Electrodynamic Simulations Research Articles

Electrodynamic Simulation of Interaction of Microwaves with Anisotropic Nanomaterials Based on 3D Lattices of Carbon Nanotubes

Electrodynamic Simulation of Interaction of Microwaves with Anisotropic Nanomaterials Based on 3D Lattices of Carbon Nanotubes

Biphasic Approach for the Synthesis of Au Nanoparticles via Diffusional Release of Gallic Acid from Supramolecular Hydrogels to Aqueous Solutions: Implications for Biomedical Applications

Owing to the unique optical and chemical properties of plasmonic nanoparticles, new synthetic approaches for their fabrication are constantly being developed to control their size, shape, colloidal stability, and surface functionalization. This work presents a biphasic method based on the controlled release of a reducing agent from a supramolecular polymer hydrogel to produce gold nanoparticles (Au NPs). Specifically, Au NPs are obtained by gallic acid (GA) diffusion from a poly(vinyl alcohol) (PVA)/GA hydrogel to a Au3+ solution using the reducing properties of polyphenols. The formation process of Au NPs is investigated under different conditions, such as changing the PVA/GA stochiometric ratio and Au3+ concentration. It is demonstrated that a subtle change in the conditions leads to different Au NPs sizes and morphologies. Au NPs of small size can be obtained (<20 nm) using this method, in contrast with Au NPs produced by simply mixing GA with Au3+ at any GA/Au3+ concentration ratio. For instance, when 5 wt % GA in the PVA-GA hydrogels in combination with 0.25 mM Au3+ solution is used, spherical and highly monodispersed (7 ± 2 nm) GA-functionalized Au NP colloidal dispersions can be obtained, giving a suitable substrate for biomedical applications. The results are rationalized in terms of different standard diffusional models, morphological characterization of the PVA-GA hydrogels, and UV–vis spectroscopy in combination with electrodynamic simulations.

Multiscale models of plasmonic structural colors with nanoscale surface roughness.

Plasmonic coloration arises from resonant interaction between visible light and metallic nanostructures, which causes wavelength-selective absorption or scattering of light. This effect is sensitive to surface roughness that can perturb these resonant interactions and cause observed coloration to deviate from coloration predicted by simulations. We present a computational visualization approach that incorporates electrodynamic simulations and physically based rendering (PBR) to investigate the effect of nanoscale roughness on the structural coloration from thin, planar silver films decorated with nanohole arrays. Nanoscale roughness is modeled mathematically by a surface correlation function and parameterized in terms of roughness that is either out of or into the plane of the film. Our results provide photorealistic visualization of the influence of nanoscale roughness on the coloration from silver nanohole arrays in both reflectance and transmittance. Out-of-plane roughness has a significantly greater effect on coloration than in-plane roughness. The methodology introduced in this work is useful for modeling artificial coloration phenomena.

Influence of Geometrical Parameters on the Optical Activity of Chiral Gold Nanorods

AbstractChiral metal nanoparticles (NPs) offer a powerful means of inducing and harnessing optical activity. However, due to the incomplete knowledge of the underlying growth mechanisms, there is still limited control over the achievable morphological detail and, consequently, over the resulting optical activity. Therefore, theoretical modeling is needed to guide experimental development toward optimizing the plasmonic chiroptical response. Toward filling this gap, herein an extensive parametric analysis is presented, via computer‐aided‐design (CAD) models and full‐wave electrodynamic simulations, which aims at systematically analyzing the influence of structural changes on the plasmonic circular dichroism (CD) spectra of rod‐shaped gold NPs comprising helical indentations on achiral nanorod cores. From this analysis, interesting patterns in the plasmon‐mediated resonant behavior are identified and cause–effect relationships are drawn that may serve as a go‐to recipe for the understanding and fabrication of these NPs and their applications, such as spectroscopic (bio)detection including CD spectral shifts and surface‐enhanced Raman optical activity.

Effects of edge cracks on the thermomagnetic instabilities of type-II superconducting thin films.

Thermomagnetic instability is a crucial issue for the application of superconductors. Effects of edge cracks on the thermomagnetic instability of superconducting thin films are systematically investigated in this work. Dendritic flux avalanches in thin films are well reproduced through electrodynamics simulations, and relevant physical mechanisms are revealed from dissipative vortex dynamics simulations. It is found that edge cracks sharply decrease the threshold field for the thermomagnetic instability of superconducting films. Spectrum analysis shows that the time series of magnetization jumping displays scale-invariance and follows a power law with an exponent around 1.9. In a cracked film, flux jumps more frequently with lower amplitudes compared with its crack-less counterpart. As the crack extends, the threshold field decreases, the jumping frequency gets lower, while its magnitude gets larger. When the crack has extended long enough, the threshold field increases to even larger than that of the crack-less film. This counterintuitive result originates from the transition of the thermomagnetic instability triggered at the crack tip to the one triggered at the center of the crack edges, which is validated by the multifractal spectrum of magnetization jumping sequences. In addition, with the variation of crack lengths, three different modes of vortex motion are found, which explains the different flux patterns formed in the avalanche process.

Impact of plasmonic substrate of different geometries on the enhancement factor of hyper‐Raman scattering process

Effective plasmonic heterotrimers of different geometry have been proposed as an active substrate to substantially enhance the scattered signal of hyper-Raman scattering process. The three-particle systems are constructed independently from asymmetric nanoparticles of spherical, cylindrical, and spheroidal shapes. The interacting metallic characters in each one of the three trimers are arranged collinearly in a J-aggregate configuration and illuminated by a longitudinally polarized light. The finite-difference time-domain electrodynamic simulation tool is employed to simulate optical cross-section and nearfield intensity associated with the excited plasmonic bands. The extinction profile of each one of the three trimers exhibits the excitation of two plasmonic modes. The first is a superradiant mode, which resulted from the in-phase coupling between the dipole moments induced in each one of the three plasmonic elements. The second mode excited at a shorter wavelength, which is a combination between bright and dark modes (Fano interference). The resonance wavelength of the former mode matches the incident one; meanwhile, the resonance energy of the Fano profile is equivalent to the second-order Stokes condition. From the values of the highly enhanced nearfield associated with the observation of these modes, the enhancement factor of the hyper-Raman scattering process is calculated. The simulation results highlight the influence of the selected particle's geometry on both the quality of plasmonic coupling and calculated enhancement factor. The maximum value of the enhancement factor can reach as high as 3.83 × 1017 when the heterotrimer is constructed from spheroids. For nanoparticles of spherical shape, the calculated value of the factor is comparable to the maximum limit; meanwhile, the rod shape demonstrates the minimum limit of the enhancement factor.

Estimation of the electrodynamics parameters of the multilayer strip-slot transition based on the approximation of its equivalent circuit by the chebyshev filter of the first kind

The basic structural element of volume-modular microwave devices is a multilayer strip-slot transition, which provides electromagnetic coupling between its layers in a wide frequency range. The mathematical model of the multilayer strip-slot transition is known. It is based on the use of circuit analogies of its design elements. However, in the papers related to the study of its equivalent circuit only the amplitude-frequency dependences are reflected without analyzing other electrodynamics parameters, such as: phase-frequency characteristics and dependences of the input impedance on frequency. The purpose of this paper is to estimate the electrodynamics parameters of the multilayer strip-slot transition based on its equivalent circuit, approximated by the Chebyshev filter of the first kind and to conduct a comparative analysis of the results of its circuit and electrodynamics simulations. At the same time, the following simulation results are compared: 1) amplitude-phase characteristics of the functions of the reflection coefficient and transmission coefficient; 2) change in input impedance in the frequency range. As a result of the study it has been established that the equivalent circuit of the multilayer strip-slot transition can be approximated by the Chebyshev filter of the first kind. It allows to determine the necessary characteristic impedances of strip and slot resonators by given return/insertion losses in the pass band and their cutoff frequencies, while reflecting all electrodynamics characteristics correctly.

Simulation and Experimental Study of a Linear Bifocal Antenna Array

A leaky-wave bifocal linear array focused in the Fresnel zone is considered. The structural parameters provide perfect focusing of the field at two points corresponding to two different frequencies. An approximate theory of antenna arrays is used to analyze the bifocal array, to study the shape of the frequency scanning arc, and to determine the field distribution in the focal spot. An electrodynamic simulation is performed for a strip array with slotted radiators. The scattering parameters of the array and its radiation characteristicsare presented. The design features of a strip antenna that suppress the effect of normal are discussed. The results of an experimental study of a strip bifocal array are presented. The possibility of implementation of a symmetrical scanning arc and a relatively low level of optical aberrations is shown.

Signal Integrity Analysis of the Structure with Single Modal Reservation Before and After Failures

To improve the reliability of radio electronic devices, engineers use redundancy of the most relevant circuits. The simplest and most common solution is classical cold redundancy in which the primary and redundant circuits are located far from each other. Meanwhile, locating the primary and redundant circuits close together makes it possible to use modal distortions to protect both circuits from unwanted ultra-wideband interference. This approach is known as modal redundancy or modal reservation (MR). This paper presents the results of signal integrity analysis for a structure with single MR before and after failures. It is shown that the bandwidth after failures changes. This affects the rate of data that can be excited to the input of the device without significant distortion of the useful signal. The authors used pseudorandom binary sequences with a data rate of 50 and 200 Mbps. It is experimentally shown that failures in the redundant circuit do not significantly degrade the quality of signal integrity in the bandwidth. Thus, the data-dependent jitter changes from 12 (before failures) to 19 ps (after the worst failure). In all investigated variants, the eye stays open; as a result, the bit error rate will be low. The mismatch of the redundant circuit has the largest effect on the noise amplitude. As a result, the signal-to-noise ratio decreases from 57 (before failures) to 28 (after the worst failure). To confirm the measurements, we performed electrodynamic simulations. Their results are in good agreement with the experimental data.

Моделирование и экспериментальное исследование четырехпроводной полосковой структуры с трехкратным модальным резервированием при отказах её элементов

The relevance of research on the rational use of redundancy by applying triple modal redundancy (MR) techniques to increase the resistance to ultra-wideband interference is highlighted. An experimental evaluation of the change in maximum voltage of decomposed pulses in a bilateral printed circuit board (PCB) with triple MR after failures has been performed. A special prototype of a double-sided PCB with triple MR with optimum parameters has been fabricated. SMD resistors were used as loads. At the far end of the redundant conductor of the PCB, a decomposition into 4 pulses at different input durations is shown. The maximum voltage changes before and after failures at input durations of 120 ps and 240 ps are 26.8% and 28%, respectively. The above comparison of the experimental and electrodynamic simulation results in the time and frequency domains showed their consistency. The maximum voltage and delay differences are 3.69 and 5.6 %, respectively.

Dimers of Plasmonic Nanocubes to Reach Single-Molecule Strong Coupling with High Emission Yields.

Reaching reproducible strong coupling between a quantum emitter and a plasmonic resonator at room temperature, while maintaining high emission yields, would make quantum information processing with light possible outside of cryogenic conditions. We theoretically propose to exploit the high local curvatures at the tips of plasmonic nanocubes to reach Purcell factors of >106 at visible frequencies, rendering single-molecule strong coupling more easily accessible than with the faceted spherical nanoparticles used in recent experimental demonstrations. In the case of gold nanocube dimers, we highlight a trade-off between coupling strength and emission yield that depends on the nanocube size. Electrodynamic simulations on silver nanostructures are performed using a realistic dielectric constant, as confirmed by scattering spectroscopy performed on single nanocubes. Dimers of silver nanocubes feature Purcell factors similar to those of gold while allowing emission yields of >60%, thus providing design rules for efficient strongly coupled hybrid nanostructures at room temperature.

Local phase center hodograph of radiating element’s navigation antenna Glonass

The main methods for the determination of the object position using code phase and signal carrier phase measurements are given. An accuracy of pseudorange measurements of both carrier phase and group delay code signal depends on local phase center variations for the signals coming from different directions. The relation of group delay and local phase center of the antenna is determined wherein the key factor is frequency dependence of phase or local phase center variation in solid angle of the radiation pattern. The definition of local phase center whose coordinates depends on the direction of radio signal is given. The single helical antenna analysis of local phase center position is carried out considering that the radio technical performances of the navigation antenna’s spacecraft «Glonass» depends on the same ones of its radiating elements. The features and the technique for hodographs computation of the helical radiator’s navigation phased antenna array using electrodynamic simulation tools are considered. The offset estimation of local phase center is obtained. The features of that offset in relation to the radiator construction depending on the operating frequency are detected.

Anisotropic metallic heterotrimer systems for an ultrahigh plasmonic-based improvement of hyper-Raman scattering signal

An anisotropic metallic trimer is proposed as an active plasmonic substrate for an ultrahigh enhancement in the spectroscopic signal of the hyper-Raman scattering (HRS) process. The suggested three-particle system is composed from non-aligned asymmetric nanoparticles of a cubic shape. The interacting resonators are made of gold material and illuminated by a longitudinally polarized light. The non-alignment condition in the heterotrimer is achieved by shifting the intermediate cube transversely away from the interparticle axis. Optical cross-section, nearfield distribution and charge density are calculated by using the finite-difference time-domain electrodynamic simulation tool. The enhancement factor of the HRS is calculated theoretically from the nearfield intensity associated with the resonance phenomenon of the considered trimer. The extinction profile of the illuminated system exhibits the excitation of two plasmonic modes. A superradiant mode observed in the longer wavelength region which resulted from the in-phase coupling between the plasmonic modes excited in each one of the three resonators. The second mode is a subradiant band emerged from the interference between bright and dark modes. The resonance wavelength of these two modes matches the excitation one and the second-order Stockes condition, respectively. After optimizing the value of both the transverse shift and the gap spacing, the enhancement factor of the HRS can reach as high as a value never reported before of 1 × 1018.

Cathodoluminescence Spectroscopy of Complex Dendritic Au Architectures for Application in Plasmon‐Mediated Photocatalysis and as SERS Substrates

AbstractComplex 3D metallic nanostructures with large surface areas and broadband absorption are attractive candidates for efficient photocatalysis and spectroscopy. Here, hierarchical Au dendrites are self‐assembled over centimeter‐scales by electrodeposition and the plasmon modes are locally mapped using cathodoluminescence spectroscopy. A correlation between the spatial and spectral distribution of the plasmonic “hot‐spots” and the morphology of these structures are demonstrated. Electrodynamic simulations show that the spectra of the plasmon modes are determined by the local geometry of sharp features. Their performance as both surface‐enhanced Raman scattering (SERS) substrates and as photocatalysts for the N‐demethylation reaction of methylene blue is investigated. High hot‐spot densities result in larger SERS enhancement, while the sample with the lowest hot‐spot density has a reaction yield 136% larger than the sample with the highest density. These findings indicate that maximizing the hot‐spot density is not sufficient to optimize plasmonic substrates for all applications. The spectral and spatial distribution of the plasmon resonances will modify the hot electron generation efficiency and need to be considered for plasmon‐enhanced photocatalysis. This work extends the understanding of light–matter interactions in complex 3D structures and provides direction for the rational design of plasmonic architectures for different applications.

Comparison of Electromagnetic Forces Evaluated in Yee-Lattice Finite-Difference Time-Domain and Lattice-Boltzmann Method

Electrodynamic simulations in the Yee-lattice finite-difference time-domain method (YL-FDTDM) require half-unit-cell and half-time-unit offsets between electric and magnetic field components. To evaluate energy, momentum, and force in the YL-FDTDM, electric and magnetic fields must be colocated in both space and time. Colocation operations can be computationally intensive while introducing implementation complexity and numerical error. As an alternative to the YL-FDTDM, the Hauser and Verhey electrodynamic lattice-Boltzmann method (HV ELBM) is implemented on a single lattice representing electric and magnetic fields at the same location and time. Since colocation is not required on a single lattice, its central differences are simpler to implement and more accurate. Continuous plane-wave scattering of electric and magnetic fields from a nondispersive dielectric sphere is solved using the YL-FDTDM, the HV ELBM, and Mie scattering analysis. The YL-FDTDM and HV ELBM are compared in terms of numerical error and computational time representing the analytical Mie solution for the Einstein and Laub time-averaged force density. The HV ELBM requires less computational time to evaluate a time-averaged force density with half the error of the YL-FDTDM. At a similar absolute error, the HV ELBM evaluates a time-averaged force density in less than 10% of the computational time.

Ionization-induced optical heterogeneity and ion-like direct emission in 1-nm silicon nanoparticle grains: Prospect for fast optical modulation

Silicon, a highly symmetric and homogeneous material, does not exhibit fast optical modulation. Recent classical electrodynamics simulations, however, demonstrated transient optical heterogeneity in silicon nanostructures, in which a high-density of excitonic electron–hole pair plasma and charge is created. The phenomenon, however, requires a specific particle size (∼100 nm diameter) and a high-density (1023/cc) plasma. We examine here the quantum aspect of the heterogeneity in 1-nm Si nanoparticles. Due to the small number of atoms, 1 nm nanoparticles are amenable to the Hartree–Fock first principle atomistic quantum theory simulations procedure, while single ionization events are sufficient to provide high charge density (2 × 1021/cc). The simulations show that the charge distribution in singly charged 1-nm particles is nonlinear and heterogeneous, accompanied with structural distortion that produces an electric dipole moment. Electronically, the simulations show that the single charge induces stationary Coulomb states that riddle the bandgap of the neutral particle, with dipole-allowed transitions, effectively inducing partial conducting-like behavior. Optically, when the charge is produced by ionizing UV radiation, the ionized particle survives and exhibits both extended (wide-band) as well as atomic- or ion-like sharp emission, in agreement with infrared polarimetry and spectroscopy observations in the solar coronal holes, as well as under synchrotron irradiation. Not only do ionized Si nanoparticles (charged nanosilicon grains) afford fast optical modulations, but they may also prove pivotal for understanding features of interstellar medium, observed throughout the Milky Way and other galaxies, including spectroscopic and material composition, as well as neutral hydrogen abundancy.

General Approaches to Solving Problems of Analysis and Synthesis of Directional Properties of Antenna Arrays

The work carried out the calculation and synthesis of antenna arrays used in radio-electronic complexes on unmanned aerial vehicles. The analytical model is designed to find asymptotic estimates of the polarization components of the electric field of the grating in the far zone of the carrier surface; the results obtained with its use are the initial data for constructing a technique for the numerical solution of a boundary value problem for a grating on a carrier surface in the CST MWS electrodynamic simulation environment. The synthesis of gratings with the maximum achievable coefficient of directional action is carried out with the control of radiation patterns at a given set of angles. A two-mirror antenna system has been calculated. It is shown that with an increase in the number of re-reflections taken into account, the convergence of the result for the calculated characteristics of the antenna is observed. To test the proposed method, the same antenna was calculated using the integral equation method. The comparison showed a high degree of agreement between the results obtained by two different methods. The results of the simulation based on a software algorithm designed to quantify the input matching at the input of a multichannel frequency-scanning antenna array power divider are presented. It was found that when performing wide-angle scanning in a relative frequency band of more than a few percent, the disadvantage of the known method for eliminating the normal effect is a sharp deterioration in matching in the lower and upper frequencies of the operating range. A new method is proposed based on an automated iterative process of optimizing the divider geometry, which makes it possible to obtain an acceptable match over the entire operating frequency band. The feasibility of switching from a serial power divider construction scheme to a series-parallel scheme is analyzed for wide-angle scanning in a relative frequency band of about 5%.

Time-Resolved Thickness and Shape-Change Quantification using a Dual-Band Nanoplasmonic Ruler with Sub-Nanometer Resolution.

Time-resolved measurements of changes in the size and shape of nanobiological objects and layers are crucial to understand their properties and optimize their performance. Optical sensing is particularly attractive with high throughput and sensitivity, and label-free operation. However, most state-of-the-art solutions require intricate modeling or multiparameter measurements to disentangle conformational or thickness changes of biomolecular layers from complex interfacial refractive index variations. Here, we present a dual-band nanoplasmonic ruler comprising mixed arrays of plasmonic nanoparticles with spectrally separated resonance peaks. As electrodynamic simulations and model experiments show, the ruler enables real-time simultaneous measurements of thickness and refractive index variations in uniform and heterogeneous layers with sub-nanometer resolution. Additionally, nanostructure shape changes can be tracked, as demonstrated by quantifying the degree of lipid vesicle deformation at the critical coverage prior to rupture and supported lipid bilayer formation. In a broader context, the presented nanofabrication approach constitutes a generic route for multimodal nanoplasmonic optical sensing.

Continuous flow synthesis of plasmonic gold patches and nearly-complete nanoshells on polystyrene core particles

We describe the development of a T-mixer based continuous flow process for the coating of 86–500 nm diameter spherical polystyrene particles with thin gold patches by heterogeneous nucleation and growth. After establishing a suitable flow rate for good mixing and sufficiently uniform product morphology we systematically investigate the main reaction parameters. This reveals a considerable tunability of the patch morphology and, by virtue of the localized surface plasmon resonance of gold, the optical properties of the product dispersions. In order to further widen the range of nanostructures accessible by our process, a second T-mixer was added. This introduced new gold precursor, leading to further growth of the patches that were formed after the first mixer. By this approach, nearly-complete gold nanoshells could be produced in high yield on both small and large core particles, without the unwanted production of free-standing gold nanoparticles. Due to the pronounced optical properties of nearly-complete gold nanoshells on small core particles, we could estimate from electrodynamic simulations the equivalent shell thickness to be as low as 8.6 nm. This is significantly thinner than can be routinely achieved using the standard seeded growth approach to synthesise gold nanoshells. Our results are therefore highly promising for the gram-scale synthesis of plasmon resonant nanostructures with designed optical properties.

Анализ целостности полезного сигнала для четырехслойного зеркально-симметричного модального фильтра

The article presents the results of signal integrity analysis for a four-layer reflection symmetric modal filter and determines a potential field of application for such devices. The authors performed a simulation and experimental study in the range from 0 to 1.5 GHz. The results show that the average level of insertion loss in the passband does not exceed -18 dB, which demonstrates good agreement. The insertion loss does not significantly attenuate the useful signal. After the modal filter was defined in the frequency domain, the authors used the Advanced Design System to analyze the time characteristics. Pseudorandom binary sequences with bitrates from 0.125 to 0.5 Gb/s were excited to the input of the device in the Advanced Design System. The eye diagrams show that the modal filter has an excellent performance in terms of signal integrity. Thus, at the fastest data rate, the jitter does not exceed 39.5 ps. The maximum value of amplitude noise was 71 mV. It is possible to achieve significantly lower values of jitter and amplitude noise for the modal filter using its optimal geometric and electrical parameters. The results of the electrodynamic simulation are in good agreement with the experimental results.