-signal stability of the power system with the DFIG is enhanced by installing MRAC-PSS-VI on the rotor side control (RSC) control link. The controller model is built in DigSILENT/PowerFactory. The improvement effect of the controller on the low-frequency oscillation (LFO) characteristics of the power system is verified by using eigenvalue analysis and the time domain simulation method under different transmission powers of the tie line and different installed positions of the DFIG.

Effect of plasmonic coupling in different assembly of gold nanorods studied by FDTD

The influence of the orientation of gold nanorods in different assemblies has been investigated using the Finite Difference Time Domain (FDTD) simulation method. To understand the relative orientation, we vary the size and angle in dimer geometries. Significant effects of plasmon coupling emerged in longitudinal resonances having end-to-end configurations of gold nanorods. The effect of orientational plasmon coupling in dimers gives rise to both bonding and anti-bonding plasmon modes. Effects of various geometries like primary monomer, dimer, trimer, and tetramer structures have been explored and compared with their higher nanorod ensembles. The asymmetric spectral response in a 4 * 4 gold nanorods array indicates a Fano-like resonance. The variation of gap distance in ordered arrays allowed modulation of the Fano resonance mode. The plasmon modes' resonance wavelength and field enhancement have been tuned by varying the gap distance, angular orientation, size irregularity between the nanorods, and nanorod numbers in an array. The integrated nanostructures studied here are not only significant for fundamental research but also applications in plasmon-based devices.

A Physics-Informed Deep Learning Paradigm for Transient Power Angle Stability Assessment

Transient stability assessment (TSA) plays an essential role in the safe and stable operation of power system. The traditional time-domain simulation method and direct method have great limitation in modeling and computing, which can not meet the need of fast and accurate assessment of power system stability. Therefore, combined physics model with data-driven algorithm, this paper proposes a physics-informed deep learning paradigm to solve this problem. In the data-driven algorithm, recurrent neural networks are trained to predict the rotor speed and power angle with the guiding of the power system’s physical information. Finally, the simulation on IEEE14 system is carried out by using PSS/E software, which proved that the proposed method can predict the power angle trajectory quickly and accurately.

Analyses of the light extraction efficiency of organic light-emitting diodes with ITO/mesh-Ag/ITO anode and its optical transmittance as transparent electrode

We have used the three-dimensional finite-difference time-domain simulation method to analyze the out-coupling efficiency of organic light-emitting diodes (OLEDs) with an ITO/Mesh-Ag/ITO anode. The period and grid space of the meshed Ag structure effectively affected the extraction ratio (ER) of the bottom-emitting OLED. The 3D simulation results for a simple structure of OLED with an ITO/Mesh-Ag/ITO anode showed the ER of about 26.4% and it enhanced 2.26 times that of the continuous Ag layer of about 11.7%. In addition, in respect to the transparent electrode of the OLED using the ITO/Mesh-Ag/ITO multiple layers, the Ag thickness and open ratio were important factors that affected the transmittance and the electrical conductance. The open ratio of 60% resulted in sheet resistance of less than 25 Ω/□, while keeping the transmittance nearly similar to that in the case of a single ITO layer.

Optimization of dynamic reactive power devices’ coordinate operation for minimizing HVDC’s successive commutation failure based on PSO

More and more dynamic reactive power devices (DRPDs) have been put into operation to improve voltage stability in AC/DC hybrid power system. In order to make full use of these devices, this paper presented an optimization technology for multiple DRPDs’ coordinate operation in receiving-end of AC/DC hybrid power system. DRPDs’ steady-state reactive instructions are treated as optimization variables in the proposed model, while the objective is minimizing the successive times of HVDC’s commutation failure under a number of grounding faults. Power flow constraints and electro-mechanical transient process are considered simultaneously. Based on the characteristics of the model, commutation failure of HVDC under each grounding fault could be obtained by employing time-domain simulation method. Furthermore, particle swarm optimization (PSO) is used to get DRPDs’ optimal reactive instruction in steady-state. The simulation is done by considering Jiangsu power grid in eastern china, including synchronous condenser, STATCOM and UPFC. The results show that the proposed model and algorithm are corrective and effective.

Principles of modeling the fluorescence spectral dynamics of dye molecules in solutions

FSDS is Fluorescence Spectral Dynamics Simulator, designed to investigate photoinduced charge transfer and its manifestations in time-resolved fluorescence spectra of a dye in a solvent. The project is implemented in C, using the MPI library. The computer model simulates stationary absorption and fluorescence spectra, as well as the time evolution of population distributions along the solvent reaction coordinate, to calculate spectral dynamics. Excitation and relaxation of intramolecular high-frequency vibrations are described at the quantum level. FSDS can fit the physical parameters of the dyes to experimental data. The dye excitation is consistently described accounting for the finite duration of the pump pulse. The program code implements two evolutionary models: a spin-boson model within the harmonic potential and a stochastic model based on the Smoluchowski diffusion operator for an arbitrary free energy surface. Computer simulation shows the numerical coincidence of the calculations performed in the framework of these models for the harmonic free energy surface. The detail description of the code and numerical schemes are presented. Program summaryProgram Title: Fluorescence Spectral Dynamics Simulator (FSDS)CPC Library link to program files:https://doi.org/10.17632/d39rrrkvhr.1Code Ocean capsule:https://codeocean.com/capsule/6544976Licensing provisions: GPLv3Programming language: CSupplementary material: Sample configuration files, User ManualNature of problem: The time-resolved spectra contain reach information about photochemical processes. However, the chemically important information about separate elementary processes is entangled due to they proceed in parallel. Time-resolved spectra modeling allows extracting information about these processes. The presented program code makes it possible to determine the relaxation characteristics of the solvent and the times of intramolecular relaxation.Solution method: Finite-difference time-domain and Brownian dynamics simulation methods.