Finite-difference Time-domain Research Articles

Tunable Photonic Hook Design Based on Anisotropic Cutting Liquid Crystal Microcylinder

The selective control and manipulation of nanoparticles require developing and researching new methods for designing optical tweeters, mainly based on a photonic hooks (PHs) effect. This paper first proposes a tunable PH in which a structured beam illuminates an anisotropic cutting liquid crystal microcylinder based on the Finite-DifferenceTime-Domain (FDTD) method. The PHs generated by plane wave, Gaussian, and Bessel beam are analyzed and compared. The impact of beams and LC particle parameters on the PHs are discussed. Where the influence of the extraordinary refractive index (ne) on PHs is emphasized. Our results reveal that introducing birefringence can change the bending direction of PH. Besides, the maximum intensity of the PHs increases as ne increases regardless of the beam type. The PH generated by a plane wave has a higher maximum intensity and smaller FWHM than that generated by the Gaussian and Bessel beams. The smallest FWHM and maximum intensity of the PHs generated by the Gaussian falls between that generated by the plane wave and the Bessel beam. The PH generated by a Bessel beam has the minor maximum intensity and the largest FWHM. Still, it exceeds the diffraction limit and exhibits bending twice due to its self-recovery property. This paper provides a new way to modulate PH. This work offers novel theoretical models and the degree of freedom for the design of PHs, which is beneficial for the selective manipulation of nanoparticles. It has promising applications in Mesotronics and biomedicine.

Влияние числа Куранта на результаты численного моделирования распространения сигналов в недиспергирующих однородных средах

The paper is devoted to the study of the connection between the numerical dispersion arising in FDTD modeling of electromagnetic signal propagation in nondispersive homogeneous media optically different from vacuum and the Courant number in the 2D case. The main results are formulated in the form of four statements, as well as a number of corollaries and remarks that determine the nature of the numerical dispersion, the optimal value of the Courant number and the limitations of the method. It is proved that the optimal choice of the Courant number eliminates the numerical dispersion and extends the capabilities of the developed numerical algorithm to media, which refractive index is lesser than refractive index of vacuum, as well as media with negative refraction.

Plasmon-enhanced sub-Debye-length nanogap photoelectrochemical cells for field-assisted electrolyte-free water splitting

This work studies pure water splitting (i.e., no additional electrolyte) using plasmonic sub-Debye-length nanogap photoelectrochemical cells (NPECs). With 60 nm Au gratings covered by 4 nm amorphous TiO2 under 638 nm normal incident illumination, we demonstrate up to 66 × higher photocurrent at 1.5 V than that of Au-gratings-only NPECs. In addition, we are able to efficiently drive pure water splitting at voltages as low as 1.3 V, which is very close to the theoretical thermodynamic potential threshold of 1.23 V. Our electromagnetic finite difference time domain (FDTD) simulation results (Lumerical Inc.) show a 240 × increase in the electric field intensity near the TiO2 layer compared to the incident electric field. We attribute our observations to 1) Hot spots generated by surface plasmonic resonance (SPR) on Au gratings, which greatly increase the number of electron-hole pairs; 2) The 60 nm gap distance between electrodes, which is smaller than the Debye length of pure water (∼220 nm) resulting in a much higher electric field by eliminating the shield effect and by coupling two half-reactions of water splitting together; and 3) The 4 nm amorphous TiO2, which improves hot carriers separation. Our NPEC is a promising device for efficient hydrogen production enabling the utilization of visible light.

Design of All-Optical D Flip Flop Memory Unit Based on Photonic Crystal.

This paper proposes a unique configuration for an all-optical D Flip Flop (D-FF) utilizing a quasi-square ring resonator (RR) and T-Splitter, as well as NOT and OR logic gates within a 2-dimensional square lattice photonic crystal (PC) structure. The components realizing the all-optical D-FF comprise of optical waveguides in a 2D square lattice PC of 45 × 23 silicon (Si) rods in a silica (SiO2) substrate. The utilization of these specific materials has facilitated the fabrication process of the design, diverging from alternative approaches that employ an air substrate, a method inherently unattainable in fabrication. The configuration underwent examination and simulation utilizing both plane-wave expansion (PWE) and finite-difference time-domain (FDTD) methodologies. The simulation outcomes demonstrate that the designed waveguides and RR effectively execute the operational principles of the D-FF by guiding light as intended. The suggested configuration holds promise as a logic block within all-optical arithmetic logic units (ALUs) designed for digital computing optical circuits. The design underwent optimization for operation within the C-band spectrum, particularly at 1550 nm. The outcomes reveal a distinct differentiation between logic states '1' and '0', enhancing robust decision-making on the receiver side and minimizing logic errors in the photonic decision circuit. The D-FF displays a contrast ratio (CR) of 4.77 dB, a stabilization time of 0.66 psec, and a footprint of 21 μm × 12 μm.

Plasmonic structurally colored surfaces with metal film over microsphere lattices

Structural coloring through plasmonic nanostructures involves the manipulation of light interaction with nanostructured metallic surfaces to selectively reflect specific spectral ranges within the visible spectrum. Unlike conventional pigments that absorb light based on their chemical properties, plasmonic nanostructures achieve color through their physical configuration. This study investigates the potential of generating plasmonic structural colors by utilizing self-assembled colloidal microsphere lattices that are coated with thin metal films. Finite-Difference Time-Domain (FDTD) simulations are employed on realistic models to analyze the optical reflectance of dielectric microsphere lattices ranging in sizes from 300 to 700 nm, coated with varying thicknesses (20–200 nm) of different metals (Al, Ag, Au). The reflectance spectra are then translated into specific colors using a software algorithm, with the color representations plotted on CIE1931 diagrams. Furthermore, to validate the theoretical findings, two-dimensional polystyrene microsphere arrays coated with Au and Ag films are prepared, and their reflectance spectra and color images are examined. The resulting plasmonic colors can be adjusted by altering the sphere size, the type of metal used, and the thickness of the coating, demonstrating their potential for diverse color-based applications.

Laser irradiation-induced surface-enhanced Raman scattering of Sn@SnO composite films

In this work, Sn@SnO composite films were obtained by Nd:YAG fiber pulsed laser ablation at room temperature. The effects of laser ablation on the microstructure, optical properties, and Raman scattering properties of the films were investigated. Tunable Localized Surface Plasmon resonance(LSPR)of Sn nanoparticles was realized in the wavelength range of 380–450 nm by varying the scanning rate to modulate the degree of oxidation of Sn nanoparticles. Surface enhanced Raman scattering (SERS) was also observed in the composite structure, which was demonstrated excellent stability and SERS sensitivity with the detectable concentration of methylene blue (MB) as low as 10−9 M and an enhancement factor of 1.75 × 105 as well as a good uniformity. Finally, the electric field distribution of the samples before and after the laser treatment is simulated by the finite difference in time domain (FDTD) method, and the results are consistent with those of experimental ones.

Cellulose binary coatings with spherical envelope structure via structure rearrangement in ball milling for integrated radiative cooling-electricity generation

Passive daytime radiative cooling is a zero-energy consumption cooling technology, which can dissipate heat to outer space via infrared radiation. Recently, coupling radiative cooling technology and thermoelectric devices to generate electricity has attracted much attention. However, existing radiative cooling integrated thermoelectric devices still suffer from low-temperature gradient and output voltage. Here, based on the Mie scattering and internal reflection enhancing principle, an impact-inducing geometry reconstruction approach was proposed to fabricate hierarchical nanostructured cellulosic coatings with good daytime cooling performance to achieve stable electricity generation function, which can be realized by using a scalable and facile wet ball milling technology. Guided by the theoretical simulations of the finite difference time domain method (FDTD), the cellulose and TiO2 nanoparticles can assemble into spherical envelope structured coatings drying by the shear, impact, and friction interaction in the ball milling process, dramatically enhancing the Mie scattering and internal reflection of coatings. The cellulosic coatings exhibit sunlight reflectivity of 0.962 and infrared emissivity of 0.94, resulting in a daytime radiative cooling efficiency of 5.9 °C under direct sunlight. Energy Plus stimulation demonstrated 35 % cooling energy and 468.9 kWh of cooling energy can be saved annually in China. Meanwhile, this cellulosic coating-based thermoelectric device can deliver a high voltage output of 150 mV under 1 Sun due to the strong bonding and high-temperature gradient formation (30 °C), which is higher than previous reports. This study will facilitate the development of sustainable power generation device for the goal of green future.

Thermal emitter performance of ultra-broadband solar metamaterial absorber based on metal-insulator-metal device structure

In this paper, we have designed a solar absorber with ultra-broadband absorption. The structure has seven layers with a metal-dielectric periodic film stack and a metallic copper substefficiency. The metal-dielectric periodic membrane stack and the dielectric are continuous to form a metal/dielectric composite microstructure. It has a very high absorption efficiency at 280 nm–2893 nm. The absorption efficiency in this wavelength range is 96.26% and absorption efficiency at AM 1.5 is up to 95.66% by finite difference time domain method (FDTD) simulation. The thermal radiation absorption efficiency of the structure is 93.69% at 1000K and 94.5% at 1200 K, which can also be used in the manufacture of the heat sink due to its high thermal radiation absorption efficiency. Our proposed solar absorber has good incidence angle insensitivity (0°–60°) and good polarization independence (0°–90°). These advantages enable our absorbers to be widely used for solar absorption and emission and offer a variety of design options for ideal absorbers.

Design and Simulation of a Three-Channel Plasmonic Demultiplexer in an MIM Waveguide

This study proposes a structure for optical plasmonic demultiplexers based on nanodisk/nanoring resonators in metal-insulator-metal (MIM) plasmonic waveguides. The proposed structure can transmit the desired wavelength spectra, including telecommunication and visible waves. It consists of a bus waveguide, several nanodisks and nanorings, and three drop waveguides, consisting of several filters that can be extended to N×1 channels. The FWHM of the designed structure was calculated to be approximately 18 nm. The transmission spectrum of each demultiplexer channel can be easily adjusted by changing the geometric parameters and refractive index. As the waveguide refractive index increases, the transmission spectrum shifts to higher wavelengths, and its amplitude decreases due to internal structural losses. Therefore, the differences between the waveguide and metal refractive indices gradually decrease, and the light confinement decreases in the waveguide, leading to an increase in FWHM. The outputs of each channel have two sharp resonance modes, resulting from the coupling of two interacting cavities (ring and disk) due to the Fano resonance phenomenon. This study employed a two-dimensional (2D) finite difference time domain (FDTD) numerical method and simulation by FDTD Lumerical software to evaluate the transmission properties.

A flexible nanostructured multimodal sensor based on surface plasmon resonance

Biosensors are of great significance for human health and safety monitoring, especially the sensor based on Surface Plasmon Resonance (SPR), widely used in biomolecular detection. Improving its sensitivity has always been the focus of research. In this paper, SPR in grating structures was primarily investigated and then a SPR-based nanoscale metal grating sensor was proposed. With Finite Difference Time Domain (FDTD) simulation, optimization of the sensor's duty ratio, thickness of the Gold (Au) layer, and periodic parameters was conducted. Then sensitivity and quality factor of the sensor are analyzed to obtain the optimal sensor parameters. With a grating period of 0.64 μm, Au layer thickness of 30 nm, and sensor's fill factor of 0.625, the sensor achieves a sensitivity of 0.417 μm/refractive index unit (RIU), demonstrating optimal performance. Additionally, Abaqus tensile simulations were performed on the grating imprinting section, validating the uniformity and consistency of strain in the Polydimethylsiloxane (PDMS) substrate-Au film grating structure during stretching. Finally, a combination mold based on acrylic material is designed, effectively addressing the challenging demolding characteristics of cured PDMS material, offering an efficient manufacturing approach for PDMS substrate fabrication. The SPR-based nanoscale grating multimodal sensor designed in this paper exhibits high sensitivity and potential cost-effectiveness in protein molecule detection and flexible stress-strain sensing applications, revealing promising broad prospects for practical utilization.

The damping factor improved the algorithm in semi-airborne transient electromagnetic modelling based on accelerated FDTD

3D modelling plays a crucial role in analysing semi-airborne transient electromagnetic fields, how to accurately and efficiently calculate electromagnetic response has become a significant issue. In this paper, a damping factor improved algorithm of the accelerated finite difference time domain (AFDTD) is proposed for semi-airborne transient electromagnetic modelling. Differing from the previous research, we introduce a novel function to redefine the damping factor, allowing for adaptive adjustment and enhancing calculation accuracy within the finite difference time domain (FDTD) iterative process. To verify the effectiveness of our approach, we conduct tests using homogeneous half-space models, which demonstrate that the improved damping factor significantly enhances calculation accuracy without a notable reduction in efficiency. Furthermore, we apply our improvement to typical conductivity models and polarisable conductivity models, and the results indicate that our approach achieves high accuracy in accelerated semi-airborne transient electromagnetic modelling.

Composite scattering characteristics analysis of micro-ellipsoidal periodic structure optical surface and microdefects

In order to adjust and detect micro-nano periodic structure optical surface accurately and efficiently, the problem of composite scattering between micro-ellipsoidal periodic structure optical surface and pore defects is studied use the multi-resolution time domain (MRTD) approach. A calculation model is established for the intensity distribution of composite scattering, which is modulated by the micro-ellipsoidal periodic structure optical surface and microdefects. Results are in good agreement with those obtained using CST Microwave Studio software and the finite-different time-domain (FDTD) approach, which demonstrates the effectiveness of the calculation model and method. By combining the field distribution of the micro-ellipsoidal periodic structure optical surface containing microdefects with the optical response at different wavelengths, it is necessary to study the influence of various parameters of the micro-ellipsoidal structure and microdefects on the optical system of metamaterials. The effects of the parameters such as roughness, structure of micro-ellipsoidal unit, defect sizes and buried depths on the composite scattering characteristics are analyzed numerically. The results provide technical support for the fields of functional surface design, ultrasensitive detection, scattering peak orientation and frequency selection.

Design and numerical analysis of high contrast ratio and ultra-compact all-optical 2-bit reversible comparator

In this paper, a novel interference-based nanostructure was designed and simulated to realize an all-optical 2-bit reversible comparator by employing a novel technique. The plane wave expansion (PWE) method was adopted to analyze the encoder design and frequency modes. Aside from downsizing, the finite-difference time-domain (FDTD) method was utilized for the simulation and numerical analysis of the design proposed herein. An ultra-compact nanostructure with a 129.8 μm2 footprint was utilized for the all-optical 2-bit reversible comparator. One of the noteworthy characteristics of the proposed nanostructure was its excellent contrast ratio (i.e., 13.8 dB) in comparison to other nanostructures. The bitrate and delay time in this nanostructure were 3.33 Tb/s and 300 fs, respectively. Based on the findings of the simulations conducted at a central wavelength of 1.55 μm, it is recommended to employ the nanostructure proposed herein during the third telecom window. A photonic crystal nano-resonator was utilized to design the high-performance all-optical 2-bit reversible comparator, which may also be employed in integrated optical circuits (IOCs).

High efficiency ultra-broadband absorber and thermal emitter for the composite Ag-NPs/SiO2/MXene multilayer structure on ITO substrate

n this paper, we presented an Ag-NPs/SiO2/MXene plasma model consisting of an MXene layer, a silicon dioxide (SiO2) layer with silver nanoparticles (Ag-NPs) as well as an indium tin oxide (ITO) substrate. The light absorption and thermal radiation of the Ag-NPs/SiO2/MXene plasma model were investigated by finite-difference time-domain (FDTD) and experimental methods. The results showed that due to the localized surface plasmon resonance (LSPR) generated on the surface of Ag-NPs, the maximum absorption of the Ag-NPs/SiO2/MXene plasma model in the visible and near infrared could reach 99.99 %, and the ultra-high wide-angle absorption from 0° to 60° was achieved, and the simulated data were in good agreement with the experimental data. Finally, we calculated that the Ag-NPs/SiO2/MXene model was 93.0 % ultra-efficient thermal emissivity at 1200 K using Planck’s law theory. Broadband absorption and thermal emission of the Ag-NPs/SiO2/MXene structure of importance in the field of light absorption.

Long-Range Self-Hybridized Exciton-Polaritons in Two-Dimensional Ruddlesden-Popper Perovskites.

Lead halide perovskites have emerged as platforms for exciton-polaritonic studies at room temperature, thanks to their excellent photoluminescence efficiency and synthetic versatility. In this work, we find proof of strong exciton-photon coupling in cavities formed by the layered crystals themselves, a phenomenon known as the self-hybridization effect. We use multilayers of high-quality Ruddlesden-Popper perovskites in their 2D crystalline form, benefiting from their quantum-well excitonic resonances and the strong Fabry-Pérot cavity modes resulting from the total internal reflection at their smooth surfaces. Optical spectroscopy reveals bending of the cavity modes typical for exciton-polariton formation, and absorption and photoluminescence spectroscopy shows splitting of the excitonic resonance and thickness-dependent peak positions. Strikingly, local optical excitation with energy below the excitonic resonance of the flakes in photoluminescence measurements unveils the coupling of light to in-plane polaritonic modes with directed propagation. These exciton-polaritons exhibit high coupling efficiencies and extremely low loss propagation mechanisms, which are confirmed by finite difference time domain simulations. Thus, we prove that mesoscopic 2D Ruddlesden-Popper perovskite flakes represent an effective but simple system to study the rich physics of exciton-polaritons at room temperature.

Individually adjustable emission from a quantum emitter embedded in a double bowtie-bullseye plasmonic nanoantenna

Single-photon sources have a wide range of applications in fields such as quantum computing and quantum communications. These applications require single-photon sources to possess both high emission directionality and a high Purcell factor. However, currently, there is no satisfactory solution that can meet both of these characteristics. Here, we present a novel hybrid plasmonic nanoantenna that addresses this challenge. The hybrid nanoantenna comprises a bullseye antenna with a double bowtie antenna placed at its center. This design enables the simultaneous achievement of high directionality and a high Purcell factor. Specifically, the Purcell factor can reach up to 10,000, and the emitted photons are predominantly concentrated in the 0° direction in the far-field emission pattern. To characterize the hybrid nanoantenna and investigate the influence of its dimensions on directionality and Purcell factor, we conducted Finite-Difference Time-Domain (FDTD) simulations. The results demonstrate promising findings for enhancing the performance of plasmonic antennas. Moreover, this platform holds potential for the development of efficient single-photon sources, thereby facilitating advancements in photonic integrated circuits and quantum devices.

Controllable Construction of Aptamer-Modified Fe3O4@SiO2-Au Core-Shell-Satellite Nanocomposites with Surface-Enhanced Raman Scattering and Photothermal Properties and Their Effective Capture, Detection, and Elimination of Staphylococcus aureus.

The early monitoring and inactivation of bacteria are of crucial importance in preventing the further spread of foodborne pathogens. Staphylococcus aureus (S. aureus), a prototypical foodborne pathogen, is widely present in the natural environment and has the capability to trigger a range of diseases at low concentrations. In this work, we designed Fe3O4@SiO2-Au core-shell-satellite nanocomposites (NCs) modified with aptamer for efficient capture, high-sensitivity surface-enhanced Raman scattering (SERS) detection, and photothermal therapy (PTT) against S. aureus. Fe3O4@SiO2-Au NCs with tunable Au nanocrystal nanogaps were prepared. By combining the finite-difference time-domain (FDTD) method and experimental results, we studied the electric field distribution of Fe3O4@SiO2-Au under different Au nanogaps and ultimately obtained the optimal SERS substrate FSA-60. The modification of aptamer on the surfaces of FSA-60 could be used for the specific capture and selective detection of S. aureus, achieving a detection limit of as low as 50 cfu/mL. Furthermore, Apt-FSA-60 possessed excellent photothermal properties, demonstrating the strong photothermal killing ability against S. aureus. Therefore, Apt-FSA-60 is a promising high-sensitivity SERS substrate and efficient photothermal agent and is expected to be widely applied and promoted in future disease prevention and treatment.

Enhanced light extraction efficiency for the inclined-sidewall-shaped AlGaN-based DUV LED by using an omni-directional reflector with a thin hybrid dielectric layer.

In this Letter, an omni-directional reflector (ODR) with a thin hybrid dielectric layer (hybrid-ODR) is proposed to enhance the light extraction efficiency (LEE) for inclined-sidewall-shaped AlGaN-based deep ultraviolet light-emitting diode (DUV LED) by inserting a thin diamond with high refraction index into a conventional Al/Al2O3-based ODR. The three-dimensional finite-difference time-domain (3D FDTD) simulation results show that the LEE of TM-polarized light for the DUV LED with hybrid-ODR is enhanced by 18.5% compared with Al/Al2O3-based ODR. It is because the diamond can transform the evanescent wave in Al2O3 into the propagating light wave in diamond, thereby preventing effective excitation of the surface plasmon polariton (SPP) on the surface of the metal Al. Moreover, the Brewster's angle effect causes the TM-polarized light in diamond to propagate effectively into AlGaN. Furthermore, decreasing the total thickness of the dielectric layer also improves the scattering effect of the inclined sidewall. However, the utilization of hybrid-ODR results in a slight reduction in the LEE for transverse electric (TE) polarized light because the light is confined to the diamond layer and eventually absorbed by the metal Al.

Coherent coupling of localized surface plasmons and surface plasmons in borophene-based metamaterial

The strong coherent coupling in different electromagnetic modes can control the light-matter interaction more conveniently. Here, we theoretically researched the hybridization between the borophene surface plasmon (BSP) mode and the borophene localized surface plasmon (BLSP) mode in borophene grating structure. This coupling effect leads to the emergence of multiple hybrid modes. The absorption spectra of the system are investigated through finite difference time domain (FDTD) simulation and coupled oscillator model (COM). Results show that the coherent coupling of BSP and BLSP can be achieved by adjusting the carrier density of the borophene gratings. A Rabi splitting effect with frequency of 21.6 THz can be observed. Furthermore, we investigated the effects of geometric structural parameters, incident angle, and relaxation time on the correlated coupling spectra. Our work may deepen the understanding of light–matter interactions and provide a reference for borophene-based active photonic devices in the near-infrared region.

Thermotunable mid-infrared metamaterial absorption material based on combined hollow cylindrical VO2 structure

We have designed a metamaterial absorption material based on the VO2 phase transition effect. The aim is to realize thermally tunable broadband absorption in the mid-infrared band. The metamaterial absorption material is a three-layer structure. From top to bottom, the combined hollow cylindrical VO2 resonant layer, the SiO2 dielectric layer and the Ti substrate. In this work, we perform numerical simulations using the three-dimensional time-domain finite element difference method. The simulation shows that the absorption material's average absorption intensity is adjustable by temperature between 0.09 and 0.95. At a temperature of 342 K, the absorption material has an absorption bandwidth of 17.3 μm (6.43 μm to 23.73 μm) with an absorption rate above 90%. And the average absorption in this band range is 94.95%. And this band covers the wavelength of the atmospheric transparency window (8 μm∼14 μm), which suggests that our absorption material has great application prospects. In this work, we first analyze the electromagnetic field at the absorption material's resonant wavelength and explain the absorption material's absorption mechanism. Then, we investigated the absorption effect of the absorption material under the VO2 structure with different temperatures. And we explain temperature's effect on the absorption material's absorption effect by analyzing the electric field's change on the absorption material's surface at different temperatures for the same wavelength. The effect of the absorption material structural parameters on the absorption material absorption performance to determine the optimal structural parameters was then investigated. Finally, we investigate the absorption curves of the absorption material under different columnar VO2 layers and demonstrate the innovative and unique nature of the designed absorption material. The absorption material we designed has a simple structure and is easy to configure. And the absorption material has high average absorption rate and a wide absorption bandwidth. We therefore believe that the metamaterial absorption material has great application prospects in optoelectronic detection, infrared imaging and infrared stealth.