Finite-difference Time-domain Calculations Research Articles

Coherent Phonon Dynamics in Plasmonic Gold Tetrahedral Nanoparticle Ensembles.

Coherent phonon modes supported by plasmonic nanoparticles offer prospective applications in chemical and biological sensing. Whereas the characterization of these phonon modes often requires single-particle measurements, synthetic routes to narrow size distributions of nanoparticles permit ensemble investigations. Recently, the synthesis of highly monodisperse gold tetrahedral nanoparticles with tunable edge lengths and corner sharpnesses has been developed. Herein, we characterize a size series of these nanoparticles in colloidal dispersion via transient absorption spectroscopy to examine their mechanical and plasmonic responses upon photoexcitation. Oscillations of transient absorption signals are observed in the plasmon resonance and correspond to the lowest-order radial breathing modes of the nanoparticles, the frequencies of which are affected by the edge length and truncation of the corners. Homogeneous quality factor values ranging from 24 to 34 are observed for the oscillations that convey potential utility in mass-sensing and plasmon-exciton-coupling photonics schemes. Finite-difference time domain and finite element analysis calculations establish specific optically relevant phonon modes.

A Computer Model of a Marx Generator Charging a Coaxial Switched Wave Oscillator

The paper describes an axisymmetric Finite-Difference Time-Domain computer model of a coaxial Switched Wave Oscillator integrated with a dipole antenna. The model analyzes the operation of a realistic circuit model of a multistage Marx generator that charges the oscillator to a high voltage. The initial field distribution is calculated with an electrostatic finite-difference method solver to speed up the time-domain analysis. The work presents the results of our circuit model of a Marx generator’s simulations of the charging phase, followed by the results from the discharge phase, using our axisymmetric Finite-Difference Time-Domain model. Our work describes new and fast numerical solvers that can observe the operation of Switched Wave Oscillator systems (also considering the connected antenna). The codes could be used in the process of designing such a system. Advanced boundary conditions modeling the spark gap and oscillator’s excitation set our work apart from the other attempts in the literature.

Chiral 3D structures through multi-dimensional transfer printing of multilayer quantum dot patterns

Three-dimensional optical nanostructures have garnered significant interest in photonics due to their extraordinary capabilities to manipulate the amplitude, phase, and polarization states of light. However, achieving complex three-dimensional optical nanostructures with bottom-up fabrication has remained challenging, despite its nanoscale precision and cost-effectiveness, mainly due to inherent limitations in structural controllability. Here, we report the optical characteristics of intricate two- and three-dimensional nanoarchitectures made of colloidal quantum dots fabricated with multi-dimensional transfer printing. Our customizable fabrication platform, directed by tailored interface polarity, enables flexible geometric control over a variety of one-, two-, and three-dimensional quantum dot architectures, achieving tunable and advanced optical features. For example, we demonstrate a two-dimensional quantum dot nanomesh with tuned subwavelength square perforations designed by finite-difference time-domain calculations, achieving an 8-fold enhanced photoluminescence due to the maximized optical resonance. Furthermore, a three-dimensional quantum dot chiral structure is also created via asymmetric stacking of one-dimensional quantum dot layers, realizing a pronounced circular dichroism intensity exceeding 20°.

Facile Fabrication of SERS Substrates by the Electrodeposition Method to Detect Pesticides with High Enhancement Effect and Long-Term Stability.

In this study, surface-enhanced Raman scattering substrates were investigated by the electrodeposition method to detect low concentrations of pesticides via the electrodeposition method with different agents from silver and gold precursors on APTES-modified ITO glass. A dual-potential method supplied three electrodes and was performed with a nucleation potential of 0.7 V for 2 s and a growth potential of -0.2 V for 500 s. The Ag film produced by the electrodeposition approach has great surface uniformity and good SERS signal amplification for the thiram insecticide at low concentrations. Interestingly, the ITO/APTES/Ag substrate extensively increased the sensitivity than the other investigated ones, thanks to the adequate assistance of amino groups of APTES in the denser and hierarchical deposition of Ag NPs. These observations were additionally elucidated via finite-difference time-domain (FDTD) calculation. For thiram, the detection was set at 10-8 M with an enhancement factor of up to 3.6 × 107 times. Comparing the SERS spectra of thiram at concentrations of 10-3, 10-4, and 10-5 M with a relative standard deviation (RSD) of less than 7.0% demonstrates excellent reproducibility of the ITO/APTES/Ag substrate. In addition, the special selectivity of the ITO/APTES/Ag substrate for thiram demonstrates that these nanostructures can identify pesticides with extreme sensitivity.

Underwater acoustic non-reciprocal manipulation based on dynamic-modulation structures

Abstract Underwater acoustic non-reciprocal transmission via dynamic-modulation structures with time-varying mass and stiffness is studied. The model system consists of spatiotemporally modulated discrete lattices immersed in the water background. Based on the transfer matrix method, an analytic model for the coupled continuum-discrete system is developed to calculate acoustic scattering responses in the frequency domain. Finite-difference time-domain computation is conducted for the coupled system to verify the theoretical model. Results show that acoustic non-reciprocal transmission in opposite directions appears at frequencies where there are asymmetric bandgaps in dispersion diagrams. Asymmetric transmission can be enhanced in magnitude by engineering the modulating amplitudes of time-varying parameters or increasing the number of lattice elements, while the frequency bandwidth can be broadened by cascading structural elements with different modulating frequencies due to the gap-combining effect. The model may find potential applications in underwater acoustic isolation and sonar communication.

Construction of an Au/g-C3N4/GO/Au substrate by a layer-by-layer assembly approach for surface-enhanced Raman spectroscopy

In this work, an Au/g-C3N4/GO/Au hybrid nanofilm for surface-enhanced Raman scattering (SERS) substrate application was prepared by a facile layer-by-layer assembly technique using nanosheet-structured graphitic carbon nitride (g-C3N4), graphene oxide (GO) and 40 nm-gold nanoparticles (Au NPs). The effects of composition, assembly sequence and assembly cycles on the SERS performance were optimized to achieve the SERS-active substrate. The detection limit of Au/g-C3N4/GO/Au substrate for crystal violet and rhodamine 6 G probe molecules reached as high as 10−8 M, with good stability and uniformity. Its further application in food colorants detection (erythrosine, carmine and temptation red) had successfully met the national food safety standards and the quantitative detection for erythrosine and temptation red had also been established. Based on finite-difference time-domain element simulation and density functional theory quantum chemical calculation, the mechanism of the SERS effect of the substrate was unveiled, which is attributed to the combination of electromagnetic enhancement effect of Au nanoparticle aggregates and chemical enhancement effect of GO and g-C3N4. Notably, water droplet evaporation induced Raman enhancement was observed on the Au/g-C3N4/GO/Au substrate. This can be ascribed to the inhomogeneous evaporation of the droplet that promotes the enrichment of the probe molecules into the center of the droplet. This novel enhancement strategy provides a new idea for further improving detection sensitivity of SERS substrate materials.

Far-Field Petahertz Sampling of Plasmonic Fields.

The response of metal nanostructures to optical excitation leads to localized surface plasmon (LSP) generation with nanoscale field confinement driving applications in, for example, quantum optics and nanophotonics. Field sampling in the terahertz domain has had a tremendous impact on the ability to trace such collective excitations. Here, we extend such capabilities and introduce direct sampling of LSPs in a more relevant petahertz domain. The method allows to measure the LSP field in arbitrary nanostructures with subcycle precision. We demonstrate the technique for colloidal nanoparticles and compare the results to finite-difference time-domain calculations, which show that the build-up and dephasing of the plasmonic excitation can be resolved. Furthermore, we observe a reshaping of the spectral phase of the few-cycle pulse, and we demonstrate ad-hoc pulse shaping by tailoring the plasmonic sample. The methodology can be extended to single nanosystems and applied in exploring subcycle, attosecond phenomena.

Plasmonic sensing: FDTD calculations to interpret experimental LSPR water adsorption isotherms

Finite-Difference Time-Domain (FDTD) calculations are widely used to interpret experimental Localized Surface Plasmon Resonance (LSPR) spectra. With FDTD it is possible to predict the physical parameters of the nanostructures that will theoretically give the best experimental LSPR signals. This information is crucial for optimizing sample fabrication and saving significant time. FDTD is also essential to control and calibrate various parameters of the nanostructures and to understand effects such as a Cr adhesion layer and LSPR sensor size distribution. Finally, for water adsorption sensing, FDTD confirms that the LSPR peak shift strictly follows Campbell’s model, even at submonolayer coverage.

Modeling of the synergistic anti-reflection effect in gradient refractive index films integrated with subwavelength structures for photothermal conversion.

Photothermal materials generally suffer from challenges such as low photothermal conversion efficiency and inefficient full-spectrum utilization of solar energy. This paper proposes gradient refractive index transparent ceramics (GRITCs) integrated with subwavelength nanostructure arrays and simulates the synergistic anti-reflection effect by an admittance recursive model. An innovative subwavelength structure, possessing a superior light-trapping capability, is initially crafted based on this model. Subsequently, various intelligent optimization algorithms including genetic algorithm, particle swarm optimization, and simulated annealing are employed to optimize the structure of gradient refractive index films respectively. Finally, the photothermal conversion efficiencies of devices based on different photothermal materials are calculated. The simulations and finite-difference time-domain calculations demonstrate that the three-layer GRITCs integrated with an optimal SNA exhibit outstanding full-spectrum and omnidirectional anti-reflection performance. The solar transmittance of the devices can exceed 97% for light wavelengths ranging from 300 to 2500 nm over the full angle of incidence. Our results reveal that the synergistic anti-reflection effect in the SNAs and GRITCs can enhance the photothermal conversion efficiency by more than 20%.

Direct observation of light reflection by titania particles

Abstract We report on light refraction by titania particles with a high refractive index (approximately 2.4). Clusters of titania particles modified using a fluorescent dye, rhodamine B isothiocyanate, and nonfluorescent titania particles were prepared. When the clusters were irradiated using light at the excitation wavelength of the fluorescent dye, the edges of the bound nonfluorescent particles glowed brightly. Geometric optics and finite difference time domain calculations revealed that this phenomenon was due to a lens effect caused by titania particles.

Plexcitonic Nanorattles as Highly Efficient SERS-Encoded Tags.

Plexcitonic nanoparticles exhibit strong light-matter interactions, mediated by localized surface plasmon resonances, and thereby promise potential applications in fields such as photonics, solar cells, and sensing, among others. Herein, these light-matter interactions are investigated by UV-visible and surface-enhanced Raman scattering (SERS) spectroscopies, supported by finite-difference time-domain (FDTD) calculations. Our results reveal the importance of combining plasmonic nanomaterials and J-aggregates with near-zero-refractive index. As plexcitonic nanostructures nanorattles are employed, based on J-aggregates of the cyanine dye 5,5,6,6-tetrachloro-1,1-diethyl-3,3-bis(4-sulfobutyl)benzimidazolocarbocyanine (TDBC) andplasmonic silver-coated gold nanorods, confined within mesoporous silica shells, which facilitate the adsorption of the J-aggregates onto the metallic nanorod surface, while providing high colloidal stability. Electromagnetic simulations show that the electromagnetic field is strongly confined inside the J-aggregate layer, at wavelengths near the upper plexcitonic mode, but it is damped toward the J-aggregate/water interface at the lower plexcitonic mode. This behavior is ascribed to the sharp variation of dielectric properties of the J-aggregate shell close to the plasmon resonance, which leads to a high opposite refractive index contrast between water and the TDBC shell, at the upper and the lower plexcitonic modes. This behavior is responsible for the high SERS efficiency of the plexcitonic nanorattles under both 633nm and 532nm laser illumination. SERS analysis showed a detection sensitivity down to the single-nanoparticle level and, therefore, an exceptionally high average SERS intensity per particle. These findings may open new opportunities for ultrasensitive biosensing and bioimaging, as superbright and highly stable optical labels based on the strong coupling effect.

Mid-infrared thermal radiation resonating with longitudinal-optical like phonon from n++-doped GaN–semi-insulating GaN grating structure

The mid-infrared emission mechanism of line-and-space structures of metallic plates on dielectric materials is substantiated using high conductive n-doped (n++-) GaN–semi-insulating (SI-) GaN microstripe structures on an SI-GaN epitaxial layer, which was veiled when using line-and-space structures of Au plates. The present structure exhibits a few thermal emission lines originating from electric dipoles resonating with the coherent longitudinal optical (LO) phonon-like lattice vibration, which are formed by the local depolarization electric field in the surface n++-GaN/SI-GaN/n++-GaN regions. The energies of the LO-phonon-like modes shift from the original LO-phonon energy of GaN to the lower energy region, which contrasts with the LO-phonon resonant emission from the microstructures on GaAs. These emission lines have another notable feature, i.e. the observed peak energies are independent of the polar emission angle for both s- and p-polarizations, unlike the emissions by surface phonon polaritons showing a significant directive nature of peak energies. The results show that each peak energy of the present emission lines is positioned at the zero-point of the real part of the electric permittivity comprising the components of the transverse optical phonon and other electric dipoles induced by the LO-like modes, excluding the target mode. The significant peak-energy shift of the LO-like phonons is applicable to materials with wide Reststrahlen bands, which contrasts with that of the nearly LO-phonon resonating feature of materials with narrow Reststrahlen bands, such as GaAs. The peak energy shift depending on the emission direction is observed for Au–GaN stripe structures. This property is ascribed to the imperfect Au/GaN interface with surface states through the theoretical analysis of the modified electric permittivity in the surface region, numerical simulation of the local electric field via finite-difference time-domain calculation, and experimental studies on a Ti–GaN structure and emission peaks originating from an LO-like phonon of the α-Al2O3 substrate.

Ultrasensitive SERS detection of trace polycyclic aromatic hydrocarbons in seawater using 1-propanethiol modified Fe3O4/Ag@inositol hexaphosphate nanocomposite

Polycyclic aromatic hydrocarbons (PAHs) in seawater even at trace level could continuously cause serious threats via food chain. Herein, we fabricate 1-propanethiol (C3)-modified Ag@inositol hexaphosphate (IP6)-decorated Fe3O4 composite nanomaterials, namely Fe3O4/Ag@IP6-C3. The magnetic and hydrophobic of Fe3O4/Ag@IP6-C3 could greatly enrich PAHs molecules and has giant surface enhanced Raman scattering (SERS) effect due to magnetic inducing increase of SERS “hot spots”. The optimized SERS effect of Fe3O4/Ag@IP6-C3 triggered by the spatial distributions of electromagnetic field were validated by using finite difference time domain (FDTD) calculations, and the enhancement factor (EF) of 1.26 × 107 for 4-mercaptopyridine (4-MPy) as Raman probe. Interestingly, Fe3O4/Ag@IP6-C3-based SERS protocol shows specific response to pyrene owing to the great partitioning to pyrene in the presence of C3, which is also proofed by the simulation of binding energies between PAH molecules and Ag@IP6-C3 by using DFT method. The quantitative linear range for the detection of pyrene is from 1 μg/L to 10 mg/L (R2 =0.989) and the limit of detection (LOD) of 24 ng/L (24 ppt) is reached, which meets the limitation amount (1 ppb) of PAHs in water set by USEPA and WHO. Clearly, as-proposed SERS sensing strategy paves a path to realize seashore monitoring of marine environmental and early warning for PAHs pollution based on the pyrene as an indicator.

Lasing in Zn-doped GaAs nanowires on an iron film

In this work, we demonstrate optically pumped lasing in highly Zn-doped GaAs nanowires (NWs) lying on an iron film. The conically shaped NWs are first covered with an 8 nm thick Al2O3 film to prevent atmospheric oxidation and mitigate band-bending effects. Multimode and single-mode lasing have been observed for NWs with a length greater or smaller than 2 μm, respectively. Finite difference time domain calculations reveal a weak electric field enhancement in the Al2O3 layer at the NW/iron film interface for the lasing modes. The high Zn acceptor concentration in the NWs provides enhanced radiative efficiency and enables lasing on the iron film despite plasmonic losses. Our results open avenues for integrating NW lasers on ferromagnetic substrates to achieve new functionalities, such as magnetic field-induced modulation.

Simulation for formation process of atomic orbitals by the finite difference time domain method based on the eight-element Dirac equation

Abstract Using the finite difference time domain (FDTD) method based on the eight-element Dirac equation, we found that a stable Dirac field wave packet with low velocity can be created without explicit consideration of Zitterbewegung (the rapid oscillatory motion of elementary particles), which is difficult in one-dimensional simulations. Furthermore, we successfully simulated the formation process of atomic orbitals for the first time without any physical approximations by calculating the eight-element Dirac field propagation in the central electric force potential. Initially, a small unstable orbital appears, which rapidly grows and results in a large stable orbital with a radius equal to the Bohr radius divided by the atomic number, as given by the solution of the Schrödinger equation. The FDTD calculation based on the conventional four-element Dirac equation cannot produce such reasonable orbitals owing to the spatial asymmetry of the 4 × 4 4\times 4 Dirac matrices. This method has the potential to be used for transient analyses of not only atomic or molecular orbitals but also interactions among elementary particles.

In silico assessment of histotripsy-induced changes in catheter-directed thrombolytic delivery.

Introduction: For venous thrombosis patients, catheter-directed thrombolytic therapy is the standard-of-care to recanalize the occluded vessel. Limitations with thrombolytic drugs make the development of adjuvant treatments an active area of research. One potential adjuvant is histotripsy, a focused ultrasound therapy that lyses red blood cells within thrombus via the spontaneous generation of bubbles. Histotripsy has also been shown to improve the efficacy of thrombolytic drugs, though the precise mechanism of enhancement has not been elucidated. In this study, in silico calculations were performed to determine the contribution of histotripsy-induced changes in thrombus diffusivity to alter catheter-directed therapy. Methods: An established and validated Monte Carlo calculation was used to predict the extent of histotripsy bubble activity. The distribution of thrombolytic drug was computed with a finite-difference time domain (FDTD) solution of the perfusion-diffusion equation. The FDTD calculation included changes in thrombus diffusivity based on outcomes of the Monte Carlo calculation. Fibrin degradation was determined using the known reaction rate of thrombolytic drug. Results: In the absence of histotripsy, thrombolytic delivery was restricted in close proximity to the catheter. Thrombolytic perfused throughout the focal region for calculations that included the effects of histotripsy, resulting in an increased degree of fibrinolysis. Discussion: These results were consistent with the outcomes of in vitro studies, suggesting histotripsy-induced changes in the thrombus diffusivity are a primary mechanism for enhancement of thrombolytic drugs.

Anderson localization of electromagnetic waves in three dimensions

Anderson localization is a halt of diffusive wave propagation in disordered systems. Despite extensive studies over the past 40 years, Anderson localization of light in three dimensions has remained elusive, leading to the question of its very existence. Recent advances have enabled finite-difference time-domain calculations to be sped up by orders of magnitude, allowing us to conduct brute-force numerical simulations of light transport in fully disordered three-dimensional systems with unprecedented dimension and refractive index difference. We show numerically three-dimensional localization of vector electromagnetic waves in random aggregates of overlapping metallic spheres, in sharp contrast to the absence of localization for dielectric spheres with a refractive index up to 10 in air. Our work opens a wide range of avenues in both fundamental research related to Anderson localization and potential applications using three-dimensional localized light. Whether Anderson localization of light can be achieved in three dimensions has remained an open question. Numerical calculations now show that it is possible with a random arrangement of metallic spheres, but not with dielectric ones.

Mechanism of structural colors in binary mixtures of nanoparticle-based supraballs.

Inspired by structural colors in avian species, various synthetic strategies have been developed to produce noniridescent, saturated colors using nanoparticle assemblies. Nanoparticle mixtures varying in particle chemistry and size have additional emergent properties that affect the color produced. For complex multicomponent systems, understanding the assembled structure and a robust optical modeling tool can empower scientists to identify structure-color relationships and fabricate designer materials with tailored color. Here, we demonstrate how we can reconstruct the assembled structure from small-angle scattering measurements using the computational reverse-engineering analysis for scattering experiments method and use the reconstructed structure in finite-difference time-domain calculations to predict color. We successfully, quantitatively predict experimentally observed color in mixtures containing strongly absorbing nanoparticles and demonstrate the influence of a single layer of segregated nanoparticles on color produced. The versatile computational approach that we present is useful for engineering synthetic materials with desired colors without laborious trial-and-error experiments.

Spectrally Selective Thermal Emission from Graphene Decorated with Metallic Nanoparticles

We showed in past work that nanopatterned monolayer graphene (NPG) enables spectrally selective thermal emission in the mid-infrared (mid-IR) from 3 to 12 μm. In that case, the spectral selection is realized by means of the localized surface plasmon (LSP) resonances inside graphene. Here, we show that graphene decorated with metallic nanoparticles, such as Ag nanocubes or nanospheres, also realizes spectrally selective thermal emission but, in this case, by means of acoustic graphene plasmons (AGPs) localized between graphene and the Ag nanoparticle inside a dielectric material. Our finite-difference time-domain (FDTD) calculations show that the spectrally selective thermal radiation emission can be tuned by means of a gate voltage into two different wavelength regimes, namely, the atmospherically opaque regime between λ = 5 and 8 μm or the atmospherically transparent regime between λ = 8 and 12 μm. This allows for electrical switching between a radiative heat trapping mode for the former regime and a radiative cooling mode for the latter regime. Our theoretical results can be used to develop graphene-based thermal management systems for smart fabrics.

Equations for finite-difference, time-domain simulation of sound propagation in moving media with arbitrary Mach numbers

Finite-difference time-domain (FDTD) techniques for sound propagation have become increasingly popular. In moving media, such as the atmosphere, starting equations for FDTD calculations are often limited to low Mach numbers, which may result in significant errors. In this article, two coupled equations for the sound pressure and acoustic particle velocity are derived from the linearized fluid dynamic equations. These coupled equations are valid for arbitrary Mach numbers, in the high-frequency approximation, and can be used in FDTD calculations and other methods for sound propagation in moving media. For low Mach numbers, the equations derived are valid for arbitrary frequencies and are consistent with equations from the literature.