Finite-difference Time-domain Technique Research Articles

Tungsten-Doped ZnO as an Electron Transport Layer for Perovskite Solar Cells: Enhancing Efficiency and Stability.

This study delves into enhancing the efficiency and stability of perovskite solar cells (PSCs) by optimizing the surface morphologies and optoelectronic properties of the electron transport layer (ETL) using tungsten (W) doping in zinc oxide (ZnO). Through a unique green synthesis process and spin-coating technique, W-doped ZnO films were prepared, exhibiting improved electrical conductivity and reduced interface defects between the ETL and perovskite layers, thus facilitating efficient electron transfer at the interface. High-quality PSCs with superior ETL demonstrated a substantial 30% increase in power conversion efficiency (PCE) compared to those employing pristine ZnO ETL. These solar cells retained over 70% of their initial PCE after 4000 h of moisture exposure, surpassing reference PSCs by 50% PCE over this period. Advanced numerical multiphysics solvers, employing finite-difference time-domain (FDTD) and finite element method (FEM) techniques, were utilized to elucidate the underlying optoelectrical characteristics of the PSCs, with simulated results corroborating experimental findings. The study concludes with a thorough discussion on charge transport and recombination mechanisms, providing insights into the enhanced performance and stability achieved through W-doped ZnO ETL optimization.

Facile modification method for the controlled synthesis of dumbbell-like gold nanoparticles (AuNDBs) for application in detecting glucose using the SERS method

Dumbbell-like nanoparticles, previously known for their excellent light absorption and dispersion, have diverse applications, especially in SERS. Herein, we have explored a fast and novel controlled synthesis approach to achieve a high formation of AuNDBs. Precise control of precursor amounts and seed addition order halted seed growth at the AuNDB formation stage, preventing further elongation into rod-like structures. The AuNDBs were functionalized with 4-mercaptobenzoic acid (4-MBA) to create SERS nanosubstrates and deposited on the glass slide through 3-mercaptopropyl (dimethoxy)methylsilane by the spin-coating technique. The detection of d-glucose was achieved by measuring the signal decrease of the 4-MBA Raman probe molecules on the nanosubstrates at different trace concentrations of the analyte. These novel SERS substrates have the potential to provide an enhancement factor (EF) of 2.41 × 107 and a limit of detection (LOD) as low as 1.17 nmol/L. The electromagnetic field distribution of AuNDBs with many arrangements surrounded by air and d-glucose solutions was also determined by using the finite-difference time-domain technique (FDTD). The FDTD results confirmed that d-glucose on the SERS substrates caused the local electromagnetic field to degrade, resulting in the degree of the Raman signal. Our findings suggest that dumbbell-shaped SERS substrates can effectively detect very low levels of glucose, making them ideal for designing novel anisotropic nanoparticle-functionalized surfaces with ease.

Multiphysics simulations of a cylindrical waveguide optical switch using phase change materials on silicon

This work presents the design and multiphysics simulation of a cylindrical waveguide-based optical switch using germanium-antimony-tellurium (GST) as an active phase change material. The innovative cylindrical architecture is theoretically analyzed and evaluated at 1550 nm wavelength for telecommunication applications. The dispersion relation is derived analytically for the first time to model the optical switch, while finite element method (FEM) and finite difference time domain (FDTD) techniques are utilized to simulate the optical modes, light propagation, and phase change dynamics. The fundamental TE01 and HE11 modes are studied in detail, enabling switching between low-loss amorphous and high-loss crystalline GST phases. Increasing the GST thickness is found to increase absorption loss in the crystalline state but also slows down phase transition kinetics, reducing switching speeds. A 10 nm GST layer results in competitive performance metrics of 0.79 dB insertion loss, 13.47 dB extinction ratio, 30 nJ average power consumption, and 3.5 Mb/s bit rate. The combined optical, thermal, and electrical simulation provides comprehensive insights towards developing integrated non-volatile photonic switches and modulators utilizing phase change materials.

An improved two‐dimensional (2,4) finite‐difference time‐domain method for Lorentz dispersive media

AbstractThe credible solution of discretized Maxwell's equations in spaces occupied by Lorentz dispersive media is the main subject of this work. Specifically, we introduce a finite‐difference time‐domain (FDTD) algorithm with a typical (2,4) structure that features dispersion‐relation‐preserving characteristics and produces reduced numerical errors in two‐dimensional electromagnetic simulations, compared to the standard approach with similar computational requirements. We consider the case of dispersive media with non‐vanishing absorption coefficients and investigate different options for the suitable modification of the spatial approximations, so that the accomplished accuracy is optimized for a given computational overhead. The properties of the proposed FDTD technique are thoroughly examined, both theoretically and in numerical tests, and the performance upgrade compared with the conventional solution is assessed.

Highly selective single-mode graphene bandpass filter based on Wilkinson power divider structure

In this paper, we introduce a novel graphene nanoribbon bandpass filter designed using the Wilkinson power divider structure for single-mode operation. Through the three-dimensional finite-difference time-domain (3D-FDTD) technique, we analyze transmission spectra and electromagnetic field distributions. Our primary goals are to achieve high transmission efficiency, improved adjustability, and an excellent quality factor (Q-factor). The proposed single-mode graphene nanoribbon bandpass filter exhibits outstanding performance characteristics, including a 45 THz free spectral range (FSR), 95 % transmission efficiency, and a full width at half maximum (FWHM) of 0.14 THz at the resonant frequency of 15 THz. By changing the chemical potential (or bias voltage) to manipulate the conductivity and dielectric constant of the graphene surface, we successfully tune the center frequency of the filter within the 10–20 THz range. This flexibility, combined with exceptional transmission efficiency and Q-factor, positions our filter as a prime candidate for seamless integration into compact optical devices. Additionally, by adjusting the refractive index of the substrate, we achieve a sensitivity exceeding 7 THz/RIU or 7500 nm/RIU, making our filter a promising sensor for applications requiring precise detection and measurement capabilities.

Accurate Nonstandard Path Integral Models for Arbitrary Dielectric Boundaries in 2-D NS-FDTD Domains.

An efficient path integral (PI) model for the accurate analysis of curved dielectric structures on coarse grids via the two-dimensional nonstandard finite-difference time-domain (NS-FDTD) technique is introduced in this paper. In contrast to previous PI implementations of the perfectly electric conductor case, which accommodates orthogonal cells in the vicinity of curved surfaces, the novel PI model employs the occupation ratio of dielectrics in the necessary cells, providing thus a straightforward and instructive means to treat an assortment of practical applications. For its verification, the reflection from a flat plate and the scattering from a cylinder using the PI model are investigated. Results indicate that the featured methodology can enable the reliable and precise modeling of arbitrarily shaped dielectrics in the NS-FDTD algorithm on coarse grids.

Enhancement of light absorption by ultra-thin film solar cells using graded gratings

In this study, we developed a novel method based on uniform and graded gratings on the front surface of ultra-thin film Si solar cells to enhance light absorption. The proposed gratings were designed in two configurations comprising penetration into the active layer and placement on it. These structures enhance absorption by scattering and diffracting light, and enlarging the optical path for photons. Simulations based on the finite element method and finite difference time domain technique showed that the graded gratings could significantly enhance absorption in the visible and infrared regions. The maximum current density and efficiency achieved for graded gratings placed on the top surface of the active layer were 21.7 mA/cm2 and 23.9%, respectively (47.6% and 48.4% higher compared with the reference cell).

Frequency encoded tristate Pauli X-gate using SOA assisted photonic band gap crystal

The quantum optical tristate Pauli-X gate can broadly be used due to its very fast speed of operation up to THz order, very fast information processing rate, and a wide domain of application. This Pauli X-gate is also important as it can undergo inversion operation. In this proposed scheme, the quantum tristate Pauli-X logic gate has been constructed by a closely packed square lattice of 2D air photonic band gap crystal (PhCs) composed of GaAsInP-dopped rods. The photonic band gap crystal-based tristate Pauli-X gate is performed also as an all-optical reversible quantum logic gate which has the character of low-powered output, very fast speed and a densely packed design structure. This photonic crystal-based structure is also verified by simulation experiment that this proposed scheme performs as the path of two input signals are cross-exchanged to one another at the output, retaining the path of the other input signal same. In this paper, the frequency encoding technique is used in all the designs to establish the logic of all-optical tristate Pauli X-gate towards the need of obtaining a three-input-three-output inversion system. The frequency encoding technique and the photonic crystal-based three Semiconductor Optical Amplifiers (pc-SOAs) are used here in all the schemes to establish the logic of the tristate Pauli-X gate scheme. Tristate Pauli-X gates have been designed and analyzed by Finite-Difference Time-Domain (FDTD) and Plane Wave Expansion (PWE) techniques. The Plane Wave Expansion (PWE) technique is used here in each system for determining the frequency range of band gap structure of transverse electric (TE) mode in the units of μm-1. All of these devices perform within the frequency range 0.629818≤ω2πc≤0.888037 and 1.17551≤ω2πc ≤ 1.34921 μm-1. The wafer dimension of each photonic band gap crystal-based device is 19.20 μm (length) × 16.20 μm (width) providing a very fast response time and very fast information handling (bit rate) capacity, which is found good for these THz Photonic crystal devices. Simulation results show that the proposed gate provides a response period of 0.291 ps and a bit rate of 3.44 Tbps, operating with a low input power of 3.45 mW. The reversible gate exhibits a response period of 0.289 ps and a bit rate of 3.46 Tbps, operating also with a low input power of 2.70 mW. Additionally, the alternative Pauli-X gate demonstrates a response time of 0.291 ps and a bit rate of 3.44 Tbps, operating with a low input power of 2.83 mW.

Analogy between a moving line source illuminating a metallic wire and Compton scattering experiment

AbstractThis paper presents an electromagnetic analysis of a moving infinitely long line source illuminating an infinitely long metallic wire at rest. The study uses the full‐wave numerical finite difference time domain (FDTD) method, which is based on the spatial and temporal discretization of Maxwell's equations. Movement is computed within the FDTD technique by varying the position of the line source at each time loop. An analogy is proposed between the wavelength spectrum of the simulated electromagnetic field and the Compton scattering experiment. A good agreement is obtained between theoretical analysis, numerical results, and Compton experimental data.

A tunable ultra-broadband and ultra-high sensitivity far-infrared metamaterial absorber based on VO2 and graphene.

We proposed a far-infrared tunable metamaterial absorber using vanadium dioxide (VO2) and graphene as controlling materials. The properties of the absorber are investigated theoretically using the finite-difference time-domain (FDTD) technique. It was found that when the Fermi energy level of graphene is fixed at zero, VO2 is in the insulated state, and the metasurface exhibits far-infrared broadband absorption performance, with absorptance exceeding 90% in the wavelength range of 12.6 μm to 23.2 μm. In addition, by elevating the Fermi energy level of graphene, the absorption bandwidth of the device is expanded continuously. When the VO2 is in the metallic state, the device can flexibly transform into a far-infrared narrowband absorber. The device also has the advantage of being insensitive to changes in polarization and incident angle. The origin of the absorption and the tuning principle of the device were analyzed and verified successfully by using an equivalent circuit model (ECM). Besides, we also studied the refraction index sensing characteristics of the absorber. Surprisingly, the absorber exhibits excellent sensing characteristics, and its sensitivity (S) reaches 14.108 μm per RIU and the figure of merit (FOM) is 6.13 per RIU.

Design of a High Q-Factor Label-Free Optical Biosensor Based on a Photonic Crystal Coupled Cavity Waveguide.

In recent years, there has been a significant increase in research into silicon-based on-chip sensing. In this paper, a coupled cavity waveguide (CCW) based on a slab photonic crystal structure was designed for use as a label-free biosensor. The photonic crystal consisted of holes arranged in a triangular lattice. The incorporation of defects can be used to design sensor devices, which are highly sensitive to even slight alterations in the refractive index with a small quantity of analyte. The plane wave expansion method (PWE) was used to study the dispersion and profile of the CCW modes, and the finite difference time domain (FDTD) technique was used to study the transmission spectrum, quality factor, and sensitivity. We present an analysis of adiabatically coupling light into a coupled cavity waveguide. The results of the simulation indicated that a sensitivity of 203 nm/RIU and a quality factor of 13,360 could be achieved when the refractive indices were in the range of 1.33 to 1.55.

On-Chip Integration of a Plasmonic FET Source and a Nano-Patch Antenna for Efficient Terahertz Wave Radiation.

Graphene-based Field-Effect Transistors (FETs) integrated with microstrip patch antennas offer a promising approach for terahertz signal radiation. In this study, a dual-stage simulation methodology is employed to comprehensively investigate the device's performance. The initial stage, executed in MATLAB, delves into charge transport dynamics within a FET under asymmetric boundary conditions, employing hydrodynamic equations for electron transport in the graphene channel. Electromagnetic field interactions are modeled via Finite-Difference Time-Domain (FDTD) techniques. The second stage, conducted in COMSOL Multiphysics, focuses on the microstrip patch antenna's radiative characteristics. Notably, analysis of the S11 curve reveals minimal reflections at the FET's resonant frequency of 1.34672 THz, indicating efficient impedance matching. Examination of the radiation pattern demonstrates the antenna's favorable directional properties. This research underscores the potential of graphene-based FETs for terahertz applications, offering tunable impedance matching and high radiation efficiency for future terahertz devices.

A 3D complex urban sound propagation benchmark case

Predicting sound propagation in the dense urban fabric is challenging as it involves multiple and higher-order reflections, both specular and diffuse in nature, diffraction around complex objects like building edges and roofs, and atmospheric effects especially relevant at non-directly exposed locations. Over the last decades, there has been a continuing interest in enclosed courtyards, a micro-environment with the potential to offer human restoration in a busy city. Also roofs are increasingly being designed as outdoor spaces. The sound exposure in such zones could be especially challenging to predict. To gain more understanding in the related physics and accuracy of prediction schemes, a 3D urban benchmark case is presented, consisting of 4 building blocks with a fully symmetric geometry. Receivers and acoustic sources are positioned in the streets, on the roofs, and in the courtyards. Simulations with the reference finite-difference time-domain technique show pronounced differences in the time responses, and allow assessing the impact of including (geometrical) façade details. Additional simulations show the impact of openings from within the streets towards the courtyards. The simulation results allow checking the accuracy of sound propagation models and their simplifications.

A novel strategy for specific sensing and inactivation of Escherichia coli: Constructing a targeted sandwich-type biosensor with multiple SERS hotspots to enhance SERS detection sensitivity and near-infrared light-triggered photothermal sterilization performance

Human health is greatly threatened by bacterial infection, which raises the risk of serious illness and death in humans. For early screening and accurate treatment of bacterial infection, there is a strong desire to undertake ultrasensitive detection and effective killing of pathogenic bacteria. Herein, a novel surface-enhanced Raman scattering (SERS) biosensor based on sandwich structure consisting of capture probes/bacteria/SERS tags was established for specific identification, capture and photothermal killing of Escherichia coli (E. coli). Finite-difference time-domain (FDTD) technique was used to simulate the electromagnetic field distribution of capture probes, SERS tags and sandwich-type SERS substrate, and a possible SERS enhancement mechanism based on sandwich structure was presented and discussed. Sandwich-type SERS biosensor successfully achieved distinctive identification and magnetic beneficiation of E. coli. In addition, a single SERS substrate, including capture probes and SERS tags, could also achieve outstanding photothermal effects as a consequence of localized surface plasmon resonance (LSPR) effect. Intriguingly, sandwich-type SERS biosensor demonstrated a higher photothermal conversion efficiency (50.03 %) than the single substrate, which might be attributed to the formation of target bacterial clusters. The superior biocompatibility and the low toxicity of the sandwich-type biosensor were confirmed. Our approach offers a fresh method for constructing sandwich-type biosensor with multiple SERS hotspots based on extremely effective hybrid plasmonic nanoparticles, and has a wide range of potential applications in the recognition and treatment of bacteria.

Design and analysis of a group-III nitride superlattice structure based semiconductor laser diode for blue region emission

The demand for high-power blue laser diodes (LDs) in the range above 2 W has been steadily increasing due to their applications in solid-state lighting, projection displays, high-density optical data storage and underwater communication. However, current designs face limitations in terms of achieving both high power output and efficiency. This work focuses on the design, development and numerical analysis of a blue LD utilizing group-III nitride superlattice structures. The present study aims to overcome design challenges by investigating the fundamental factors affecting the performance of blue LDs based on superlattice InGaN structures through careful device parameter optimization. The results show that our device successfully emits at around 430 nm wavelength and is capable of achieving a differential quantum efficiency of 46.91%, with a maximal optical power output of 2.18 W for 1.71 A of current for a strip width of 15 µm. However, when the strip width is increased to 20 µm, 4.6 W optical power is achieved with 3 A of injection current. Numerical studies are performed with several calibrated physics models and finite-difference time-domain techniques. Our results provide an insight into the potential of using superlattice group-III nitride structures to enhance the performance of blue LDs, opening up new possibilities for high-power and high-efficiency devices in the future.

A novel and computationally efficient subgrid technique for modeling scattering and Doppler effects of rough dynamic sea surfaces using the finite-difference time-domain method

Over the last decade, the improvements in computational performance that multiprocessing, solid-state hard drives, and fast memories have brought to desktop workstations have made modeling and simulation methods that were previously inaccessible into the office workspace. These methods were previously restricted to large servers or computer clusters because of their demanding computation and large memory requirements when applied to underwater acoustic simulation scenarios. One such method is the Finite-difference Time-domain (FDTD) technique. The FDTD method is a discrete numeric model that provides the full pressure wave solution solved directly in the time domain. This makes it an ideal model for investigating scattering and Doppler effects of boundaries with complex geometries and motion. A fully developed sea surface generated using the Pierson–Moskowitz wavenumber spectra meets these criteria. A novel subgrid technique has been developed to improve the accuracy of signals scattered from fully developed sea surfaces. This method remains computationally efficient because the subgrid is applied only along the rough sea surface as it varies over time. The FDTD subgrid method has been applied to simulations that investigate the “frozen” sea surface assumption that is utilized by other traditional models that incorporate boundary motion.

A 2D GaAs-Based Photonic Crystal Biosensor for Malaria Detection

Gallium arsenide (GaAs) composite semi-conductive rods with an air background lattice act as the building blocks for the photonic crystal structure used of a biosensor. The study presents a biosensor of a two-rod nano-cavity for identifying distinct stages of plasmodium falciparum in red blood cells (RBCs) in the early detection of malaria. The proposed biosensor enables the creation of a label-free biosensing environment in which optical and dispersion properties are investigated using plane wave expansion (PWE) and finite-difference time-domain (FDTD) techniques. The biosensor, with a sensing region for an analyte, is utilized to detect a change in refractive index to differentiate between normal RBCs and plasmodium falciparum-infected cells. The results show that the biosensor has a high sensitivity of 798.143 nm/RIU, a high Q-factor of 9881.926, a low detection limit (δ) of 222.4 × 10-6 RIU, a high FOM of 4496.079 RIU-1, and a compact area of 46.14 µm2.

Triple plasmon-induced transparency and polarization-insensitive optical switch based on monolayer patterned graphene metamaterial

This study introduces a single-layer patterned graphene metamaterial, composed of a graphene ring (GR), two parallel graphene strips (TPGSs), two vertical graphene strips (TVGSs), and a graphene block (GB), which achieves triple plasmon-induced transparency (TPIT) in the terahertz (THz) frequency range using coupled mode theory (CMT) and the finite-difference time-domain (FDTD) technique. Moreover, a synchronized electro-optical switch with four modulation modes is implemented by dynamically adjusting graphene’s Fermi level, exhibiting modulation degrees of amplitude (MDA) of 88.3%, 94.7%, 86.9%, and 89.0% at 1.69[Formula: see text]THz, 3.14[Formula: see text]THz, 3.95[Formula: see text]THz, and 4.67[Formula: see text]THz, respectively. In addition, the modes excited by TPGSs and TVGSs interconvert due to the proposed structure’s symmetry, accounting for the polarization insensitivity of TPIT. The enhanced MDA and polarization insensitivity of the electro-optical switch presented in this study significantly surpass those of comparable electro-optical switches. As a result, the polarization insensitivity of multiple optical switches to incident light fields offers a promising approach for designing optoelectronic devices.

Effect of defects and doping on the transmission spectrum enhancement of photonic crystal based on 2D-MoS2

In this paper, we present the structural and photoluminescence (PL) spectra of 2D-MoS2 and we report a theoretical investigation of the influence of Ag- and Au-doping on the transmission in 2D-MoS2 photonic crystal structure (PCS). A finite-difference time-domain (FDTD) technique and plane wave expansion method (PWE) have been introduced and adapted for periodic and aperiodic structures to extract the PC-bandgap and study the behavior of the transmission spectrum. The impact of the size of defects, the Ag- and Au-doping on the 2D-PC band structure and transmission spectrum has been visualized under the inclusion of the distribution of electromagnetic fields and the associated field maps. It has been shown that the Au-doping changes significantly the 2D-MoS2 PCS and as a result the transmission spectrum increases considerably. We can predict that 2D-MoS2 PC doped with Au is more convenient than doped with Ag in most applications involving optical transmission and waveguides for optoelectronics devices.

Mode division (de)multiplexer based on transverse mode conversion in photonic crystal using asymmetric directional couplers.

Mode division multiplexing technology has the potential to increase the channel capacity of a single wavelength carrier. Attaining cost-effective high-bandwidth-density devices with small footprints is a concern, and photonic crystal based devices are promising for ultra-small on-chip communications. This paper presents a 2D photonic crystal based mode division (de)multiplexer on a silicon on insulator platform. The device comprises two coupling regions of asymmetric directional couplers that perform mode conversion operations between the fundamental mode and higher-order modes. Each coupling section is dedicated to converting a specific mode. Mode conversion is achieved by designing a multimode waveguide to satisfy the phase-matching condition of the desired mode with the single mode waveguide. Two linear adiabatic tapers are introduced for the smooth transition of modes between waveguide sections. The device is designed and simulated for three-channel modes at 1550nm using the finite-difference time-domain technique. The obtained insertion loss and cross talk are <0.41d B and <-20.14d B, respectively. The overall size of the proposed mode division (de)multiplexer is 328.5µm 2. A fabrication tolerance study for the proposed device is performed by varying the rod radius and position in the device structure's taper and bus waveguide regions. The proposed 2D photonic crystal based mode division (de)multiplexer has the potential to be used in large-capacity optical communication systems.