Time-domain Simulations Research Articles

ADAPTATION OF THE FEEDBACK TRANSFER FUNCTION FOR OSICILATION BASED TESTING OF SECOND-ORDER ACTIVE RC FILTERS

This paper, explores possible solutions for the feedback transfer functions applied in active RC filters for oscillation-based testing (OBT). Three types of filter transfer functions, namely low-pass (LP), high-pass (HP) and band-pass (BP) are examined. Active filters are realized with the realistic operational amplifier model targeting the 180nm CMOS technology. To confirm the theoretical insights, three realizations of LP, HP, BP second-order filterers with proposed feedbacks are designed and simulated. One of the solutions, namely the BP filter, is further examined by inserting defects in the circuit. Based on the time domain simulations, the key parameters are extracted from the oscillating filter’s output signal. The fault dictionary has been created and appropriate classification of the defects is performed.

High-efficiency ultra-thin GaAs solar cells based on ITO/Ag/ITO transparent electrodes and photonic crystals

Ultrathin photovoltaic technology has great potential to improve efficiency and reduce costs. However, achieving optical thickness requires an effective light capture strategy. In this study, for GaAs thin film solar cells with an active layer thickness of 500 nm, we adopt a combination strategy of front-end ITO/Ag/ITO transparent electrodes and back-end photonic crystals (PC). By employing the finite difference time domain (FDTD) simulation, we verify that ITO/Ag/ITO transparent electrodes and AlGaAs PC can effectively enhance the light-harvesting capacity of cells. The proposed cell structure has a spectral absorption rate (SAR) of 0.9781 in the visible wavelength range. To further optimize the photovoltaic performance parameters, we optimize the doping concentration of the pn junction in the cell. Finally, the short circuit current density Jsc, open circuit voltage Voc, fill factor (FF) and photoelectric conversion efficiency (PCE) of GaAs thin film solar cells are successfully increased to 31.10mAcm−2, 1.28V, 88.18% and 35.12%, respectively. This provides crucial experimental validation and theoretical guidance for advancing ultra-thin photovoltaic technology.

Integrated numerical investigation of a 15 MW semi-submersible wind turbine at moderate water depth

Numerical simulations of dynamic responses of 15 MW offshore floating wind turbine are performed by an integrated FAST-OrcaFlex coupling model. An integrated model is established for an IEA 15 MW reference wind turbine (RWT) mounted over a semi-submersible floating platform, where the mooring line system and dynamic cable are designed at a moderate water depth h = 50m. The dynamic responses of 15 MW FOWT, mooring line system and dynamic cable are discussed, and the characteristics of load, displacement, mooring tension and bending curvature of dynamic cable at different IEC design load cases are investigated by the integrated time-domain simulations. The results indicate that the mean response of 15 MW FOWT is found to be mainly depended by the wind load, while the standard deviation of response is mainly influenced by the wave load. The mooring line system remains within the safe design range during the extreme environmental conditions. Moreover, the dynamic response of dynamic cable is influenced by the combined effects of the floating platform and ocean environmental factors. Large curvatures are observed in the buoyancy and boundary-layer sections, and high tensions are observed at the hang-off point, decline section and anchor point.

Nano-pixel polarization rotator for a photonic integrated breath sensor

In response to the global aging population, a photonic-integrated-circuit sensor is investigated for the detection of disease markers within human breath content. The device relies on cavity-ring-down spectroscopy with an amplifying medium and loop feedback to secure sufficient sensitivity down to ppm-order concentration detection. This configuration, however, might cause unwanted oscillation, and the polarization rotation method has been proposed to prevent this issue. We have researched a waveguide-based polarization rotator using nano-pixels. The device consists of two regions: (1) From TE00 mode TE10 modes conversion and (2) TE10 to TM00 modes conversion. As the intermediary TE10 mode quality is key to realizing polarization rotation performance, the purpose of this study is to realize high-quality TE10 by employing the mean-squared-error criterion for waveguide design optimization. A finite-difference time-domain simulation with this method reveals a TE10 mode with 1% accuracy that results in a polarization extinction ratio improved from 4.3 to 8.6 dB.

High-performance passive impedance network model for grid-tie inverters in steady state

Nowadays, the high penetration of inverter-interfaced distributed energy resources (DERs) has raised a well-founded concern regarding the interaction of these devices with the upstream grid. The harmonic interaction phenomenon between these players can be described by time-domain simulations, using high computational effort switched and/or average models often impractical for large power systems. This paper proposes an innovative low-computational-burden model named passive impedance network (PIN), capable of accurately representing DERs in electric power systems (EPSs). Through passive RLC components, the PIN model can describe DER dynamic behavior at a certain frequency of interest, making it able to represent harmonic flow within the EPS nodes. The proposed model is compared with the switched model, average model, and an experimental setup under different grid distortions and injected current profiles. A comparison of the total demand distortion (TDD) between the proposed PIN model and the experimental results is carried out. The effect of DER parameter deviations on PIN model accuracy is addressed, in which the absolute error of the PIN model TDD values are 0.65% and 0.06% for the experimental and switched model, respectively. Besides, the PIN model presents an improved computational performance compared to the time-domain-based switching model, requiring 99% less computational burden.

Hierarchical coordination scheme for voltage-aware fast frequency provision with flexible loads

Fast-frequency reserve (FFR) is required to counteract the limited system damping in renewable-rich power systems. Several distributed resources, such as batteries, PVs, and thermal loads, suit this role. However, since the devices are located in distribution grids, local constraints must be considered in the control design. In this work, a coordination scheme is developed to provide FFR without local frequency measurements while using limited communication to the central agent and a neighborhood support system to support local voltages. The proposed scheme is tested through time-domain simulations under diverse grid conditions. The coexistence with other local droop control is evaluated, demonstrating how different types of units with flexible active and reactive power production/consumption can coexist with this scheme. The results highlight that the coordination scheme is a promising alternative to local droop control and can coexist with other schemes.

Co-optimizing transmission and BESS expansions with system strength constraints

The increased penetration of renewables through converter interfaced generation (CIG) and the associated displacement of synchronous generators (SGs) can significantly reduce system strength. Although some network enhancements, such as the installation of battery energy storage systems (BESS), can partially counterbalance the reduction in strength, current network planning methodologies overlook system strength effects. In this context, this work proposes a new optimization model for co-optimizing investments in network expansions and battery systems to meet, at minimum cost, system strength requirements quantified through short circuit levels (SCL). Recognizing that SCL needs vary on a case-by-case basis, this model incorporates a logistic regression analysis trained on offline time-domain dynamic simulations; this approach effectively correlates SCL with system stability, providing a tailored strategy for each power system. To solve the optimization, we also propose an algorithm to iteratively find the optimal solution for the co-optimized portfolio of new transmission and energy storage investments. Through various case studies, we demonstrate the importance of considering system strength requirements in planning studies with significant penetration of CIG.

Enhancement of Quantum Efficiency in Perovskite Solar Cells Through Whispering Gallery Modes from Titanium Oxide Micro‐Resonators

With the aid of 3D full‐field finite difference time–domain simulations, model configurations for thin‐film solar cell devices that include periodically arranged microspheres, exhibiting resonating whispering gallery modes (WGMs), are proposed. The microspheres present, either immersed in perovskite or coated with perovskite layer, between the electron‐ and hole‐transport layers show enhanced current‐conversion efficiency. The presence of WGMs lead to enhancement in the absorption of layer. The incoming electromagnetic wave couples with microsphere and forms confined resonating modes. Different designs are examined for deciding the appropriate position of WGM exhibiting spheres with respect to thin‐film perovskite solar cell (PSC) featuring back reflector and optimized antireflectance coating. Since the incoupling element is lossless, energy stored in microspheres is absorbed efficiently by the underlying active material. This directly contributes to the increment in the current density of the solar cell. Thus, the devices show a higher current density of 23.62 mA cm−1, while that in planar solar cell device shows current density of 13.68 mA cm−1, for the same thickness of perovskite layer. This leads to more than 70% enhancement in the short‐circuit current density than the conventional PSCs device of similar size.

Analysis of faulted power system with fault current limiter using compensation current method

Fault current limiters (FCL) keep the fault current levels below equipment ratings in expanding power systems. Analyzing a faulted power system with FCL using symmetrical components becomes computationally intensive due to the coupling between FCL impedance and the need to recalculate the bus impedance matrix. This paper proposes a fault analysis method based on compensation currents to minimize the computation efforts. The paper presents the mathematical formulation for the compensation currents to model a fault and an FCL. This paper discusses the coupling between sequence networks for various FCL activation cases and identifies corresponding interconnections. The results of the proposed method for various fault types and FCL activation are validated with time-domain simulations. In addition, the computational efforts required by the proposed method are discussed with an application to a large power system. The results show that the proposed method requires minimum computations. The analysis of FCLs with non-linear impedances is beyond the scope of this paper.

Amino acid-mediated chiral Ag@Au with circularly polarized light induced photothermal and photodynamic synergistic photocatalytic disinfection

As the deterioration of the environment, microbiological contamination of drinking water is threatening human health. Therefore, it is crucial to investigate rapid, reliable, and safe sterilization techniques. In this study, chiral Ag@Au nanoparticles (L/D-NPs) were synthesized using an amino acid-mediated protocol. The L/D-NPs demonstrated outstanding photothermal and photodynamic capabilities when irradiated with linearly polarized (LP) light. The photothermal and photodynamic synergism demonstrated by L/D-NPs resulted in high sterilization capability against Staphylococcus aureus (S. aureus) and Escherichia coli (E. coli). It was noteworthy that these results were chirality-independent. With the interaction of chirality-matched circularly polarized light (CPL) and chiral NPs, excellent sterilization performance could be achieved even at a much lower concentration. The mechanisms were investigated using Finite-Difference Time-Domain (FDTD) simulations and photocurrent-time measurements. In addition, under identical conditions, chiral NPs were found to maintain excellent sterilization performance for the complex microbial system of actual river water. This work offers novel guidance and insights into photocatalytic disinfection, including sterilization of water and public facilities.

Mueller matrix-based calculation model for extracting polarization parameters in full-wave simulation of all-dielectric Pancharatnam–Berry (PB)-phase metasurfaces

In this study, the Mueller matrix of all-dielectric Pancharatnam–Berry (PB)-phase metasurfaces was retrieved using finite-difference time-domain (FDTD) simulation, Stokes–Mueller calculus, and Lu–Chipman decomposition. The polarization properties are discussed for nanofin-shaped PB-phase metasurfaces made of GaN and Nb2O5 and L-shaped PB-phase metasurfaces made of GaN. The results show that both the nanofin-shaped and L-shaped PB-phase GaN metasurfaces are optically equivalent to a linear-diattenuating retarder, and the nanofin-shaped PB-phase Nb2O5 metasurface is optically equivalent to a linear phase retarder. The PB-phase GaN metasurface exhibits stronger diattenuation and lower transmittance than the Nb2O5 metasurface due to the inherent defect absorption in the visible range. With a similar refractive index but barely any absorption, Nb2O5 is a more suitable material for PB-phase metasurfaces in the visible range than GaN.

Bowtie Nanoantenna LSPR Biosensor for Early Prediction of Preeclampsia.

The concentration of the placental circulating factor in early pregnancy is often extremely low, and the traditional prediction method cannot meet the clinical demand for early detection preeclampsia in high-risk gravida. It is of prime importance to seek an ultra-sensitive early prediction method. In this study, finite-different time-domain (FDTD) and Discrete Dipole Approximation (DDA) simulation, and electron beam lithography (EBL) methods were used to develop a bowtie nanoantenna (BNA) with the best field enhancement and maximum coupling efficiency. Bio-modification of the placental circulating factor (sFlt-1, PLGF) to the noble nanoparticles based on the amino coupling method were explored. A BNA LSPR biosensor which can specifically identify the placental circulating factor in preeclampsia was constructed. The BNA LSPR biosensor can detect serum placental circulating factors without toxic labeling. Serum sFlt-1 extinction signal (Δλmax) in the preeclampsia group was higher than that in the normal pregnancy group (14.37 ± 2.56 nm vs. 4.21 ± 1.36 nm), p = 0.008, while the serum PLGF extinction signal in the preeclampsia group was lower than that in the normal pregnancy group (5.36 ± 3.15 nm vs. 11.47 ± 4.92 nm), p = 0.013. The LSPR biosensor detection results were linearly consistent with the ELISA kit. LSPR biosensor based on BNA can identify the serum placental circulating factor of preeclampsia with high sensitivity, without toxic labeling and with simple operation, and it is expected to be an early detection method for preeclampsia.

All-optical multichannel switch and slow light based on dynamically tunable plasmon-induced transparency

The triple plasmon-induced transparency (PIT) effect based on a metal–insulator–metal waveguide structure comprising two groups of big and small disk resonators (BSDRs) is investigated theoretically and numerically. As a tool employed to explain the PIT, N-order coupled mode theory (CMT), is established, and the calculated results of the triple-PIT effect exhibit excellent consistency with finite-difference time-domain simulations. The influence of the separation between the small disk resonators on the triple-PIT response is discussed in detail through the dynamical equation. Further research shows that the central wavelengths of the triple-PIT transmission window can be adjusted with extremely low pump intensity and ultrafast optical response when monolayer graphene covers the surface of the BSDRs. Meaningfully, light traveling at resonant wavelengths can be effectively slowed down, with the highest group index reaching 368. Based on the PIT effect, a low-power and ultrafast switch is realized with a modulation amplitude of more than 93% at the corresponding wavelengths of the eight depressions. Thus, not only do the insights put forward new ideas, to the best of our knowledge, for highly tunable optoelectronic devices, but the results from the N-order CMT also offer new theory progress and references in the plasmonic waveguide structures.

Analytical and numerical optimization of broaching tool tooth distribution

This paper presents a detailed mathematical analysis of the effect of tooth distribution on the stability of broaching operations. Analytic and numeric techniques are employed to analyse a simplified one-degree-of-freedom mechanical model of variable pitch broaching, to assess its stability, and to find the optimal tooth distances maximizing robustness against harmful self-excited chatter vibrations. A novel modelling approach, considering a theoretical infinitely long broaching tool, draws parallels with variable pitch milling, and analytic formulas for tuning milling cutters are extended to broaching tools. For a further increase of robustness, and to take feasibility constraints into account, a goal function based on the semi-discretization and spectral collocation techniques is implemented in a direct numeric optimisation framework. A new, detailed derivation of the corresponding parameter gradients is presented to enable the use of gradient descent techniques. Since broaching is inherently a time-limited process, optimal parameters found in this manner are then validated via time domain simulations to show the desirable transient behaviours achievable by this ideal tuning of the tool geometry.

Utilizing Polarization and Multidimensional Spatial Angle‐Resolved Spectroscopy to Reveal the Optical Anisotropy of Few‐Layer Bi2O2Se

AbstractThe novel ternary 2D material Bi2O2Se exhibits numerous unique properties in the optical discipline due to its distinctive structure, rendering it of significant research importance and application potential in the domains of new‐generation microelectronics, optoelectronic sensors, and multidimensional optical recognition. However, the optical anisotropy and multidimensional angular resolution of Bi2O2Se have yet to be investigated under the condition of 3D spatial variation of incidence angle. Herein, the spatial optical sensing of the few‐layer Bi2O2Se is investigated under 3D spatially varying incidence conditions using a combination of polarized Raman spectroscopy and multidimensional angle‐resolved spectroscopy. By manipulating the incident angle conditions and adjusting the sample placement angle, a comprehensive optical characterization of materials from diverse spatial orientations is conducted. It is observed that few‐layer Bi2O2Se exhibits a systematically transformed multidimensional spatial photoresponse within visible and near‐infrared wavelengths. This demonstrates the capability of Bi2O2Se as photosensitive semiconductors for 3D stereo optical detection. Furthermore, finite difference time domain simulations reveal that the spatial optical anisotropy of few‐layer Bi2O2Se can be manifested under 3D variable‐angle incidence conditions. The work serves as a crucial guide for the future of Bi2O2Se in anisotropic integrated optics applications, based on polarization‐based chiral recognition.

Non-linear impedance boundary condition from linear piecewise B-H curve applied to induction heating systems

Purpose This paper aims to apply the non-linear impedance boundary condition (IBC) for a linear piecewise B–H curve in frequency domain simulations to find the equivalent impedance of a simple induction heating system model. Design/methodology/approach An electromagnetic description of the inductor system is performed to substitute the effects of the induction load, for a mathematical condition, the so-called IBC. This is suitable to be used in electromagnetic systems involving high conductive materials at medium frequencies, as it occurs in an induction heating system. Findings A reduction of the computational cost of electromagnetic simulation through the application of the IBC. The model based on linear piecewise B–H curve simplifies the electromagnetic description, and it can facilitate the identification of the induction load characteristics from experimental measurements. Practical implications This work is performed to assess the feasibility of using the non-linear boundary impedance condition of materials with linear piecewise B–H curve to simulate in the frequency domain with a reduced computational cost compared to time domain simulations. Originality/value In this paper, the use of the non-linear boundary impedance condition to describe materials with B–H curve by segments, which can approximate any dependence without hysteresis, has been studied. The results are compared with computationally more expensive time domain simulations.

A sensitivity-based estimation method for investigating control co-design relevance

Abstract. Control co-design is a promising approach for wind turbine design due to the importance of the controller in power production, stability, load alleviation, and the resulting coupled effects on the sizing of the turbine components. However, the high computational effort required to solve optimization problems with added control design variables is a major obstacle to quantifying the benefit of this approach. In this work, we propose a methodology to identify if a design problem can benefit from control co-design. The estimation method, based on post-optimum sensitivity analysis, quantifies how the optimal objective value varies with a change in control tuning. The performance of the method is evaluated on a tower design optimization problem, where fatigue load constraints are a major driver, and using a linear quadratic regulator targeting fatigue load alleviation. We use the gradient-based multi-disciplinary optimization framework Cp-max. Fatigue damage is evaluated with time-domain simulations corresponding to the certification standards. The estimation method applied to the optimal tower mass and optimal cost of energy show good agreement with the results of the control co-design optimization while using only a fraction of the computational effort. Our results additionally show that there may be little benefit to using control co-design in the presence of an active frequency constraint. However, for a soft–soft tower configuration where the resonance can be avoided with active control, using control co-design results in a taller tower with reduced mass.

Strong coupling in an asymmetric two-dimensional guided mode resonance metasurface structure and enhancement for optical third-harmonic generation

A two-dimensional guided mode resonance structure supports a transverse magnetic (TM) resonant mode in the direction of incident polarization and a transverse electric (TE) resonant mode in the direction perpendicular to the polarization. In this work, the coupling between the transverse magnetic and the transverse electric resonant modes in an asymmetric two-dimensional dielectric metasurface structure is investigated. The asymmetric structure consists of a two-dimensional square nanohole array etched in a titanium dioxide thin film on a transparent silica substrate. With finite difference time domain simulations, anti-crossing of the resonant spectra of the TM and TE modes is observed by adjusting the asymmetry of the structure. The anti-crossing indicates that the interaction between TM and TE resonant modes results in a strong coupling state. A coupled harmonic oscillator model is used to explain the strong coupling effect. The results of the coupled harmonic oscillator modeling agree well with the results of numerical simulations. Furthermore, it is shown that the strong coupling can significantly enhance the third harmonic generation intensity compared with the uncoupled TM and TE resonant modes.

Multifunctional terahertz device based on plasmon-induced transparency

Enhancing light-matter interaction is crucial in optics for boosting nanophotonic device performance, which can be achieved via plasmon-induced transparency (PIT). In this study, a polarization-insensitive PIT effect at terahertz frequencies is achieved using a novel metasurface composed of a cross-shaped graphene structure surrounded by four graphene strips. The high symmetry of this metasurface ensures its insensitivity to changes in the polarization angle of incident light. The PIT effect, stemming from the coupling of graphene bright modes, was explored through finite difference time domain (FDTD) simulations and coupled mode theory (CMT) analysis. By tuning the Fermi level in graphene, we effectively modulated the PIT transparent window, achieving high-performance optical switching with a modulation depth (88.9% < MD < 98.0%) and insertion losses (0.17 dB < IL < 0.51 dB) at a carrier mobility of 2 m2/(V·s). Furthermore, the impact of graphene carrier mobility on the slow-light effect was examined, revealing that increasing the carrier mobility from 0.5 m2/(V·s) to 3 m2/(V·s) boosts the group index from 126 to 781. These findings highlight the potential for developing versatile terahertz devices, such as optical switches and slow-light apparatus.

Flat optics of nonuniform phase gradient metasurfaces

Flat optics of uniform phase gradient metasurfaces based on the generalized Snell’s law has been extensively studied. The optics of nonuniform phase gradient metasurfaces (NPGMs) is less clear. Here based on Huygens’ principle and finite-difference time-domain (FDTD) simulation, we explore the optical properties of NPGMs made of nanorods (meta-atoms), which can be tuned by modulation of propagation phase and geometric phase. It is found that the nonuniformity of phase gradient may lead to multichannel anomalous reflection/refraction. The multiple beam splitters with arbitrary intensity ratio can be achieved by using the amplitude/phase modulation. Based on our design principle, we can obtain different anomalous reflection patterns (with channels ±1, ±2, both ±1 and ±2, or no reflection at anomalous angle) by different arrangement of just two types of meta-atoms. In addition, we are able to achieve chiral anomalous reflections for designed NPGMs made of nanorods and L-shaped nanoparticles. Our formulism provides general design guidance for NPGMs and can be used to realize the beam splitting function with adjustable angle and intensity ratio.