FDTD Simulations Research Articles

Photonic crystal fibers based on Dirac point-guiding

—In this work photonic crystal fibers of different cross-sectional patterns are studied. Dirac points of these various lattices are explored, propagation diagrams showing positions of the Dirac spectrums are obtained, and their application in fiber guiding is discussed. A quasi-3D FDTD simulation method, which is simpler and more efficient than a commercial 3D FDTD simulation software, is developed for photonic crystal fiber. This method is then applied to the new photonic crystal fiber proposed in [Fiber guiding at the Dirac frequency beyond photonic bandgaps, Light Sci. & App. 4 (2015) e304.]. Dirac-point guidance, which is based on Dirac localization of photons, is verified for these fibers.

Dynamically tunable highly sensitive sensor based on graphene and black phosphorus composite metamaterial

Focusing on the realization of multiple-detection-point sensors in the mid-infrared band, a dynamically tunable high-sensitive index refraction sensor based on graphene and black phosphorus (BP) composite metamaterial is proposed. By adjusting the height of the grating, the strength of the structural plasmon-induced absorption (PIA) can be enhanced within a certain range. The energy transfer mechanism of the system is analyzed by the coupled mode theory (CMT), the theoretical data of CMT fit well with the FDTD simulation results, which proves that the analysis of the system using the CMT model is correct. The resonant wavelengths of the PIA can be efficiently modulated by changing the Fermi level of graphene and the carrier density of BP. It is worth mentioning that under different environmental refractive indexes, our proposed system has excellent sensing characteristics in the mid-infrared band. The maximum sensitivity and the maximum figure of merit (FOM) are up to 5.174 μm/RIU and 26.449, respectively. This research could play an important role in mid-infrared optical sensors.

Synergistic antibacterial effects of selenium-doped gold nanosheets with notable near infrared-II photothermal conversion against multidrug-resistant bacteria

Non-metallic nanoparticles are increasingly acknowledged for their broad-spectrum antibacterial properties, which often face limitations due to structural instability and potential biotoxicity. To address these challenges, integrating non-metallic nanoparticles with plasmonic metal nanostructures offers a promising solution. This fusion not only enhances structural stability but also reduces the required dosage. The synergistic coupling of the inherent antibacterial properties of non-metallic nanoparticles with the plasmonic absorption and photothermal conversion capabilities of metal nanostructures significantly amplifies antibacterial efficacy. In this study, we validate this concept by presenting the successful synthesis of selenium-doped gold nanosheets (AuSe NSs) that demonstrate robust absorption in the second near-infrared (NIR-II) window and exhibit efficient photothermal conversion, followed by testing their antibacterial activity. Notably, Au NSs were synthesized with high shape purity, followed by the introduction of SeO2 and ascorbic acid to facilitate Se doping. The resulting products displayed a uniform distribution of both Au and Se elements, while maintaining the NIR-II absorbance characteristic of Au nanosheets unaffected by the Se doping as evidenced by both experimental and FDTD simulation results. Subsequent assessments of the AuSe NSs demonstrated their potent synergistic antibacterial effects against multidrug-resistant bacteria at minimal inhibitory concentrations (MICs), which suggests that lower dosages can be employed and benefit minimizing potential side effects and optimizing treatment efficiency. Our findings indicate a synergistic boost between the photothermal attributes originating from Au's NIR absorption and Se's inherent antibacterial properties—an aspect minimally explored in prior research and offering a novel solution to tackling antibiotic-resistant bacterial strains.

Harnessing surface plasmon polaritons from nanogroove plasmonic lenses to enhance the EM fields around plasmonic nanoantennas

In this paper, we present a device—consisting of a bowtie nanoantenna surrounded by either an asymmetric plasmonic nanogroove or a symmetric plasmonic nanogroove—that allows improved excitation of the bowtie nanoantenna via interaction between the bowtie nanoantenna and the surface plasmon polaritons (SPPs) generated from the nanogrooves. We study the effect of both a linear plasmonic nanogroove as well as a circular plasmonic nanogroove (i.e., a circular nanogroove plasmonic lens) on the EM fields around a bowtie nanoantenna placed at a certain distance from the nanogroove. We show that the electric field enhancement of a bowtie nanoantenna can be significantly improved by the interaction between the bowtie nanoantenna and the SPPs generated from the nanogrooves. We employed FDTD simulations to calculate quantities such as the electric field enhancements and power coupling into SPPs as well as a finite difference eigenmode solution to obtain dispersion relation of the nanogrooves. The geometrical parameters of the symmetric nanogroove and asymmetric nanogroove were optimized to couple maximum light into SPPs. We show that the bowtie nanoantenna surrounded by a single asymmetric nanogroove plasmonic lens produces a SERS electromagnetic enhancement factor (EMEF) of 1010—even when the gap between the arms of the bowtie nanoantenna is as large as 10 nm—which is three orders of magnitude higher than SERS EMEF of a stand-alone bowtie nanoantenna and one order of magnitude higher than the SERS EMEF of a bowtie nanoantenna surrounded by a single symmetric nanogroove plasmonic lens. In addition, the effect of the radius of nanogroove plasmonic lenses is studied. The calculation of collection efficiencies of Raman signal from the proposed nanostructures shows that ∼62% Raman signal can be collected from the bowtie nanoantenna surrounded by a symmetric nanogroove plasmonic lens or an asymmetric nanogroove plasmonic lens (PL) compared to 10% Raman signal collected from only a bowtie nanoantenna on a silica substrate.

Anisotropic growth of Ag nanonails induced by plasmon-mediated chemical reactions in cross-shaped nanocavity array

The Ag nanonail rooted in Au cross-channel nanocavity is fabricated by plasmon-mediated chemical reactions (PMCRs). The Au cross-channel nanocavity is prepared based on two-dimensional polystyrene (PS) nanosphere template through wet etching, and the shape of Au cross-channel nanocavity changes from trapezoidal to triangular, depending on the wet etching time. The growth of Ag nanostructures in Au channels by PMCRs indicates the morphology of Ag nanostructures has changed from Ag nanoparticles to nanonails, which is regulated by the shape of Au nanocavities and the exciting light polarization. The hotspots distributions results in the anisotropic growth of Ag nanostructures induced by PMCRs, which leading to the different Ag nanostructures depending on the shape of Au nanocavity and light polarization, as confirmed by FDTD simulations. In comparison with others reports, our nanocavity provide more parameters to control the shapes of Ag nanostructures, the nanocavity shape, the light polarization and the growth time. Our work provides a profound insight into the construction of periodic nanostructures and reconstruction of hotspots.

Modeling aerosol transmission spectra from n(λ) and k(λ) infrared optical constants measurements of organic liquids and solids

The effects of light scattering and refraction play significantly different roles for aerosols than for bulk materials, making it challenging to identify aerosolized chemicals using traditional spectral methods or spectral reference libraries. Due to a potentially infinite number of particle morphologies, sizes, and compositions, constructing a database of laboratory-measured aerosol spectra is not a practical solution. Here, as an alternative approach, the measured n/k optical vectors of two example organic materials (diethyl phthalate and D-mannitol) are used in combination with particle absorption / scattering theory (Mie theory and FDTD) and the Beer-Lambert law to generate a series of synthetic infrared transmission / scattered light spectra. The synthetic spectra show significant differences versus simple slab transmission spectra, even for small changes in particle size (e.g., 5 vs. 10 µm) for both single particles and ensembles, potentially serving as useful reference data for aerosol sensing. For spherical single particles with diameters of 1 to 10 µm, FDTD simulations predict changes in the magnitudes of spectral shifts and the shapes of the peaks vs. particle size with only small deviations from Mie theory predictions, yet reliably capture the direction of the shifts. Typical spectral peak shifts in the longwave infrared correspond to Δλ ∼0.20 µm (∼34 cm-1) when compared to corresponding slab transmission spectra. Additionally, synthetic spectra generated from the n/k values derived using two different methods (KBr pellet transmission and single-angle reflectance) are compared using the Mie theory model.

Broadband light absorption enhancement in a–Si:H ultrathin film solar cells with nanophotonic light trapping structures

Light trapping over a wide wavelength range is critical for applications such as thin-film solar cells (TFSCs). Therefore, in this paper, we report a plasmonic light trapping scheme as a solution to overcome the limited absorption of light in hydrogenated amorphous silicon (a-Si:H) thin film solar cells. Here we propose plasmonic nanostructures, which have periodic nanodisks added to the front surface of the cell. These structures combine a variety of photonic effects to reduce reflection, improve light trapping, and increase absorption over a broadband solar spectrum. Initially, to test whether the suggested plasmonic nanoparticles can enhance light-trapping within the absorber layers of the cell, we used the 3D finite difference time domain method to simulate the propagation of electromagnetic waves and the absorbed light in each layer of the cell. Additionally, the obtained optical properties after the FDTD simulation are used to design electrical model simulations using SCAPS software. The results show that the front surface of nanodisk-shaped solar cells with only a 100-nm-thick a-Si:H layer can absorb light at wavelengths ranging from 300 to 800 nm, which is greater than that of flat a-Si:H film devices. With optimized structural parameters, the short circuit current density Jsc could be enhanced by 98 % and 71 % with the addition of Au and Cu nanodisks, respectively. Additionally, the conversion efficiency is improved by 28 % and 26 % for Au and Cu, respectively. The proposed design of the light trapping structure can provide guidelines for ultra-TFSCs with high performance.

A method to efficiently and rapidly approximate the vectorial fields generated by large area metasurfaces

In order to calculate the electromagnetic fields that are produced after light passes through a metasurface, simulation methods such as the Finite-Difference Time-Domain method are often employed. While these provide a good approximation to the fields, the level of detail at which the volume of space that the light is propagating in needs to be modelled and the time for which simulations need to run, mean that as the area of the metasurface is increased these simulations rapidly become unwieldy. In this paper we show how the result of a FDTD simulation of a unit cell can be used to generate a good approximation of the vectorial field that large area metasurfaces will generate, but using a fraction of the computational resources. This approach can provide an intermediate design step, allowing potentially interesting designs to be rapidly identified or discarded.

Active manipulation for Goos-Hänchen shift of guided-wave via a metasurface of silicon-nanoscale semi-spheres on SOI waveguide

Goos-Hänchen shift of total internal reflection (TIR) is the light beam movement without external driving, so envisioned to have potential manipulation of optical beams. In this article, with a silicon-on-insulator (SOI) waveguide corner structure, a variable equivalent permittivity of guided wave is modelled, and then the equivalent electric polarizabilities and the Goos-Hänchen shift of guided wave are modelled. Consequently, with a 2.0-µm SOI waveguide corner structure and an abrupt phase change of ∼0.5π caused by a vertically inserted metasurface of nanoscale semi-spheres having a 450-nm radius can reach the GH shifts of 2.1 µm for TE- and TM-mode, respectively, which are verified by both the FDTD simulation results of 1.93 µm with a reflectivity of about 62% and the experimental results of 2.0 µm with ∼60%. Therefore, this work has efficiently modelled the optical feature response of semi-sphere metasurface to guided wave and the active manipulation for the GH shifts of guided-wave, opening more opportunities to develop the new functionalities and devices for Si-based photonic integrated circuit (PIC) applications.

Sensitive surface plasmon resonance biosensor by optimized carboxylate functionalized carbon nanotubes/chitosan for amlodipine detecting

The adoption of biophotonic sensing technologies holds significant promise for application in health care and biomedical industries in all aspects of human life. Then, this piece of writing employs the powerful effective medium theory and FDTD simulation to anticipate the most favorable state and plasmonic attributes of a magnificent nanocomposite, comprising carboxylate functionalized carbon nanotubes and chitosan (CS). Furthermore, it thoroughly explores the exhibited surface plasmon resonance behaviors of this composite versus the quantity of CS variation. Subsequently, enlightening simulations are conducted on the nanocomposite with a delicate layer and a modified golden structure integrating as a composite. The intricate simulations eventually unveil an optimal combination to pave the way for crafting an exceptional specific biosensor that far surpasses its counterpart as a mere Au thin layer in terms of excellence. The proposed biosensor demonstrated linear behavior across a wide range from 0.01 μM to 150 μM and achieved a detection limit of 10 nM, with a sensitivity of 134◦RIU−1.

Control of the coupling degree between surface plasmon and microcavity mode in dual-cavity OLEDs

The metal/dielectric/metal (MDM) multilayer resonator is of increasing interest because of its functionality of surface plasmons generated at the interfaces. We aimed to control the EL spectra of OLED with an MDM electrode by actively utilizing the plasmon-microcavity coupling. The change in EL spectra of OLED with MDM electrode was simulated depending on the design of MDM structures by FDTD simulations. The strength of the plasmon-microcavity coupling can be controlled by the thickness of the Ag film in AZA, which can determine the tuning range of the EL peak position.

Utilizing waste corn straw to photodegrade methyl orange and methylene blue: Photothermal effect of biochar enhances photodegradation efficiency

The burning of corn straw will cause serious air pollution, and the utilization of corn straw can avoid the burning of corn straw and reduce environmental pollution. The pyrolysis of corn straw into biochar (BC) is a promising approach to apply corn straw in environmentally sustainable development. Dye wastewater such as methyl orange (MO) and methylene blue (MB) is a pollution that poses a threat to human health and the environment. Herein, biochar/TiO2 (BC/TiO2) and biochar/ZnO (BC/ZnO) were prepared by pyrolysis of corn straw for the photodegradation of MO and MB in water. The MO removal rates of BC/TiO2 and BC/ZnO are up to 83.3 % and 76.2 %, respectively, and the MB removal rates of BC/TiO2 and BC/ZnO are 78.6 % and 71.4 %, respectively. The photothermal effect of BC improves the surface temperature of photocatalysts and promotes the separation and transfer of photogenerated electron-hole pairs, enhancing the photodegradation of MO and MB efficiencies. Three-dimensional (3D) FDTD simulations were performed to investigate the mechanism of photothermal effect in BC/TiO2 and BC/ZnO. This work provides a promising approach to the utilization of corn straw.

Polarization‐Enabled Tuning of Anapole Resonances in Vertically Stacked Elliptical Silicon Nanodisks

This work presents the polarization‐dependent behavior of the anapole state in stacked amorphous silicon (a‐Si) nanodisks with elliptical geometries. Using SiO2 as a spacer layer between the a‐Si disks, the high index contrast between these materials can be used to significantly reduce the fabrication complexity of the system compared to traditional methods that require additional etching of the spacers. A novel way of continuous tuning of the electric dipole anapole excitation within elliptical stacked a‐Si nanoresonators is demonstrated. By rotating the incident electric field's polarization angle, the anapole state can be selectively excited at two distinct wavelength positions separated by 80 nm. Experimental results show characteristic dips in the reflectance of the fabricated elliptical a‐Si stacks with wavelength positions between 1135 and 1217 nm depending on the polarization angle of the incident field which is corroborated by FDTD simulations. Through simulating the internal electric field in the resonators and using multipole decomposition, it is shown that the reflectance dips are due to anapole excitation in the individual disks. The capability to excite anapoles at two distinct wavelengths in the same structure has promising implications for the development of tunable sensors, frequency converters, and quantum memory applications.

Fiber end integrated surface plasma lens for circular airy beam shaping

This paper presents a circular Airy beam generator based on a fiber end integrated surface plasma lens. The surface plasma lens consists of a nano-annular slot and an array of series concentric circular grooves etched on the gold film coated on the fiber end. When the fiber light field illuminates the nano-annular slot, surface plasmon polaritons (SPPs) will be excited, then the SPPs propagates along the surface of the gold film, and will finally be decoupled into free space to generate circular Airy beam by the array of concentric circular grooves. The effect of the parameters of the plasma lens on the properties of the output circular Airy beam, such as self-focusing, focal spot size, is studied in detail via FDTD simulations. In addition, we found that the proposed plasma lens has a high tolerance to manufacturing errors. The case that instead of the nano-annular slit with a circular wheat-spike shaped structure is also investigated. In this case, due to the different photon spin response of the circular wheat-spike shaped structure, the device can generate circular Airy beam when the input fiber light field is right-handed circularly polarized (RHCP) light, and subwavelength Bessel-like nondiffracting beam when the input fiber light field is left-handed circularly polarized (LHCP) light. These results provide a highly integrated all-fiber circular Airy beam and subwavelength Bessel-like nondiffracting beam generation scheme, which may be useful in the areas of fiber end structured light beam shaping and fiber-integrated photonic devices.

Synthesis, characterization, and FDTD simulations of Ag-enriched RGO nanosheets for catalytic reduction of 4-nitrophenol

Reduced graphene oxide-based nanocomposites are eminent materials having diverse applications including environmental remediation. The present work emphasizes the facile one-step co-reduction method for synthesizing silver nanoparticle (Ag NPs) decorated reduced graphene oxide (RGO) for catalytic reduction of 4-nitrophenol. FDTD simulation studies justify the experimental results, and XRD studies confirmed the reduction of graphene oxide and the formation of Ag NPs with reduced graphene oxide (AgRGO) composite. Raman analysis complements the structure, crystallinity, and defects in the fabricated material. XPS analysis verifies the reduction of graphene oxide (GO) into RGO and the decoration of metallic Ag on the surface of RGO. FESEM image showed the decoration of Ag NPs on the surface of RGO. AgRGO exhibited appreciable catalytic performance for reducing 4-nitrophenol in the presence of sodium borohydride compared to GO and RGO.

InGaN‐Based Whispering Gallery Mode Laser with Lateral Nanoporous Distributed Bragg Reflector Exhibits Superior Mode Selectivity

AbstractWhispering gallery mode (WGM) laser with single‐mode is important for optoelectronic applications because of the higher stability during the light signal transmission. But till now, for InGaN‐based WGM laser, most reports of lasing behavior are multimode lasing. To achieve single‐mode WGM laser, an InGaN‐based micro‐ring laser is proposed and demonstrated with lateral nanoporous distributed Bragg reflector (DBR), where the high‐order lasing modes are strongly suppressed. The side mode suppression ratio (SMSR) is more than 9 times larger than that of the micro‐disk laser with the same radius. Combined with FDTD simulation, the results show that the enhancements are originated from the suppression of the high‐order resonant mode because of the inner boundary of micro‐ring. This work provides a new solution to achieve stable single‐mode WGM laser for future photonic integration.

Physics‐informed surrogates for electromagnetic dynamics using Transformers and graph neural networks

AbstractA novel use case for two data‐driven models, namely, a Transformer and a convolutional graph neural network (CGNN) is proposed. The authors propose to use these models for emulating the dynamics of electromagnetic (EM) propagation and scattering. The Transformer translates a past sequence into a future sequence by constructing representations from the past and using it to predict the future, taking all of its own previous predictions as input at each step of prediction. The CGNN updates the current state of attribute vectors of each node by passing it information (messages) from all of its neighbouring nodes. We train these models with FDTD simulations of plane waves propagating and scattering from PEC objects. The authors demonstrate that, within the bounds of computational resources, the Transformer can be utilised as a surrogate for EM dynamics, providing 14× speed‐up, while the CGNN can be utilised as a next‐frame predictor, providing 9× speed‐up. When comparing the accuracy of these two models with the authors’ previously developed Encoder‐Recurrent‐Decoder (ERD) model, it is observed that the error for both the Transformer and the CGNN remains within the same bound for the ERD model. To the best of the authors’ knowledge, this work is the first to utilise the Transformer as a surrogate for EM dynamics.

Remote-controllable refractive-index-sensitive plasmonic waveguide and rake-like switch: designs and FDTD simulations

A refractive-index-sensitive plasmonic waveguide, whose transmission characteristics could be controlled remotely by a rake-like switch design, is theoretically studied in the reported work. The distance from the remote control unit to the bus waveguide is more than 0.25 μm, and it still possesses great efficiency even when the distance is increased to 0.535 μm. The switch basically contains two main sections. The first is on the bottom and next to the bus waveguide which functions as a plasmonic resonator that can induce localized surface plasmon resonance (LSPR) and restrict wave propagation at corresponding resonant frequency. The second is on the top and far away from the bus waveguide which functions as a remote controller that can modulate LSPR frequency in the first section. The refractive-index-dependent transmission spectra of this filter were simulated using finite-difference time-domain method. The results have shown that even at a distance as far as 0.5 μm, the ON/OFF switching of the wave propagation in a bus waveguide can still be modulated by adjusting the refractive index of a remote rectangular controller. With only 0.08 difference in refractive index, it could be obtained an on–off switching ratio of 18.7, 20.4 and 25.7 respectively for different waveguides at visible and near infrared wavelength, which shows great potential applications in refractive index sensors and remote-controllable band-stop filters.

Optimum Parameter Selection for Accurate FDTD Simulations in Dispersive Media

Optimum Parameter Selection for Accurate FDTD Simulations in Dispersive Media

Fabrication of multifunctional g-C3N4-modified Au/Ag NRs arrays for ultrasensitive and recyclable SERS detection of bisphenol A residues.

Monolayer g-C3N4-modified Au/Ag nanorods (g-C3N4/Au/Ag NRs) array is fabricated as a dual-function platform with high surface-enhanced Raman scattering (SERS) response and excellent photocatalytic degradation ability for bisphenol A (BPA) residues. FDTD simulation results of Au/Ag NRs proves that the electromagnetic field intensity is significantly enhanced at the gap of Ag NRs and Au NPs and the protrusion of Au NPs, which endows the arrays with excellent SERS activity. The arrays exhibit high sensitivity for rhodamine 6G (R6G) (LOD = 1.1 × 10-11 mol/L) and high SERS enhancement (EF = 9.2 × 107). In addition, the g-C3N4/Au/Ag NRs could degrade ˃90% of BPA adsorbed on the substrate surface within 140 min under visible light irradiation, and maintains its SERS activity after repeated use for 4 times. The dual-function platform with high SERS response and excellent recycling capability is proved to be reliable and is very promising for monitoring of BPA residues in food.