Finite-difference Time-domain Research Articles

Fano mode based plasmonic sensor for temperature and chemical pollutant detection

Plasmonic sensors provide great sensitivity to minute quantities of analytes and provide excellent detection. In present context of environmental monitoring, plasmonic sensor can prove to be an excellent choice in chemical pollutant and temperature detection. Plasmonic sensors can provide immediate results, allowing for monitoring temperature changes in ecosystems or climate studies and quick decision-making in emergency situations related chemical pollution incidents. They are compact and can be integrated into portable devices for on-site analysis. In this investigation, a plasmonic refractive index sensor based on key ring shaped resonator consisting of a microring resonator and two rectangular resonator is proposed. Finite-Difference Time-Domain (FDTD) method is used to study the transmittance characteristics of the sensor. The device exhibits quadruple Fano resonance with highest sensitivity of 2521.7 nm/RIU. Other performance parameters such as figure of Merit (FOM), Quality (Q) factor and Detection limit (DL) are also been calculated, with values 98.8 RIU−1, 99.6 and 0.01 respectively. Additionally, the effects of different geometrical configurations is also studied, providing insights into the design principles in context of potential fabrication complexities. Further, the simulated Fano characteristic is validated against the theoretical value. The application of the proposed sensor is investigated for different types of analyte such as chemical pollutants and temperature sensing.

Advancing early detection of biological events by digital holographic microscopy and simulation of microorganisms

There is a global need to advance bio-aerosol sensing for CBRN (Chemical, Biological, Radiological, and Nuclear) applications by compact and cost-effective devices. Employing digital holographic microscopy (DHM) and deep learning, we developed a system called HoloZcan to automate the analysis of airborne microbial pathogens and particles. DHM provides valuable information, but obtaining data from biological specimens for robust investigations is challenging. This paper introduces a custom simulation approach using the open-source software Meep and the finite-difference time-domain (FDTD) method to overcome limitations of existing Mie-based simulators, especially when dealing with complex microbial shapes. The simulation tool enables the modelling of specific microorganisms, offering a safer and more flexible alternative for CBRN research by bypassing ethical and logistical constraints associated with live pathogens. The study details the simulation workflow, built upon the construction of a database of optical properties of biological materials, for realistic simulations of light-microbe interactions. Evaluations on homogeneous and non-homogeneous objects demonstrate the tool’s limited intrinsic errors and superior sensitivity to refractive index changes compared to traditional Mie-based simulations. This work significantly advances our capability to accurately simulate and analyse CBRN-related scenarios, enhancing comprehensive research in bio-aerosol sensing.Graphical abstract

Design of a switchable triple-waveguide-based TE-converted polarization beam splitter based on lithium niobate

The lithium-niobate-on-insulator (LNOI) platform has been the new force to drive the developments of integrated photonics, where efficient polarization management and mode conversion are the fundamental issues to be solved. In this work, we proposed a switchable TE-converted polarization beam splitter (PBS) based on optical phase change material (PCM) and a triple-waveguide in simulation. To optimize and analyze the proposed design, the 3D finite difference time domain (3D-FDTD) method is applied in this work. The simulated results indicate that when PCM is in the amorphous state, the device can effectively separate the TM and TE modes, achieving a low insertion loss (<0.5dB) for both modes at 1550 nm. While the PCM is in the crystalline state, the proposed device can realize the efficient conversion from the input TE0 mode to output TE1 mode, which obtains a mode conversion efficiency of 99.5%, crosstalk of −26dB, and insertion loss of 0.5 dB at 1550 nm. We hope such a device can make compact integration and capacity improvement for the integrated photonics in LNOI.

Prediction of optical properties of uniaxial hyperbolic nanospheres via artificial neural network

In this study, absorption and scattering multilayer perceptron models are developed and validated to predict the optical spectra of uniaxial hyperbolic nanospheres. The finite difference time domain method is used to generate the dataset of the absorption and scattering optical spectra. The models’ optimal performance is achieved for 5 hidden layers and 80 neurons for the absorption and the same hidden layers with 120 neurons for the scattering. The predictions by the model on the test dataset give a low average mean squared error of 0.000145 for the absorption and 0.00071 for the scattering. We also performed a robustness test by using parameters outside the initial parameters used for the training and the predictions are in good agreement with the actual datasets for both absorption and scattering. This research shows the application of artificial neural networks to predict the optical properties of hyperbolic materials and lays the groundwork for developing more complicated neural network models to predict complex phenomena in hyperbolic metamaterials, which are faster and computationally less expensive than using conventional simulation methods. Hyperbolic metamaterials offer unique optical properties that can be used to design new optical devices.

Nucleus-Spike 3D Hierarchical Superstructures via a Lecithin-Mediated Biomineralization Approach.

3D hierarchical superstructures (3DHSs) are key products of nature's evolution and have raised wide interest. However, the preparation of 3DHSs composed of building blocks with different structures is rarely reported, and regulating their structural parameters is challenging. Herein, a simple lecithin-mediated biomineralization approach is reported for the first time to prepare gold 3DHSs composed of 0D nucleus and 1D protruding dendritic spikes. It is demonstrated that a hydrophobic complex by coordination of disulfiram (DSF) with a share of chloroauric acid is the key to forming the 3DHSs. Under the lecithin mediation, chloroauric acid is first reduced to form the 0D nucleus, followed by the spike growth through the reduction of the hydrophobic complex. The prepared 3DHSs possess well-defined morphology with a spike length of ≈95nm. Notably, the hierarchical spike density is systematically manipulated from 38.9% to 74.3% by controlling DSF concentrations. Moreover, the spike diameter is regulated from 9.2 to 12.9nm by selecting different lecithin concentrations to tune the biomineralization process. Finite-difference time-domain (FDTD) simulations reveal that the spikes form "hot spots". The dense spike structure endows the 3DHSs with sound performance in surface-enhanced Raman scattering (SERS) applications.

High-Q fano resonance metasurface sensor for refractive index and methane gas concentration measurement

High-quality-factor Fano resonances excited by metasurfaces have been playing an important role in the field of optical sensing. A high-performance sensor for refractive index and methane concentration measurement is designed and prepared in this paper. The symmetry of the structure is broken by altering the size of the rectangular holes in the tetramer, resulting fano resonance Q-factor of 34176 and modulation depth of 100%. The finite-difference time-domain (FDTD) method is used to simulate the structure. The results show that the refractive index and methane concentration sensitivity are 325 nm/RIU and −1.44 nm/%, respectively. The FOM can achieve 10156 RIU−1. Furthermore, experiments are carried out on the gas application of the structure and the sensitivity of methane concentration is −1.03 nm/%. Therefore, the proposed metasurface has more practical and feasible significance in background refractive index and gas environment monitoring.

High-performance SERS substrate based on gold nanoparticles-decorated micro/nano-hybrid hierarchical structure

Owing to its excellent localized surface plasmon resonance (LSPR) effect, noble metal nanoparticles (NPs) find extensive application in the preparation of surface-enhanced Raman scattering (SERS) substrates. However, due to process limitations, the practicality and testing effectiveness of SERS substrates still leaves much to be desired. Here, a wafer-scale gold (Au) NPs silicon (Si) micro/nano-hybrid structured substrate is prepared. This is achieved through two-step etching, followed by decorating Au-NPs onto the structure via self-assembly process induced by de-wetting. Finite-difference time-domain (FDTD) simulations reveal that the significant enhancement of local electric field in the voids between Au-NPs and Si is crucial for enhancing SERS. Using Rhodamine 6G (R6G) as the probe molecule, performance of the fabricated SERS substrate is investigated. It demonstrates a minimum detection limit of 10−11 M, with a calculated enhancement factor of 4.40 × 108, indicating its high sensitivity. The minimum relative standard deviation for the substrate is 5.254 %. After 20 days of placement, the SERS performance show tiny variation. Even after four cycles of cleaning experiments, it still maintains outstanding SERS performance. This demonstrates its excellent stability, uniformity and reusability. Our research provides guidance for the efficient and low-cost fabrication of high-performance SERS substrates.

Silver nanoparticles modified on cicada wings affect fluorescence and Raman signals based on electromagnetic and chemical enhancement mechanisms

The electromagnetic enhancement (EM) mechanism is one of the important theories that need to be mastered to study surface-enhanced fluorescence (SEF) and surface-enhanced Raman scattering (SERS). However, the charge transfer in the chemical enhancement (CM) mechanism has different effects on the two. The study developed a simple and environmentally friendly composite structural substrate by depositing silver (Ag) on the rough surface with regular columnar protuberances of bio-material cicada wings (CW) using magnetron sputtering technology. The formation of Ag nano-islands enhanced the fluorescence and Raman signals of rhodamine 6G (R6G), while the addition of silver nanoparticles (AgNPs) quenched the fluorescence. To explore the magical phenomenon, the electromagnetic field distribution on the surface of the substrate was simulated through three-dimensional finite difference time domain (3D-FDTD), which verified the experimental results of SEF and SERS. The results calculated by Gaussian16 with Density Functional Theory (DFT) and time-dependent density functional theory (TDDFT) explained the effect of adding AgNPs on the surface of the substrate on the fluorescence intensity. The work not only contributes experimental data and theoretical results to the EM mechanism and CM mechanism in enhanced spectra, but also provides ideas for preparing new SEF and SERS substrates.

Compensation of staircase-induced boundary absorption errors in FDTD simulation

Finite-difference time-domain (FDTD) methods are popular for wave-based simulations because their regular grid enables highly efficient implementation, especially on parallel processing architectures. On the flip side, these methods exhibit errors caused by the staircase approximations that result when arbitrary boundary geometries are approximated on the fixed grid. Of particular concern in room acoustics is the staircasing of absorptive boundaries, as this has been shown to cause errors as large as 40% in computed reverberation times that do not reduce when the grid is refined. This paper analyses the effectiveness of a previously proposed compensation method that consists of modifying the admittance boundary conditions based on the staircased surface area. A theoretical analysis shows that for special boundary orientations the numerical reflection coefficient converges to the exact result at low frequencies. Numerical tests complement this analysis to show that the compensation yields accurate results at intermediate orientations as well. A spherical resonator case further demonstrates that a close match between numerically and analytically computed relaxation times is also obtained for curved boundaries.

Empirical validation and engineering model comparison of outdoor long-range 3D GPU FDTD acoustic simulations with turbulence

We build upon previous work which introduced an acoustic GPU-accelerated 3D Finite-Difference Time-Domain solver, Empapu, equipped with a dynamic moving frame and turbulence modeled as a position-dependent refractive index for long-range outdoor sound propagation simulations. This physics-based model would serve as a benchmark to assess different engineering models, e.g., ISO 9613-2, CNOSSOS-EU and sonX, in the presence of uneven terrain with different ground properties and an inhomogeneous atmosphere. The solver now integrates an improved model of turbulence reproducing the kinetic spectral energy of velocity fluctuations, derived from wind velocity fields, with an eddy (quasiwavelet) approach, relevant in long-range propagations. This study validates our model by comparing level differences in third-octave band spectra at receiver points up to 300 m to analytical methods and measurements in a mostly flat topography comprised of asphalt and grassland. Further analysis is done by comparing results in different locations in Switzerland in the presence of uneven ground and barriers. This work will provide a physics-based basis for evaluating and comparing engineering models for outdoor sound propagation in lieu of measurements.

Nanoscale grating-based perovskite solar cell with improved efficiency

MAPbI3 perovskite-based solar cell (PSC) has attracted much attention due to its high absorption rate. The top flat electrode degrades the behavior of PSC due to limiting the light reached to the absorber layer, which reduces the efficiency. In this study, the influence of texturing the top FTO electrode surface with a triangular saw-tooth grating on both the optical and electrical performance is reported. The interference effects are also considered in this work by modeling the PSC structure as a Fabry–Perot resonator. In this regard, the finite difference time domain method is utilized to precisely simulate the optical characteristics of the nano-structural design. Also, the optical behavior of PSC is studied at different triangular grating (TG) structures and dimensions at which the light absorption is maximized. Furthermore, the effect of absorber thickness and defect density on the optoelectronic performance is investigated. We configured the conversion efficiency (η) of the proposed PSC structure by using the bulk and Langevin recombination mechanisms. The proposed grating structure enhances the light coupling, and hence the light absorption and the generated current density are increased. For absorber thickness of 350 nm, we reported a maximum conversion efficiency (η) of 19.5% for the proposed triangular grating (TG) structure with an enhancement of 19.6% compared to the structure with a flat FTO layer. As the defect density is increased from 1012 cm−3 to 1018 cm−3, the efficiency of the optimum TG PSC is reduced from 19.5% to 10.1%, respectively. The simulation results, therefore, contribute to the understanding of the PSC-based MAPbI3 design and can be used to improve its physical behavior.

Influence of incorporated particles on the optical performance of VO2 nanoparticle-based thermochromic films

Vanadium dioxide (VO2) nanoparticle-based thermochromic films are widely used in smart windows because of their favorable thermochromic and optical properties. The introduction of additional particles into VO2 NP-based films enables the integration of their properties with those of the original films. This combination serves to modulate the optical performance of the films further. However, the impact of the incorporated particles on the optical performance of VO2 NP-based thermochromic films has not been elucidated. In this study, the influence of particles with varying refractive indices, volume fractions, and sizes, when incorporated into VO₂ NP-based films, was calculated using the finite-difference time-domain (FDTD) method. The results indicate that an increase in the refractive index or the size of the incorporated particles led to an enhanced solar modulation ability (ΔTsol), albeit at the cost of reducing the luminous transmittance (Tlum). Conversely, increasing the volume fraction of the particles did not significantly enhance the ΔTsol. Additionally, the introduction of additional particles into the film further reduces the total energy consumption by a maximum of 2.6 MJ/m2 compared to VO2 NP-based films, according to EnergyPlus software analysis. Our findings offer new perspectives and recommendations for the development and practical implementation of thermochromic smart windows.

All-optical AND, NAND, OR, NOR and NOT logic gates using two nested microrings in a racetrack ring resonator

Boolean logic gates are essential components for optical computing and communication systems. However, most existing methods for realizing them require complex structures, high power consumption, or multiple devices. Here, we propose a simple and compact system that can realize five Boolean logic gates, including AND, NAND, OR, NOR, and NOT, by applying different polarization modes and tuning intensities to the input signals within a SOI resonator system, formed by two nested micro rings in a racetrack ring resonator (TNMIRTR). A formula was derived for the optical transfer function of the system using the delay-line-signal method and the logic gates were simulated using the variational finite-difference time domain (varFDTD) method. The proposed structure operates by combining amplitude and polarization-conversion. TNMIRTR gate has several advantages, such as its micro-scale size, low cost, and ability to realize multiple logic gates within a single layout.

Plasmonic Nanogap-Enhanced Tunable Three-Dimensional Nanoframes in Application to Clinical Diagnosis of Alzheimer's Disease.

Advancements in nanotechnology led to significant improvements in synthesizing plasmon-enhanced nanoarchitectures for biosensor applications, and high-yield productivity at low cost is vital to step further into medical commerce. Metal nanoframes via wet chemistry are gaining attention for their homogeneous structure and outstanding catalytic and optical properties. However, nanoframe morphology should be considered delicately when brought to biosensors to utilize its superior characteristics thoroughly, and the need to prove its clinical applicability still remains. Herein, we controlled the frameworks of double-walled nanoframes (DWFs) precisely via wet chemistry to construct a homogeneous plasmon-enhanced nanotransducer for localized surface plasmon resonance biosensors. By tuning the physical properties considering the finite-difference time-domain simulation results, biomolecular interactions were feasible in the electromagnetic field-enhanced nanospace. As a result, DWF10 exhibited a 10-fold lower detection limit of 2.21 fM compared to DWF14 for tau detection. Further application into blood-based clinical and Alzheimer's disease (AD) diagnostics, notable improvement in classifying mild cognitive impairment patients against healthy controls and AD patients, was demonstrated along with impressive AUC values. Thus, in response to diverse detection methods, optimizing nanoframe dimensions such as nanogap and frame thickness to maximize sensor performance is critical to realize future POCT diagnosis.

Computational Study of Moth-Eye Structures for Silicon Solar Cells Lights Harvesting Improvement

Abstract Recently Moth-Eye nanostructure is widely used in solar cell light harvesting enhancement, in this computational work, three different designs; rectangular, triangular, and semi-circular structures were introduced as anti-reflect structures placed above a silicon solar cell, anti-reflected nanostructured modeled and optimized for silicon ultra-thin film solar cells using the finite difference time domain method for optical, and Lumerical devise software for electrical properties study. The effect of the geometrical dimension of the three structures was investigated. It is found that light-harvesting and solar cell performance can be enhanced by choosing a suitable structure and dimension for the suggested structure. The optimum efficiency enhancement achieved was by a semi-circular structure with a radius of 200nm from 8.81% to 11.95%.

Different built-in electric fields for transmission-mode GaAs photocathodes through doping engineering: Design and modeling

In order to enhance the emission performance of transmission-mode GaAs photocathodes used in optoelectronic field, different exponential doping structures are designed for the GaAs emission layer to generate different types of built-in electric fields. These built-in electric fields include the constant, increasing and decreasing types, which can help photoelectron transport toward the emission surface. By solving the one-dimensional continuity equation using the finite difference method, the electron concentration distribution and quantum efficiency of the three types of exponential doping GaAs photocathodes are derived. Meanwhile, the light absorption distributions in the emission layer are simulated by the finite difference time domain method. By comparing the optical absorption distribution and the electron concentration distribution, it is concluded that, among the three types of exponential doping structures, the exponential doping structure generating the increasing built-in electric field has the most sufficient long-wave light absorption capacity and the strongest photoelectron transport ability, thereby achieving the highest quantum efficiency. This theoretical work can help understand the mechanism of improving the photoemission performance of GaAs photocathodes through doping engineering.

Enhanced Antireflection and Absorption in Thin Film GaAs Solar Cells Using Dielectric Composite Nanostructures

This study investigates the application of dielectric composite nanostructures (DCNs) to enhance both antireflection and absorption properties in thin film GaAs solar cells, which are crucial for reducing production costs and improving energy conversion efficiency in photovoltaic devices. Building upon previous experimental validations, this work systematically explores the underlying theoretical mechanisms using the finite difference time domain (FDTD) method to analyze the light interaction with the proposed DCNs. The results show that the combination of Mie resonance, Fabry–Perot resonance, and guided resonance, induced by the surface structuring of the DCNs, significantly enhances light absorption in the active layer, particularly at longer wavelengths. For solar cells featuring a 500-nm-thick absorber layer, SARL-decorated solar cells demonstrated an average reflectivity of 12.18%, whereas those incorporating DCNs exhibited a significantly reduced average reflectivity of 4.52%. These findings indicate that DCNs structures are highly effective in enhancing the performance of thin and ultra-thin GaAs solar cells by minimizing surface reflection and increasing photon utilization, offering a promising solution for high efficiency, cost effective photovoltaic devices.

Plasmonic MIM waveguide based FR sensors for Refractive Index sensing of Human Hemoglobin

Fano resonance (FR) is a universal phenomenon that is used to attain electromagnetic-induced transparency (EIT), high absorption and sensitivity, and low-power photonic devices. This work presents dual FR refractive index (RI) sensor models on a plasmonic metal-insulator-metal (MIM) waveguide system. The FR phenomenon is attained by including circular and elliptic nanorod defects in the bus waveguides. The resonances originate from the defect's narrow discreteness and the rectangular resonator's broad state. Analytical methods such as finite difference time domain (FDTD) and multimode interference coupled mode theory are adopted to analyze the FRs. The shapes of the Fano line and resonance peak amplitude can be tuned independently by controlling the diameter of the defects, the separation between the defects, and the coupling (between the resonator and the bus waveguide) distance. Moreover, the proposed structures detect the RI (human hemoglobin) variation in the bus waveguide and resonator. The obtained results with circular nanorod defect verify the autocorrelation coefficient of 99.92%, ensuring the device's linearity and high performance. However, an autocorrelation of 99.7% is attained by using two elliptic nanorod defects.

CSEM Optimization Using the Correspondence Principle

Traditionally, 3D modeling of marine controlled-source electromagnetic (CSEM) data (in the frequency domain) involves high-memory demand, requiring solving a large linear system for each frequency. To address this problem, we propose to solve Maxwell’s equations in a fictitious dielectric medium with time-domain finite-difference methods, with the support of the correspondence principle. As an advantage of this approach, we highlight the possibility of its implementation for execution with GPU accelerators, in addition to multi-frequency data modeling with a single simulation. Furthermore, we explore using the correspondence principle to the inversion of CSEM data by calculating the gradient of the least-squares objective function employing the adjoint-state method to establish the relationship between adjoint fields in a conductive medium and their counterparts in the fictitious dielectric medium, similar to the approach used in forward modeling. We validate this method through 2D inversions of three synthetic CSEM datasets, computed for a simple model consisting of two resistors in a conductive medium, a model adapted from a CSEM modeling and inversion package, and the last one based on a reference model of turbidite reservoirs on the Brazilian continental margin. We also evaluate the differences between the results of inversions using the steepest descent method and our proposed momentum method, comparing them with the limited-memory BFGS (Broyden–Fletcher–Goldfarb–Shanno) algorithm (L-BFGS-B). In all experiments, we use smoothing by model reparameterization as a strategy for regularizing and stabilizing the iterations throughout the inversions. The results indicate that, although it requires more iterations, our modified momentum method produces the best models, which are consistent with results from the L-BFGS-B algorithm and require less storage per iteration.

Finite-Difference Time-Domain Methods: From Fields and Potentials to Gauge-Invariant Field-Impulses With Universal Current-Impulse Source

Finite-Difference Time-Domain Methods: From Fields and Potentials to Gauge-Invariant Field-Impulses With Universal Current-Impulse Source