Time-domain Simulations Research Articles

Utilizing focused field as a probe for shape determination of subwavelength structures via coherent Fourier scatterometry

Nanopillars are widely used for various applications and require accurate shape characterization to enhance their performance and optimize fabrication processes. In this paper, we employ coherent Fourier scatterometry (CFS) combined with rigorous three-dimensional finite-difference time-domain simulations to accurately determine the shapes of nanopillars with various geometries, including cylindrical, triangular, square, and rectangular shapes. The nanopillars considered here have lateral dimensions (a) ranging from 100 to 1000 nm. Our methodology utilizes the preferential excitation of the nanostructures by a tightly focused beam and leverages their inherent symmetry to capture far-field signatures that vary periodically with rotation. This approach allows us to distinguish between different nanopillar shapes based on these rotational signatures. Our results demonstrate that the CFS method can reliably characterize nanopillars with lateral dimensions a≥300 nm, surpassing the conventional diffraction limit of 351 nm. However, the method reaches its fundamental limits for a≤200 nm, as also confirmed by simulations, where we approach the dipole approximation regime (a≪λ). This constraint is not observed for rectangular nanopillars, owing to their constant breadth (b=1000 nm), which prevents such a regime. Furthermore, our method successfully differentiates nanopillars transitioning from rectangular to square shapes. We also explored the method’s limitations concerning nanostructure height (h), finding that triangular and square nanopillars could be characterized accurately for h≥50 nm and h≥150 nm, respectively. Furthermore, the method remains robust against shape distortions such as edge roundness. The method is primarily effective in determining the lateral (top-down) shape of nanopillars, it does not resolve longitudinal features. The ability to accurately characterize nanostructure shapes has significant implications in fields such as photonics and biosensing, where geometry critically influences device performance. Published by the American Physical Society 2025

Vulnerability analysis on random matrix theory for power grid with flexible impact loads

The stochastic volatility of the rail transit load brings greater uncertainty to the vulnerability of the power grid. To solve the problem of the inaccurate results caused by the incomplete time-domain simulation model of the power system with rail transit load integration, this paper proposes a vulnerability analysis method for the power system with rail transit load integration based on the random matrix theory. In this paper, we first constructed a rail transit load model based on Deep Convolutional Generative Adversarial Networks (DCGAN) to simulate the situation that massive rail transit load merged into the Grid Scenario. Then, we generate a high-dimensional random matrix based on the power flow of the grid-connected system under different rail transit loads. Then, we construct a vulnerability analysis model combining the random matrix theory and the real-time separation window. Finally, we take the IEEE-39 bus system and a regional power grid in China as examples to evaluate the vulnerability of the grid-connected system. The results show that our method quantifies not only the impact of the rail transit load volatility on the system vulnerability, but the system endurance under different capacities of the rail transit load connected to grid. Moreover, it also provides a new way for system planning and safety monitoring in the power system with rail transit load integration.

Research on the Weakening Process at the Interface of Bonded-Layer Composite Structures Using Ultrasonic Longitudinal Waves

The interface weakening process of bonded-layer composite structures is calculated, simulated, and experimentally investigated using ultrasonic longitudinal waves. Firstly, the reflection coefficients of echoes are calculated theoretically. Subsequently, a time-domain simulation model of bonded-layer composite structures is established. The propagation law of ultrasonic waves in bonded-layer composite structures is obtained. The relationship between different bonding interface states and the ultrasonic reflection characteristics are investigated through ultrasonic experiments on bonded composite structures. The theoretical calculation, simulation, and experimental results are as follows: when the bonding strength of the bonding layer changes from weak to strong, the amplitude of the first echo gradually decreases, the amplitude of the second echo progressively increases, and the amplitude of the third echo is basically unchanged; when the bonding strength of the upper interface changes from weak to strong, the amplitudes of the first and the second echoes are same as in the previous variation whereas the amplitude of the third echo slightly increases; when the bonding strength of the lower interface changes from weak to strong, the amplitudes of the first and the third echoes remain essentially unchanged, but the amplitude of the second echo progressively increases in the experiment compared with the theoretical calculation and simulation. In addition, the time of the first echo remains broadly unchanged, and the times of the second and the third echoes gradually decrease under all conditions.

Optimization Control of Sub-Synchronous Oscillations in Doubly Fed Generators with Wind Turbines Using the Genetic Algorithm

The sub-synchronous oscillation accident of large-scale doubly fed wind turbines connected to a grid through series compensation has caused a serious impact on the power system. By optimizing the parameters of the doubly fed wind turbines control system, the system impedance can be effectively improved to solve the problem of sub-synchronous oscillation. However, owing to the complexity of a grid-connected system of doubly fed generators connected to wind turbines and the influence of the time-varying oscillation characteristics of the system, it is often difficult to achieve a successful suppression. To solve this problem, this paper proposes an optimized additional damping method for the rotor- and grid-side controllers, which can achieve efficient suppression of the sub-synchronous oscillation. The parameters of the proposed additional damping method are optimized for all variable operation conditions using a genetic algorithm under the established frequency–domain impedance model. The detailed time–domain simulation model was constructed with the RTLAB platform to verify the proposed method. The experimental results show that the optimized control strategy can effectively and quickly suppress the sub-synchronous oscillation under different operating conditions, and the amplitude suppression rate reached 85.99%, which effectively improved the grid-connected stability of the wind turbines.

Enhancement of Light Extraction Efficiency Using Wavy-Patterned PDMS Substrates

This study introduces an organic light-emitting diode (OLED) light extraction method using a wavy-patterned polydimethylsiloxane (PDMS) substrate created via oxygen (O2) plasma treatment. A rapid fabrication process adjusted the flow, pressure, duration, and power of the O2 plasma treatment to replicate the desired wavy structure. This method allowed the treated samples to maintain over 90% total transmittance and enabled controlled haze adjustments from 10% to 70%. Finite-difference time-domain (FDTD) simulations were employed to determine optimal amplitudes and periods for the wavy structure to maximize optical performance. Further experiments demonstrated that bottom-emitting green fluorescent OLEDs constructed on these substrates achieved an external quantum efficiency (EQE) of 3.5%, representing a 97% improvement compared to planar PDMS OLEDs. Additionally, color purity variation was minimized to 0.044, and the peak wavelength shift was limited to 10 nm, ensuring consistent color purity and intensity even at wide viewing angles. This study demonstrates the potential of this cost-effective and efficient method in advancing high-quality display.

Dual-Task Optimization Method for Inverse Design of RGB Micro-LED Light Collimator

Abstract: Miniaturized pixel sizes in near-eye digital displays lead to pixel emission patterns with large divergence angles, necessitating efficient beam collimation solutions to improve the light coupling efficiency. Traditional beam collimation optics, such as lenses and cavities, are wavelength-sensitive and cannot simultaneously collimate red (R), green (G), and blue (B) light. In this work, we employed inverse design optimization and finite-difference time-domain (FDTD) simulation techniques to design a collimator comprised of nano-sized photonic structures. To alleviate the challenges of the spatial incoherence nature of micro-LED emission light, we developed a strategy called dual-task optimization. Specifically, the method models light collimation as a dual task of color routing. By optimizing a color router, which routes incident light within a small angular range to different locations based on its spectrum, we simultaneously obtained a beam collimator, which can restrict the output of the light emitted from the routing destination with a small divergence angle. We further evaluated the collimation performance for spatially incoherent RGB micro-LED light in an FDTD using a multiple-dipole simulation method, and the simulation results demonstrate that our designed collimator can increase the light coupling efficiency from approximately 30% to 60% within a divergence angle of ±20° for all R/G/B light under the spatially incoherent emission.

All-polarization asymmetric silicon nitride photonic crystal slab waveguides by combining a single-polarization bandgap and birefringence

Abstract Photonic crystal (PhC) waveguides on the silicon nitride (SixNy) platform currently are capable only of transmission of a single light polarization, severely limiting polarization diversity applications. In this work, we propose a scheme that combines a single-polarization photonic band gap (SPBG) formed by the first two bands with the same polarization, along with large birefringence between the first two bands with different polarizations. In simple terms, one polarization in the PhC slab waveguide is guided by the SPBG, while the other polarization is index guided, and overlap between the frequency bands for the two polarizations in which propagation is forbidden, dubbed the no-polarization-mode region (NMR), tuned by the aspect ratio of PhC slabs. This approach theoretically achieves PhC waveguides on asymmetric SixNy slabs that guide both polarizations within the NMR. Three-dimensional (3D) planewave-method calculations show that through simple adjustment of the aspect ratio of the PhC slab, the first two bands with different polarizations can be adjusted to be shifted in frequency to form the NMR, where neither TE- nor TM-like modes can propagate along the in-plane directions. The NMR is then systematically investigated and optimized for various refractive indices and slab thicknesses, and using the optimized-NMR design, line-defect waveguides supporting propagation of both polarizations are theoretically demonstrated. A 3D finite-difference time-domain simulation shows that an all-polarization bandwidth as large as 180 nm can be obtained in an optimized PhC waveguide with SixNy refractive index of 2.4, which can be tuned by x and y. In addition, our investigation shows that an NMR can also be obtained in an asymmetric rod-type PhC slab even when the refractive index is as low as 1.8, offering significant flexibility in engineering of the structure and refractive index of all-polarization devices on SixNy and, by extension, in other moderate-index platforms.

Designing Fluorescent Interfaces at Hotspots in a Plasmonic Nanopore for Homologous Optoelectronic Sensing.

In this work, a site-selective functionalization strategy is proposed for modifying fluorescent dyes in the plasmonic nanopore, which highlights building optoelectronic dual-signal sensing interfaces at "hotspots" locations to construct multiparameter detection nanosensor. Finite-difference time-domain (FDTD) simulations confirmed the high-intensity electromagnetic field due to plasmonic nanostructure. It is demonstrated that adjusting the distance between the nanopore inner wall and fluorophore prevented the fluorescence quenching, resulting in more than a thirty fold fluorescence enhancement. Upon binding with the target analyte, the sensor produces homologous yet independent optoelectronic dual-signal responses that cross-validate one another, providing highly accurate analysis even in the presence of multiple interferences. The platform demonstrates precise, adaptable detection with linear responses to extracellular pH changes at the single-cell level, making it a versatile tool for a range of biosensing applications. By enabling the functionalization of fluorescent interfaces in the "hotspots" of metal nanopores, this interface design strategy efficiently exploits the enhancement of electromagnetic fields to achieve high-precision dual-signal measurements and greatly improves the sensitivity of biosensing applications.

Time-domain analysis of mode competition in ZnO nanowire lasers in inhomogeneous environments

Zinc oxide (ZnO) nanowire lasers are increasingly integrated into complex optoelectronic devices as a source of coherent radiation. To enable the rational design of these devices, it is crucial to understand how both the nanowire resonator and its surrounding environment influence mode competition and the three-dimensional structure of lasing modes. Additionally, realistic models of the lasing process must account for transient gain dynamics. In order to investigate the impact of an inhomogeneous environment, composed of various materials and structures, on mode competition, we conducted Finite-Difference Time-Domain (FDTD) simulations of the dominant lasing modes in different ZnO nanowire laser configurations. Our model describes how key parameters such as nanowire diameter, length, and substrate choice affect the field distribution in the lasing regime. We show that metallic substrates support lasing in thin nanowires in two distinct coupling regimes. Furthermore, we show that metallic particles attached to the nanowire end facets as a result of established nanowire growth techniques significantly influence lasing threshold, field distribution and competition between counter-propagating modes. We show that attaching an aluminum particle at the end facet of a ZnO nanowire leads to a threshold reduction, a switching of the dominant lasing mode and a mono-directional power flow inside a large segment of the nanowire.

Enhanced broadband quantum efficiency in LWIR T2SL detectors with guided-mode resonance structure.

Type-II superlattice (T2SL) detectors are emerging as key technologies for next-generation long-wavelength infrared (LWIR) applications, particularly in the 8-14 µm range, offering advantages in space exploration, medical imaging, and defense. A major challenge in improving quantum efficiency (QE) lies in achieving sufficient light absorption without increasing the active layer (AL) thickness, which can elevate dark current and complicate manufacturing. Traditional methods, such as thickening the absorber, are limited by the short carrier lifetime in T2SLs, necessitating alternative solutions. In this study, we introduced a guided-mode resonance (GMR) structure into T2SL LWIR detectors to enhance QE while maintaining a thin AL for efficient carrier collection. The GMR structure was fabricated by introducing a grating array on the surface of the detector and an Au mirror beneath the absorber. This configuration enhanced light trapping, which introduced additional resonance modes. The optimized grating design, with a 5 µm period and a fill factor of 0.6, significantly increased absorption, as predicted by finite-difference time-domain (FDTD) simulations and confirmed experimentally. The GMR-enhanced T2SL detector demonstrated a 2.58-fold improvement in QE over conventional LWIR detectors and a 1.33-fold increase compared to Fabry-Pérot (FP) resonance-based detectors in the 6-11 µm range. Despite exhibiting an almost identical dark current density, the GMR LWIR detector demonstrated superior performance, featuring a broader cut-off wavelength of 9.3 µm and higher QE compared to FP LWIR detectors.

Decoupling Carrier Dynamics and Energy Transport in Ultrafast Near-Field Nanoscopy.

Ultrafast near-field optical nanoscopy has emerged as a powerful platform to characterize low-dimensional materials. While analytical and numerical models have been established to account for photoexcited carrier dynamics, quantitative evaluation of the associated pulsed laser heating remains elusive. Here, we decouple the photocarrier density and temperature increase in near-field nanoscopy by integrating the two-temperature model (TTM) with finite-difference time-domain (FDTD) simulations. These results reveal that the electron-phonon coupling in a silicon film after femtosecond laser excitation is most pronounced within approximately 3 ps─substantially shorter than the photocarrier decay time scale at tens of picoseconds. Moreover, the coupled TTM-FDTD method indicates that ultrafast laser heating can cause up to a 14% variation in the near-field signal at a 220 μJ/cm2 pump pulse fluence. Our numerical results are further validated by transient experiments, highlighting the potential of this method for investigations of carrier and thermal phenomena in emerging nanomaterials and nanodevices.

Improving the Efficiency of Bulk-heterojunction Solar Cells through Plasmonic Enhancement within a Silver Nanoparticle-Loaded Optical Spacer Layer.

We investigate the enhancement in the efficiency of organic bulk heterojunction solar cells enabled by plasmonic excitation of Ag nanoparticles (NPs) of different diameters (10, 20, and 30 nm), randomly incorporated within an optical spacing layer of TiO2 placed between the organic medium and the Ag cathode. Such structures significantly increase the optical absorption and the photocurrent within the device system, leading to a power conversion efficiency of more than 4%, over 2.5 times that of the control bulk heterojunction cell. This corresponds to a 61% increase in J SC and a 6.3% in fill factor. 3D Finite-difference time-domain simulations were utilized to investigate the plasmonic field coupling within the nanogap medium of TiO2. They show that coupling between the Ag nanoparticle and the Ag thin film cathode extends the wavelength range of the local field enhancement beyond that obtained for isolated NPs, providing a better overlap with the absorption spectrum of the organic medium.

Coupling mechanisms of plasmon resonance and Bi3+ emission in YAG: Bi, Ce, Yb epitaxial films at low temperatures

This paper is devoted to the investigation of the plasmonic effect of metal nanoparticles (NPs) formed on the surface of the YAG: Bi, Ce, Yb phosphors in a temperature range between 4 and 300 K. Combination of a thin conversion layer with silver plasmonic nanostructures leads to increase of sensitizer absorption and emission efficiency. Enhancement of Bi3+ luminescence in YAG epitaxial films with Ag NPs was observed upon cooling the samples below 200 K. High enhancement factors were associated with closely matching the maximum of plasmon extinction and Bi3+ emission bands. The maximum value of enhancement factor near 170% at 4 K was obtained. It is shown that temperature decrease causes an increase in the EM field intensity around the NPs, the probability of spontaneous recombination, the penetration depth of the localized surface plasmon resonances (LSPR) into the substrate, and the adjustment of the position of the LSPR. Simultaneous action of all these factors leads to Bi3+ emission intensity enhancement. Comparative analysis of the Finite-Difference Time-Domain (FDTD) simulation data vs. experimental results of the temperature behavior of plasmon absorption spectra, luminescence spectra of Bi3+ ions, and their decay kinetics confirms the correctness of the proposed mechanisms.

A molecularly imprinted ratio fluorescence sensor based on metal-enhanced fluorescence of core-shell structure CaF2-silver nanoparticle for visual detection of dicamba.

Although fluorescence analysis methods are widely used in pesticide residue detection, improving their sensitivity and selectivity remains a challenge. This paper presents a novel ratio fluorescence sensor based on the molecular imprinting polymers (MIPs) and metal-enhanced fluorescence for visual detection of dicamba (DIC). Calcium fluoride (CaF2) quantum dots (QDs) were immobilized on the surface of Ag@MIPs, resulting in a blue fluorescence response signal (Ag@MIPs-CaF2). The MIPs shell, which possesses a specific recognition capability, serves as an isolation layer to adjust the distance between Ag nanoparticles and CaF2 QDs for enhancing the fluorescence of CaF2 QDs by up to 7.1 times under optimal conditions. In the presence of DIC, the blue fluorescence was selectively quenched, while the reference signal red fluorescence from cadmium telluride QDs coated with silicon dioxide (CdTe@SiO2) remained relatively stable, resulting in a color change from blue to red. The sensor had a detection limit of 0.16μM for DIC in the range of 1.0 to 50.0μM, recovery rates of 85.4 to 103.5% for actual samples, and an imprinting factor of 3.28. The 3D finite-difference time-domain simulation revealed that the fluorescence enhancement was due to the local electric field amplification. Therefore, the developed sensing system in this work offers a broad application prospect for visual detection of DIC in food samples.

Pulse duration dependent effects of ultrafast laser induced damage on a 1030 nm multi-layer dielectric mirror for high repetition rate, high average power laser systems

High repetition rate, high peak, and average power laser systems are crucial for next-generation particle accelerators, inertial confinement fusion, and secondary particle sources. These applications demand durable laser optics, particularly interference coatings on optics lasting millions of shots at high fluence. This study focuses on designing, testing, and simulating multi-layer dielectric (MLD) mirrors for pulse durations of 260 fs, 77 fs, and 25 fs at 1030 nm wavelength and 45-degree incidence angle with p-polarization. S-on-1 laser-induced damage thresholds (LIDT) for varying pulse numbers were determined, with single-shot LIDT values of 0.98 Jcm-2, 1.63 Jcm-2, and 2.3 Jcm-2 for 25 fs, 77 fs, and 260 fs respectively. A strong correlation between blister shape and local fluence was observed, implying that the layer expansion in a blister depends on local fluence. We have also examined mechanisms responsible for laser-induced stress generation and energy release rates in blister formation. Damage mechanisms are further explored by finite-difference time-domain (FDTD) simulations, incorporating Keldysh strong field ionization, whose predictions were in excellent agreement with the onset of damage determined experimentally. These findings offer insights for enhancing MLD coating technology, promising more efficient and resilient laser systems for diverse scientific and industrial applications.

Controlling Raman enhancement in particle-aperture hybrid nanostructures by interlayer spacing.

Here we show how surface-enhanced Raman spectroscopy (SERS) features can be fine-tuned in optically active substrates made of layered materials. To demonstrate this, we used DNA-assisted lithography (DALI) to create substrates with silver bowtie nanoparticle-aperture pairs and then coated the samples with rhodamine 6G (R6G) molecules. By varying the spacing between the aperture and particle layer, we were able to control the strength of the interlayer coupling between the plasmon resonances of the apertures and those of the underlying bowtie particles. The changes in the resulting field enhancements were confirmed by recording the Raman spectra of R6G from the substrates, and the experimental findings were supported with finite difference time domain (FDTD) simulations including reflection/extinction and near-field profiles.

Fabrication of Ag based Surface Enhanced Raman Scattering substrates with periodic mask arrays by electron beam deposition.

Surface-enhanced Raman scattering (SERS) has attracted much attention as a powerful detection and analysis tool with high sensitivity and fast detection speed. The intensity of the SERS signal mainly depended on the highly enhanced electromagnetic field of nanostructure near the substrate. However, the fabrication of high-quality SERS nanostructured substrates is usually complicated, makes many methods unsuitable for large-scale production of SERS substrates. A new strategic preparation method is needed, which is simple and inexpensive in the preparation process, can achieve large-scale mass production, and also ensures the good performance of the SERS substrate. For the first time, we propose constructing a silver (Ag) surface-enhanced Raman scattering (SERS) substrate with different angular quadrilateral periodic arrays by combining a mask plate and vapor deposition. The mask plate can precisely control the period and angle of the nano-arrays, and thus regulate the intensity of the local "hot spots". The analytical results show that the tetragonal periodic Ag SERS substrate with a 15° angle of the mask plate exhibits a significant enhancement effect in the detection of Rhodamine 6G (R6G), Rhodamine B (RhB), and Methyl Orange (MO) probe molecules, and the limit of detection (LOD) of this substrate is low as 10-11mol/L for all the three solutions. The enhancement factors (EFs) are 6.5×106, 4.2×105, and 2.6×105, respectively. The accuracy of the experimental results was further verified through a finite-difference time-domain (FDTD) simulation. In this paper, we propose that the SERS substrate was fabricated using the new strategy with a lower detection limit for multiple dye molecules simultaneously. The results show that the SERS substrate prepared in this study has great potential for applications in high-performance SERS sensors. Additionally, this new preparation method may also be applicable to other metal materials, offering broad research prospects.

Tuning of plasmonic surface lattice resonances: on the crucial impact of the excitation efficiency of grazing diffraction orders.

Metallic nanoparticles exhibit remarkable optical properties through localized surface plasmon (LSP) resonances. When arranged in arrays, nanoparticles form surface lattice resonances (SLRs) affected by inter-particle distance. SLRs can offer narrower bandwidths and stronger electric field enhancements than LSP modes, crucial for efficient optical device development. Among important aspects, grazing diffracted orders crucially impact SLR properties, facilitating long-range nanoparticle interactions. This study explores how these photonic modes propagating at the substrate surface influence SLRs, by using experimental and theoretical approaches, including Finite Difference Time Domain simulations on gold disk arrays. Results show SLR properties strongly rely on diffracted mode efficiency controlled by the grating constant along the incident polarization. Notably, large inter-particle spacing along the incident polarization can strongly reduce the SLR red-shift. Understanding and managing long-range interactions in engineered plasmonic structures are highlighted, providing insights for enhanced performance in advanced photonic and optoelectronic devices.

A Non-Sampling Time-Domain Simulation Framework for Transient Stability Assessment Under Stochastic Continuous Disturbances

A Non-Sampling Time-Domain Simulation Framework for Transient Stability Assessment Under Stochastic Continuous Disturbances

Innovative MIM diplexer with neural network enhanced refractive index detection for advanced photonic applications

This study introduces a high-performance 4-channel Metal-Insulator-Metal (MIM) diplexer, employing silver and Teflon, optimized for advanced photonic applications. The proposed diplexer, configured with two novel band-pass filters (BPFs), operates across four distinct wavelength bands (843 nm, 1090 nm, 1452 nm, 1675 nm) by precisely manipulating the passband dimensions. Utilizing Finite-Difference Time-Domain (FDTD) simulations, the designed diplexer achieves exceptional sensitivity values of 3500 nm/RIU, 4250 nm/RIU, 3375 nm/RIU, and 4003 nm/RIU, along with high figures of merit (FOM) ranging from 113.4 to 124.7 1/RIU. Also, the compact design (400 nm × 830 nm) underscores its suitability for integrated photonic circuits and advanced sensing applications. Furthermore, to further enhance accuracy in detecting refractive index (RI) changes, a multilayer perceptron (MLP) neural network was employed, ensuring the highest sensor accuracy. The accuracy of the MIM diplexer’s RI measurements was statistically validated through a one-sample t-test, confirming the sensor’s reliability. Comparative analysis with existing sensors highlights the diplexer’s superior sensitivity and efficiency, setting a new benchmark in optical communication and photonic sensing technologies. This work paves the way for future advancements in miniaturized, high-sensitivity optical devices, offering robust solutions for next-generation communication and sensing systems.