Finite-difference Time-domain Simulation Method Research Articles

Simulations of Femtosecond-Laser Near-Field Ablation Using Nanosphere under Dynamic Excitation.

Femtosecond lasers have garnered widespread attention owing to their subdiffraction processing capabilities. However, their intricate natures, involving intrapulse feedbacks between transient material excitation and laser propagation, often present significant challenges for near-field ablation predictions and simulations. To address these challenges, the current study introduces an improved finite-difference time-domain method (FDTD)-plasma model (plasma)-two-temperature model (TTM) framework for simulating the ablation processes of various nanospheres on diverse substrates, particularly in scenarios wherein dynamic and heterogeneous excitations significantly influence optical-field distributions. Initially, FDTD simulations of a single Au nanosphere on a Si substrate reveal that, with transitions in the excitation states of the substrate, the field-intensity distribution transforms from a profile with a single central peak to a bimodal structure, consistent with experimental reports. Subsequently, simulations of a polystyrene nanosphere array on a SiO2 substrate reveal that different excitation states of the nanospheres yield two distinct modes, namely near-field enhancement and masking. These modes cannot be adequately modeled in the FDTD simulations. Our combined model also considers the intrapulse feedback between the electromagnetic-field distribution resulting from near-field effects and material excitations. Furthermore, the model can quantitatively analyze subsequent electron-phonon coupling and material removal processes resulting from thermal-phase transitions. Consequently, our model facilitates predictions of the femtosecond-laser ablation of single nanospheres or nanosphere arrays with varying sizes and materials placed on substrates subjected to near-field effects.

Grating assisted temperature insensitive micro-ring resonator biosensor

A grating-assisted temperature-insensitive micro-ring resonator bio-sensor is reported using the finite-difference time-domain simulation method. In addition to choosing a negative thermo-optic coefficient (TOC) material, water, to nullify temperature-induced wavelength shift, optimization of the opto-geometric parameters provided a precisely zero-temperature sensitivity (TS) state of the structure. The negative TOC of water counterbalances the positive shift in resonance wavelength originating from Si waveguides. The grating structure delocalizes the core field into the analyte region, which enhances field–matter interaction to offer high bulk refractive index (RI) and affinity sensing. The advantage of incorporating grating structure in the ring has been demonstrated by comparing the bulk RI sensitivity and affinity sensitivity with and without the grating structure. Without the circular grating array, the bulk RI sensitivity is 214 nm RIU−1, and the affinity sensitivity is 0.139 nm nm−1. Using grating in the ring structure offers an improved bulk RI sensitivity of 525 nm RIU−1 and an adlayer sensitivity of 0.515 nm nm−1. We calculated the system limit of detection of the structure as ∼2 RIU. The fabrication tolerance of geometrical parameters, in the form of 10% variation in hole radius and 5% variation in other widths of our sensor, reveals a nominal TS of ∼6 pm ∘C−1. Changing the cladding analyte from pure water to other aqueous solutions, e.g. urine samples, also has a small impact on TS. The present work is useful in RI-dependent monitoring of the constituents present in aqueous medium-based bio-analytes samples in a variable ambient temperature environment.

Graphene Multi-Frequency Broadband and Ultra-Broadband Terahertz Absorber Based on Surface Plasmon Resonance

When surface plasmon resonance (SPR) occurs, the incident light is absorbed by the surface of the SPR structure, thus minimizing the intensity of the reflected light. Therefore, the SPR method is adopted in this paper to achieve perfect absorption of the absorbent. In this paper, we first propose a multi-frequency broadband absorber structure based on graphene SPR, which uses the continuous resonance of patterned graphene surface plasmon in the frequency spectrum to form a multi-frequency broadband absorption. In this simulation, a sandwich-stack structure was adopted, whereby the patterned graphene is situated on top of the SiO2 layer and the metal layer. The broad-band absorption bands of the absorber were obtained as 4.14–4.38 THz, 5.78–6.36 THz, and 7.87–8.66 THz through the analog simulation of finite-difference time-domain method (FDTD) solutions. Then, based on the multi-layer resonant unit structure, through the superposition and combination of absorbing units responding to different frequency bands, the perfect absorption of ultra-wideband is achieved. The data results illustrate that the total absorption bandwidth of the absorber is 2.26 THz, and the relative absorption bandwidth Bw is equal to 28.93%. The electric field in X-Y direction of the absorber in the perfect absorption band is analyzed, respectively, and the dynamic tunability of the absorber is studied. Finally, we studied whether the absorbing structure still has efficient absorption characteristics for the two polarization modes when the incident angle is changed from 0° to 70°. The structure model proposed has potential value for application in terahertz photoelectric detection, filtering, and electromagnetic shielding.

All-optical control of ultrafast plasmon resonances in the pulse-driven extraordinary optical transmission

Understanding ultrafast processes in their natural timescale is crucial for controlling and manipulating nanoscale optoelectronic devices under light–matter interaction. Here, we demonstrate that ultrafast plasmon resonances, attributed to the phenomenon of extraordinary optical transmission (EOT), can be significantly modified by tuning the spectral and temporal properties of the ultrashort light pulse. In this scheme, all-optical active tuning governs the spatial and temporal enhancement of plasmon oscillations in the EOT system without device customization. We analyze the spectral and temporal evolution of the system using two approaches. First, we develop a theoretical framework based on the coupled harmonic oscillator model, which analytically describes the dynamics of plasmon modes in the coupled and uncoupled states. Later, we compare the evolution of the system under continuous-wave and pulsed illumination. Further, we discuss the time-resolved spectral and spatial dynamics of plasmon modes using a 3D finite difference time-domain simulation method and wavelet transform. Our results show that optical tuning of the oscillation time, intensity, and spectral properties of propagating and localized plasmon modes yields a three-fold enhancement in the EOT signal. The active tuning of the EOT sensor through ultrashort light pulses paves the way for the development of on-chip photonic devices employing high-resolution imaging and sensing of abundant atomic and molecular systems.

Design of Tunable Far-Infrared Plasmonic Absorber Based on Chalcogenide Phase Change Materials

We propose a wide-angle metamaterial absorber with more than 90% absorption in the far-infrared (F-IR) and terahertz (THz) regimes. Our metal-dielectric metamaterial absorber consists of a phase change layer (Ge2Sb2Te5), a dielectric spacer (MgF2), and a bottom refractory metal layer (TiN). We numerically designed the structure by finite-difference time-domain simulation method and demonstrated a perfect absorption in the spectral range from 10 μm to 50 μm (30 THz to 6 THz). Furthermore, it shows a broad peak with maximum absorption of 93% at the resonant wavelength of 22.5 μm when the phase change layer is in the amorphous (disorder) state. In contrast, the peak resonance is red-shifted to 29.5 μm when the Ge2Sb2Te5 switches to the crystalline (order) state, demonstrating a resonant band tunability of Δλ= 7μm. The proposed structure shown here is a simple planner structure, lithographic-free and easy to fabricate with spectral band tunability, which offers great potential for ultra-cooled detection, imaging, security scanning, gas leakage detection, and remote monitoring applications.

Multiple poles resonances coupling with high sensitivity sensing for multiple bulging black phosphorus-based metasurface

Herein, a multiple bulging black phosphorus (BP)-based metasurface is proposed for studying its reflection responses and sensing performances through the finite-difference time-domain simulation method. It is shown that, the reflection dips are caused by the coupling between dipole resonance modes and poly-poles resonance modes. Moreover, the dipoles resonance modes and poly-poles resonance modes can mutually enhance and inhibit each other, and tunable reflection spectra can be realized by symmetrically and asymmetrically adjusting the bulging of the proposed BP-based metasurface. In addition, the reflection spectra as a function of the polarization of incident light are discussed. We can find that a dipole resonance mode on the vertical side at the direction of ZZ for BP is gradually fully excited, resulting in an additional obvious reflection dip as the polarization angle θ increases from 0° to 90°. Especially, the sensing performance with the maximum of sensitivity S = 1.5 μm/RIU can be realized in the proposed BP-based metasurface. The results may provide a way to design micro-nano plasmonic devices.

Experimental demonstration of inverse-designed silicon integrated photonic power splitters

Abstract The on-chip optical power splitter is a common and important device in photonic integrated circuits (PICs). To achieve a low insertion loss and high uniformity while splitting the guided light, multi-mode interferometer-based structures utilizing a self-imaging principle are widely used mainly in the form of a 1 × 2 configuration. Recently, an inverse design method for nanophotonic devices has emerged to overcome the limited capability of the conventional design methods and make it possible to explore the vast number of design parameters. Because of the non-intuitive shape of inverse-designed structures, they allow us to discover interesting and complex optical responses which are almost impossible to find with conventional design methods. Here, we report two kinds of inverse-designed 1 × 4 optical power splitters composed of silicon bars of different lengths, which are fabricated with a standard CMOS-compatible process. The particle swarm optimization method was used to minimize the insertion loss and divide the power evenly into each output port with finite-difference time-domain method simulation. The first optical power splitter has a compact size of 8.14 × 12 μm2 and the second optical power splitter has an even more compact size of 6.0 × 7.2 μm2. With the inverse designed structures, we fabricated the chip with a CMOS-compatible fabrication process. Experimental verification of the structures is provided and good agreement with the numerical results is obtained. The first 1 × 4 optical power splitter has a low insertion loss of less than 0.76 dB and uniformity of less than 0.84 dB, and the second more compact optical power splitter has a low insertion loss of less than 1.08 dB and uniformity of less than 0.81 dB. As the complexity of on-chip photonic systems has steadily increased, the inverse design of photonic structures holds great potential to be an essential part of advanced design tools.

Design and research of multiparameter tunable optical filter based on phase-change Bragg grating

On-chip filters are widely used in integrated optical systems and are indispensable for expanding the communication capacity. A silicon waveguide filter is fabricated into a subwavelength Bragg grating structure using the optical phase-change material Ge2Sb2Se4Te1 (GSST). Three-dimensional finite-difference time-domain method simulations showed that this structure performed multichannel filtering in the communications C-band (1530 to 1565 nm), demonstrating a minimum insertion loss of 0.238 dB and a full width at half-maximum of 3.4 nm with an incident TE0 mode. The number and width of channels can be adjusted using structural parameters. Furthermore, the overlapping frequencies of passbands and stopbands are obtained by the phase transition of GSST. This structure could potentially find use as an optical signal modulation device in on-chip integrated systems.

Numerical Study of Periodic Composite Structure Absorber Based on MoS2 Material

In this paper, a broadband absorber with a periodic composite structure based on numerical simulation of finite-difference time-domain method (FDTD) is proposed. The absorber base is titanium (Ti), on which iron tetroxide (Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> ) nanospheres are laid. The nanospheres are coated with molybdenum disulfide (MoS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ) and lanthanum fluoride (LaF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> ) composites, respectively, and are periodically arranged in symmetrical rectangular arrays. The numerical analysis results show that the absorber designed in this paper had an average absorption rate of 95.00% in the light wavelength range of 200 nm to 2500 nm, with the absorption bandwidth reaching 2300 nm, which was a significant improvement compared with other absorbers, and 100.00% absorption was achieved at 300 nm and 650 nm. According to the analysis of the coupling effect of the surface plasmon resonance (SPR) and the localized surface plasmon resonance (LSPR), the ultra-broadband absorber achieved perfect absorption performance. The research results indicate that the absorber proposed in this paper is insensitive to light polarization, has a wide-angle characteristic, and is easy to realize recycling. Therefore, it has a great potential value for the application in photothermal conversion, seawater desalination, and rapid collection of sea salt.

Optoplasmonic MOFs film for SERS detection

Optoplasmonic hybrid structures composed of photonic and plasmonic elements with excellent optical properties are of great significance for the development of surface-enhanced Raman spectroscopy (SERS) substrates. In this work, the optoplasmonic hybrid structure is composed of SiO2 microsphere and two-dimensional (2D) plasmonic- metal organic frameworks (MOF) film. Among them, the 2D plasmonic-MOF film is prepared from silver nanoparticles encapsulated by zeolitic imidazole acid framework (AgNP@ZIF-8) by self-assembly method. This optoplasmonic hybrid structure with gas adsorption properties could be used as a SERS substrate for 4-Mercaptophenol (4-MP) gas detection. Experimental data show that this substrate is dependent on the thickness of the ZIF shell and the size of the SiO2 microspheres. In addition, it is confirmed by the electromagnetic field simulation of finite-difference time-domain method (FDTD). The optoplasmonic hybrid microstructures exhibit good uniformity for detection of 4-MP gas molecules. This work not only broadens the understanding of our optoplasmonic hybrid structure, but also has broad application prospects in SERS and gas sensing related fields.

Engineered near- and far-field optical response of dielectric nanostructures using focused cylindrical vector beams

Near- and far-field optical properties of silicon nanostructures under linear polarization (Gaussian beam) and azimuthally or radially focused cylindrical vector beams are investigated by finite-difference time-domain method (FDTD) in Meep open-source software. A python toolkit allowing FDTD simulations in Meep for using those excitation sources is provided. In addition to the preferential excitation of specific electric or magnetic resonance modes as a function of the excitation beam polarization, it is shown in the case of spheroids that shape anisotropy affects the resonance wavelength and the dipole orientation of the magnetic or electric dipole mode. Depending on the spheroid symmetry axis with respect to the electric field orientation, the electric dipole resonance can be split into two peaks, giving quasi-unidirectional scattering, separated by an anapole mode. The optical properties in both far-field (scattering pattern) and near-field (electric and magnetic field hot spots) can be tuned by changing the excitation polarization at a fixed wavelength and selecting properly the spheroid shape and dimensions. These numerical simulations are extended to top-down fabrication-friendly nanostructures such as nanocylinders with circular or elliptic sections.

Analyses of the light extraction efficiency of organic light-emitting diodes with ITO/mesh-Ag/ITO anode and its optical transmittance as transparent electrode

We have used the three-dimensional finite-difference time-domain simulation method to analyze the out-coupling efficiency of organic light-emitting diodes (OLEDs) with an ITO/Mesh-Ag/ITO anode. The period and grid space of the meshed Ag structure effectively affected the extraction ratio (ER) of the bottom-emitting OLED. The 3D simulation results for a simple structure of OLED with an ITO/Mesh-Ag/ITO anode showed the ER of about 26.4% and it enhanced 2.26 times that of the continuous Ag layer of about 11.7%. In addition, in respect to the transparent electrode of the OLED using the ITO/Mesh-Ag/ITO multiple layers, the Ag thickness and open ratio were important factors that affected the transmittance and the electrical conductance. The open ratio of 60% resulted in sheet resistance of less than 25 Ω/□, while keeping the transmittance nearly similar to that in the case of a single ITO layer.

Implementation of polarization rotation using microring resonator and its application to design all-optical switch

A compact microring resonator (MRR) and its implementation for polarization rotation-based all-optical switching are proposed. The polarization rotation has been achieved by enhancing geometry-induced birefringence of the device and with the insertion of polarization rotator in the race-track-shaped MRR. The proposed device has been modeled using Jones matrix and the finite-difference time-domain simulation method. The insertion loss and the extinction ratio have been found to be −0.13 and 19.39 dB, respectively.

FDTD simulation on transmittance of silica microsphere thin films with varying embedding in an optical adhesive

Moth-eye inspired nanostructure is widely used in optical applications. In this numerical study, a new type of anti-reflection film is designed using finite difference time domain method (FDTD). A three-dimensional model is established, simulating a monolayer of close-packed silica nanoparticle film embedded in a layer of polymeric optical adhesive. FDTD simulation is carried out in the visible band of wavelength, in order to investigate the transmittance of light propagation through the silica nanoparticle and adhesive layer. The effect of size of nanostructure as well as the embedding degree of optical adhesive are compared on different substrates, including visible transparent substrates with lower refractive index, i.e., fused silica glass, PMMA, PS, and visible opaque silicon substrate, with high refractive index. The numerical results reveal that we can obtain the best optical performance with nanoparticle size of 200 nm in diameter, while the immersion degree has different effects on different substrates.

Nonpolarizing narrow band metamaterial transmission filter based on electromagnetically induced transparency at visible wavelengths

A nonpolarizing narrow band metamaterial transmission filter based on EIT operating at visible wavelengths is demonstrated theoretically and experimentally. The two separated resonance components that are able to generate EIT effect are connected using a SiO2 film layer and the hardness of the metamaterial filter is further improved by adding a glass substrate. The theoretical results are calculated by using the finite-difference time-domain simulation method and a metamaterial filter sample is fabricated and measured to verify the simulation spectrum. The full width at half-maximums of simulation and measured spectra are 0.74 nm and 1.61 nm respectively. Electric field distributions are also calculated to illustrate the physical mechanism. The novel method might be helpful to improve the processability of the narrow band metamaterial transmission filter, and also to further promote the applications of this kind of metamaterial filter.

A balsam pear-shaped CuO SERS substrate with highly chemical enhancement for pesticide residue detection.

Simple and traditional hydrothermal fabrication of a novel balsam pear-shaped CuO with high SERS enhancementis presented. XRD (X-ray diffraction), SEM (scanning electronic microscopy), TEM (transmission electron microscope), HRTEM (high-resolution transmission electron microscope), UV-Vis, and Raman are adopted to ensure that this balsam pear-shaped CuO with dense nanoparticle protuberance is successfully prepared. The LOD of this CuO SERS substrate is 4.79μgL-1 with R6G as molecular probe. By using DFT (density functional theory) calculation and FDTD (finite difference time domainmethod) simulation, both EM (electromagnetic enhancement) and CM (chemical enhancement) mechanisms are investigated, and the results show that these two-enhancement mechanisms can coexist in this balsam pear-shaped CuO. Finally, the prepared substrate has been applied for the determination oftrace levels of paraquat in solution, and results show that its LOD for paraquat is275μgL-1 (optimum Raman band: 1646cm-1 Raman shift), which is better than the government standard in China. Adexterous and facile way for fabrication of CuO SERS-active substrates with low cost and high performance, quite promising in detection of chemically hazardous substances and pesticide residueis provided. Graphical abstract.

Light Extraction Efficiency of GaN-Based Micro-Scale Light-Emitting Diodes Investigated Using Finite-Difference Time-Domain Simulation

We conducted a systematic investigation of the light extraction efficiency (LEE) of GaN-based vertical micro-scale light-emitting diode (μ-LED) structures using three-dimensional finite-difference time-domain (FDTD) simulations. The LEE of μ-LED structures was found to have a strong dependence on structural parameters such as the shape of the chip cross section, chip dimension, and p-GaN thickness. The LEE of a μ-LED with a circular cross section was lower than that of a μ-LED with a square cross section by 5∼10% owing to the coupling of light with high-quality-factor whispering gallery modes. The LEE of a μ-LED structure decreased as the chip dimension increased, which could be attributed to the increased portion of trapped light inside the LED chip and increased light absorption in the GaN with an increasing chip dimension. In addition, the LEE varied significantly with the thickness of the p-GaN layer, which could be explained by the strong dependence of the angular distribution of the emission pattern on the p-GaN thickness. The FDTD simulation method presented in this study is expected to be advantageously employed in designing μ-LED structures with high LEE.

Sensitive and label-free shell isolated Ag NPs@Si architecture based SERS active substrate: FDTD analysis and in-situ cellular DNA detection

In recent years there has been increasing interest to elicit the molecular dynamics at the single-cell level. In this purview, we report the fabrication of SiO2 capped silver nanoparticles (Ag NPs) on silicon (Si) substrate to understand and divulge the fingerprint of cellular DNA by a label-free method. For the in-situ label free identification thin layer of SiO2 deposited on Ag NPs@Si by e-beam evaporation method which forbids the surface degradation of NPs in culture medium. The hot spots generation in the metal-dielectric nano junction assembly from aggregates of identical and non-identical Ag NPs have been evaluated by Finite-Difference Time-Domain simulation method. The coverage of non-identical Ag NPs with a various layer thickness of SiO2 is shown to be the greatest hotspot rather than an identical particle, and appropriate non-identical NPs are validated with experimental evidence at different excitation wavelengths. The SERS efficiency of analyte molecule Rhodamine 6G is evaluated and the enhancement factor of 107 with the low detection limit up to 10−10 M concentration is achieved. The SiO2-Ag NPs@Si substrate could be an ideal platform for the detection of DNA at cellular level.

Investigating extraordinary optical transmission and sensing performance through periodic bilayer magneto-plasmonic structure

This paper proposes the phenomenon of extraordinary optical transmission via a magneto-plasmonic nanostructure, which combines magnetic and plasmonic functionalities. The structure includes an active magnetic film magnetized perpendicular to its surface and a plasmonic metal film, perforated with subwavelength circular annular arrays, with a ring placed in the middle of each annular circle. We use the finite element method and the finite-difference time-domain method for simulation of the structure. Numerical analysis shows an improvement in the Faraday rotation and optical transmission, simultaneously, in a magneto-plasmonic structure based on a silver- and bismuth-substituted ferrite garnet. Simultaneous improvement is achieved by coupling the TE and TM waveguide-plasmon modes. The amount of enhancement is adjusted by changing the dimensions, the periodicity of the hole arrays, and the refractive index of the materials filled in the holes. The influence of excitation of the two kinds of plasmon modes and the application of the external magnetic field are used to enhance the optical response. The resulting investigation shows two resonance peaks in the near-infrared range of the Faraday effect spectrum. Because of the strong Faraday rotation coinciding with the dual-band transmission of approximately 90%, the maximum figure of merit can also be obtained. Finally, this structure is investigated as a sensor in different reflective indexes from 1 to 1.5 RIU, and sensitivity of 45.97 nm/RIU was achieved. The potential applications of these nanostructures include, for example, subwavelength optics, optoelectronic devices, biosensing devices, and magneto-optical devices.

Broadband NIR absorber based on square lattice arrangement in metallic and dielectric state VO2

In this Letter, we propose a broadband near-infrared (NIR) absorber based on the phase transition material VO2. By designing different arrangements of the VO2 square lattice at high and low temperatures on fused silica substrates, the absorption rate reaches more than 90% in the entire 1.4–2.4 μm range. Using a finite-difference time-domain simulation method and thermal field analysis, the results prove that the absorber is polarization-independent and has wide-angle absorption for incident angles of 0°–70°. The proposed absorber has a smoother absorption curve and is superior in performance, and it has many application prospects in remote sensing geology.