Dipole Method Research Articles

Growth, structural, optical, z-scan, dielectric and mechanical studies of ethyl para-hydroxybenzoate crystal for optical applications

Ethyl substituted para-hydroxybenzoate known as ethyl para-hydroxybenzoate (EHB) organic single crystal is grown by solvent controlled slow evaporation system with methanol as a solvent. The grown EHB crystal unit cell geometry is investigated using a single crystal XRD system. The Plasma, Fermi and Penn energy analyses also the molecular electronic polarizabilities are evaluated by different methods such as Clausius-Mossotti, coupled dipole method and Lorentz relations. The optical absorption and transmittance spectrum are assessed by UV–visible spectrum. The different optical constants as wavelength cut-off range, optical energy gap, extinction and absorption coefficient, refractive index, the position of conduction and valance band, optical electronegativity, nonlinear absorption coefficient, etc are calculated. The dielectric nature and molecular orbital analysis are evaluated. From the z-scan analysis, third-order nonlinear optical assets are estimated.

A Curve-Shaped Beam Bistable Piezoelectric Energy Harvester with Variable Potential Well: Modeling and Numerical Simulation.

To improve the energy harvesting performance of an energy harvester, a novel bistable piezoelectric energy harvester with variable potential well (BPEH-V) is proposed by introducing a spring to the external magnet from a curve-shaped beam bistable harvester (CBH-C). First, finite element simulation was performed in COMSOL software to validate that the curved beam configuration was superior to the straight beam in power generation performance, which benefits energy harvesting. Moreover, the nonlinear magnetic model was obtained by using the magnetic dipoles method, and the nonlinear restoring force model of the curve-shaped beam was acquired based on fitting the experimental data. The corresponding coupled governing equations were derived by using generalized Hamilton’s principle, the dynamic responses were obtained by solving the coupling equations with the ode45 method. Finally, the numerical simulations showed that the proposed harvester can make interwell oscillations easier due to the spring being efficiently introduced to pull down the potential barrier compared with the conventional bistable harvester. Spring stiffness has a great impact on characteristics of the system, and a suitable stiffness contributes to realize large-amplitude interwell oscillations over a wide range of excitation, especially in the low excitation condition.

High fidelity, discrete element method simulation of magnetorheological fluids using accurate particle size distributions in LIGGGHTS extended with mutual dipole method

We simulate magnetorheological fluids (MRF) using open source LIGGGHTS soft sphere discrete element method code, extended by us to include a mutual dipole magnetic model. Our simulations take advantage of the many pair forces available in the LIGGGHTS framework, including SJKR cohesion, friction, and rolling resistance. In addition, we have included an uncoupled, Couette flow background carrier fluid. The simulated particles in this work are polydisperse, with distributions made to match the distributions used to produce magnetorheological fluids in literature, increasing the fidelity of the simulations. Using the accurate particle size distributions, high heritage contact models, and an uncoupled fluid model, we are able to match experimental MRF yield stress results more closely than with monodisperse simulations.

An extended aggregate basis function method for composite structure with periodic array

AbstractThe extended aggregate basis function method (EABFM) is proposed to analyze the electromagnetic scattering by the composite structure with periodic array. The generation of the impedance matrix in the EABFM is still time consuming. The fast dipole method (FDM) and the equivalent dipole‐moment method (EDM) are applied to accelerate the fill‐in of the matrix. Several numerical examples are performed. Simulation results validate the accuracy and efficiency of the proposed method.

Chirality-selective transparency induced by lattice resonance in bilayer metasurfaces

Chiral optical responses of bilayer metasurfaces made of twisted metallic nanorods are investigated in detail with focus on the collective effect due to lattice resonance (LR). Using an analytical approach based on the coupled dipole method (supported by full wave simulation), we find optical chirality is dramatically increased by the coupling between localized surface plasmon resonances and LR. The collective effect results in significant chiral signal even for metasurfaces made of achiral unit cells. The interlayer coupling generally destroys the Wood’s anomaly and the associated transparency. While making use of Pancharatnam–Berry (PB) phase and propagation phase, one can modulate the optical activity effectively and achieve chirality-selective transparency induced by LR in a designed structure with a g-factor of absorption as high as 1.99 (close to the upper limit of 2). Our studies not only reveal a new mechanism of modulating chiral optical response by combination effects from PB phase, propagation phase, and LR, but also give a quantitative relationship between the geometry configuration and chiral optical properties, thus providing helpful guidance for device design.

Higher-order topology in plasmonic Kagome lattices

We study the topological properties of a Kagome plasmonic metasurface, modeled with a coupled dipole method that naturally includes retarded long range interactions. We demonstrate that the system supports an obstructed atomic limit phase through the calculation of Wilson loops. Then, we characterize the hierarchy of topological boundary modes hosted by the subwavelength array of plasmonic nanoparticles: both one-dimensional edge modes and zero-dimensional corner modes. We determine the properties of these modes, which robustly confine light at subwavelength scales, calculate the local density of photonic states at edge and corner modes frequencies, and demonstrate the selective excitation of delocalized corner modes in a topological cavity, through nonzero orbital angular momentum beam excitation.

A global Curie depth model utilising the equivalent source magnetic dipole method

The depth to the Curie isotherm provides a snapshot into the deep thermal conditions of the crust, which helps constrain models of thermally controlled physical properties and processes. In this study, we present an updated global Curie depth model by employing the equivalent source dipole method to fit the lithospheric magnetic field model LCS-1 from spherical harmonic degree 16 to 100. In addition to the new field mode, we utilise all three vector components and include a laterally variable magnetic susceptibility model. We also employ an improved thermal model, TC1, to supplement the degree 1 to 15 components that are otherwise contaminated by the core field. Our new Curie depth model differs by as much as ±20 km relative to previous models, with the largest differences arising from the low order thermal model and variable susceptibility. Key differences are found in central Africa due to application of a variable susceptibility model, and shield regions, but continents with poor constraints such as Antarctica require additional improvement. This new Curie depth model shows good agreement with continental heat flow observations, and provides further evidence that Curie depth estimates may be used to constrain evaluations of the thermal state of the continental lithosphere, especially in regions with sparse or surface contaminated heat flow observations.

Fast computational optimization of TMS coil placement for individualized electric field targeting

BackgroundDuring transcranial magnetic stimulation (TMS) a coil placed on the scalp is used to non-invasively modulate activity of targeted brain networks via a magnetically induced electric field (E-field). Ideally, the E-field induced during TMS is concentrated on a targeted cortical region of interest (ROI). Determination of the coil position and orientation that best achieve this objective presently requires a large computational effort. ObjectiveTo improve the accuracy of TMS we have developed a fast computational auxiliary dipole method (ADM) for determining the optimum coil position and orientation. The optimum coil placement maximizes the E-field along a predetermined direction or, alternatively, the overall E-field magnitude in the targeted ROI. Furthermore, ADM can assess E-field uncertainty resulting from precision limitations of TMS coil placement protocols. MethodADM leverages the electromagnetic reciprocity principle to compute rapidly the TMS induced E-field in the ROI by using the E-field generated by a virtual constant current source residing in the ROI. The framework starts by solving for the conduction currents resulting from this ROI current source. Then, it rapidly determines the average E-field induced in the ROI for each coil position by using the conduction currents and a fast-multipole method. To further speed-up the computations, the coil is approximated using auxiliary dipoles enabling it to represent all coil orientations for a given coil position with less than 600 dipoles. ResultsUsing ADM, the E-fields generated in an MRI-derived head model when the coil is placed at 5900 different scalp positions and 360 coil orientations per position (over 2.1 million unique configurations) can be determined in under 15 min on a standard laptop computer. This enables rapid extraction of the optimum coil position and orientation as well as the E-field variation resulting from coil positioning uncertainty. ADM is implemented in SimNIBS 3.2. ConclusionADM enables the rapid determination of coil placement that maximizes E-field delivery to a specific brain target. This method can find the optimum coil placement in under 15 min enabling its routine use for TMS. Furthermore, it enables the fast quantification of uncertainty in the induced E-field due to limited precision of TMS coil placement protocols, enabling minimization and statistical analysis of the E-field dose variability.

Dynamic beam-steering of graphene-based terahertz cross Yagi–Uda antenna with a theoretical approach

In this paper, for the first time, a dynamic tunable graphene-based cross Yagi–Uda antenna in the terahertz region has been investigated comprehensively by two numerical methods and analytical analysis. To verify the accuracy of the analytical solution based on the coupled dipole method to obtain the directivity pattern, two numerical methods of finite-element and finite-difference time-domain have been used. Numerical results are well matched with the theoretical ones. By introducing the tunable cross Yagi–Uda antenna with graphene-coated spheres, different directivity radiation patterns such as omni-, vertical and horizontal bi- and quad-directional have been obtained with the maximum directivities of 2.42, 12.4, 12.3, and 10.5 dBi, respectively. Moreover, the effect of different element shapes including cube and cylinder on the directivity and radiation efficiency has been studied. Also, the new idea of multiple-access and controlling the user’s access to the radiated optical electromagnetic waves from the transmitting antenna has been studied as an optical wireless on-chip link. Finally, the effect of structural parameters on the directivity of the proposed antenna has been surveyed with the tolerance of ±5% to investigate the imperfections that may appear in the fabrication process.

Spin-valley locked topological edge states in a staggered chiral photonic crystal

Engineering pseudo-spin and valley degrees of freedom using quantum spin Hall and valley Hall effects has opened up remarkable possibilities for highly efficient and robust signal transport in time-reversal invariant photonic systems. Here we present a spin-valley locked photonic crystal that has distinct signs of chirality of sublattices in a honeycomb unit cell. We show that the photonic crystal has an insulating bulk dispersion and sublattice-dependent spin-valley coupled gapless edge states by exploiting a coupled dipole method and demonstrate valley-selective propagation by controlling spin state of an external dipole source. The interplay between spin, valley and sublattice shows a judicious way for one-way photon transport by using multiple degrees of freedom.

Optically Active Perovskite CsPbBr3 Nanocrystals Helically Arranged on Inorganic Silica Nanohelices.

Perovskite nanocrystals (PNCs) exhibit excellent absorption and luminescent properties. Inorganic silica right (or left) handed nanohelices are used as chiral templates to induce optically active properties to CsPbBr3 PNCs grafted on their surfaces. In suspension, PNCs grafted on the nanohelices do not show any detectable chiroptical properties. In contrast, in a dried film state, they show large circular dichroism (CD) and circularly polarized luminescence (CPL) signals with dissymmetric factor up to 6 × 10-3. Grazing incidence X-ray scattering, tomography, and cryo-electron microscopy (EM) have shown closely and helically packed PNCs on the dried helices and much more loosely organized PNCs on helices in suspension. Simulations based on the coupled dipole method (CDM) demonstrate that the CD comes from the dipolar interaction between PNC assembled into a chiral structure and the CD decreases with the interparticle distance.

Accuracy and spatial properties of distributed magnetic source imaging techniques in the investigation of focal epilepsy patients.

Source localization of interictal epileptiform discharges (IEDs) is clinically useful in the presurgical workup of epilepsy patients. We aimed to compare the performance of four different distributed magnetic source imaging (dMSI) approaches: Minimum norm estimate (MNE), dynamic statistical parametric mapping (dSPM), standardized low‐resolution electromagnetic tomography (sLORETA), and coherent maximum entropy on the mean (cMEM). We also evaluated whether a simple average of maps obtained from multiple inverse solutions (Ave) can improve localization accuracy. We analyzed dMSI of 206 IEDs derived from magnetoencephalography recordings in 28 focal epilepsy patients who had a well‐defined focus determined through intracranial EEG (iEEG), epileptogenic MRI lesions or surgical resection. dMSI accuracy and spatial properties were quantitatively estimated as: (a) distance from the epilepsy focus, (b) reproducibility, (c) spatial dispersion (SD), (d) map extension, and (e) effect of thresholding on map properties. Clinical performance was excellent for all methods (median distance from the focus MNE = 2.4 mm; sLORETA = 3.5 mm; cMEM = 3.5 mm; dSPM = 6.8 mm, Ave = 0 mm). Ave showed the lowest distance between the map maximum and epilepsy focus (Dmin lower than cMEM, MNE, and dSPM, p = .021, p = .008, p < .001, respectively). cMEM showed the best spatial features, with lowest SD outside the focus (SD lower than all other methods, p < .001 consistently) and high contrast between the generator and surrounding regions. The average map Ave provided the best localization accuracy, whereas cMEM exhibited the lowest amount of spurious distant activity. dMSI techniques have the potential to significantly improve identification of iEEG targets and to guide surgical planning, especially when multiple methods are combined.

Quantum Hall phase and chiral edge states simulated by a coupled dipole method

We present theoretical modeling of the quantum Hall phase and chiral edge states in a gyroelectric photonic crystal. The theoretical framework represented here is based on a coupled dipole method, which is a direct counterpart of the tight-binding approximation. We confirm that gyrotropic coupling produces a band gap characterized by a nonzero Chern number. Gapless edge states appear at an interface between a gyroeletric photonic crystal and a vacuum. At a domain wall between media of positive and negative gyrotropy, topological edge states that are analogous to the Jackiw-Rebbi states are observed. Propagation of the chiral edge states is also simulated in real space under excitation of an external dipole source. This analysis provides a toy model to describe the quantum Hall phase in a purely optical manner, without using a mapping or an analogy of the electron wave function in the quantum Hall effect.

Strong electromagnetic coupling in dimers of topological-insulator nanoparticles and quantum emitters

In this work, we theoretically investigate the optical properties of a topological-insulator nanoparticle--quantum emitter dimer interacting in the strong-coupling regime. The calculations are based on a recently developed semianalytical technique based on a multiple-scattering polaritonic-operator formalism in conjunction with an electromagnetic coupled dipole method. We calculate the light spectrum of the above dimer and report the emergence of a mode that stems from the coupling of the surface topological particle polariton of the topological insulator with the resonance state of the quantum emitter.

A modified magnetic force model and experimental validation of a tri-stable piezoelectric energy harvester

Nonlinear tri-stable piezoelectric energy harvesters with magnetic field coupling have attracted many researchers’ interest in ambient vibration energy harvesting and conversion. In such a tri-stable piezoelectric energy harvester, the nonlinear magnetic force generated by the external magnetic field is mainly calculated by the equivalent magnetic dipole method. However, this method will give highly erroneous results when the magnetic separation distance is small. This study presents a modified magnetic force model to precisely calculate the nonlinear magnetic force exerted on the tip of the cantilever beam. Unlike the equivalent magnetic dipole method regarding the magnet as a point dipole at its body center, this model only considers the surface magnetization current of the left and right surfaces of the magnets and replaced them by the one of the center point dipoles on these associated surfaces. With this model, the nonlinear magnetic force, the potential energy, and the transition mechanism of the tri-stable piezoelectric energy harvester are numerically investigated. Experimental validations are consequently performed to testify the numerical results. Compared with the equivalent magnetic dipole method, the modified magnetic force model has a much higher accuracy to be more applicable for different magnetic separation distances, especially when the magnetic separation distance is small.

Effect of lattice and dopant–induced strain on the conductivity of solid electrolytes: application of the elastic dipole method

Here, we studied the possibility of applying the elastic dipole method (EDM) to predict the response of defect formation and migration energy to an external strain field (ϵij) in Al-doped cubic Li6.25Al0.25La3Zr2O12 (Al-LLZO) and Li10GeP2S12 (LGPS). It is shown that EDM can quantitatively provide accurate values for Li-defect formation energy as a function of ϵij. EDM can also predict, qualitatively, how the migration barrier varies with ϵij. In both Al-LLZO and LGPS systems, the formation energy of Li+ vacancy decreases (increases) by applying a tensile (compressive) strain, which is because the lattice parameters tend to expand by formation of a Li+ vacancy. An opposite behavior is found for the formation energy of interstitial Li+. Furthermore, we found that a compressive strain decreases the diffusion barrier in Al-LLZO, while it increases it in LGPS. The lowering of migration barrier in Al-LLZO is in spite of contraction of bottleneck width of Li diffusion in this system. This finding is in line with a recent experimental study. Analysis of EDM results shows that the lowering (rising) in the migration barrier of Li in Al-LLZO (LGPS) under a compressive strain is due to tendency of the system to contract (expand) Li–O (Li–S) bond lengths in the transition states where Li ions are at the bottlenecks of diffusion pathways. We finally show that the result of Li migration barrier as a function of strain in a non-doped solid electrolyte can be used to predict the global effect of substitution/doping on the conductivity of that system.

Effect of Lattice and Dopant-Induced Strain on the Conductivity of Solid Electrolytes: Application of the Elastic Dipole Method

Here, we studied the possibility of applying the elastic dipole method (EDM) to predict the response of defect formation and migration energy to an external strain field (∊ij) in Al-doped cubic Li6:25Al0:25La3Zr2O12 (Al-LLZO) and Li10GeP2S12 (LGPS). It is shown that EDM can quantitatively provide accurate values for Li-defect formation energy as a function of ∊ij. EDM can also predict, qualitatively, how the migration barrier varies with∊ij. In both Al-LLZO and LGPS systems, the formation energy of Li+ vacancy decreases (increases) by applying a tensile (compressive) strain, which is because the lattice parameters tend to expand by formation of a Li+ vacancy. An opposite behavior is found for the formation energy of interstitial Li+. Furthermore, we found that a compressive strain decreases the diffusion barrier in Al-LLZO, while it increases it in LGPS. The lowering of migration barrier in Al-LLZO is in spite of contraction of bottleneck width of Li diffusion in this system. This finding is in line with a recent experimental study. Analysis of EDM results shows that the lowering (rising) in the migration barrier of Li in Al-LLZO (LGPS) under a compressive strain is due to tendency of the system to contract (expand) Li–O (Li–S) bond lengths in the transition states where Li ions are at the bottlenecks of diffusion pathways. We finally show that the result of Li migration barrier as a function of strain in a non-doped solid electrolyte can be used to predict the global effect of substitution/doping on the conductivity of that system.

Manipulating topological valley modes in plasmonic metasurfaces

Abstract Coupled light-matter modes supported by plasmonic metasurfaces can be combined with topological principles to yield subwavelength topological valley states of light. This study gives a systematic presentation of the topological valley states available for lattices of metallic nanoparticles (NPs): all possible lattices with hexagonal symmetry are considered as well as valley states emerging on a square lattice. Several unique effects that have yet to be explored in plasmonics are identified, such as robust guiding, filtering, and splitting of modes, as well as dual-band effects. These are demonstrated by means of scattering computations based on the coupled dipole method that encompass full electromagnetic interactions between NPs.

Three-Dimensional Scattering From Uniaxial Objects With a Smooth Boundary Using a Multiple Infinitesimal Dipole Method

The formulations for three-dimensional (3D) scattering from uniaxial objects with a smooth boundary using a multiple infinitesimal dipole method (MIDM) are introduced. The proposed technique uses two sets of infinitesimal dipole triplets (IDTs), including three co-located orthogonally polarized electric infinitesimal dipoles, distributed inside and outside of a scatterer to construct simulated fields. The dyadic Green’s functions of uniaxial materials are deployed in the MIDM so as to obtain the simulated fields. The singularity issues in using the uniaxial dyadic Green’s functions, which cannot be solved analytically so far for a general uniaxial medium, can be easily eliminated by using the proposed MIDM. In comparison to the traditional single-layered distribution scheme of IDTs, the proposed multiple-layered distribution scheme can handle the scattering from uniaxial objects accurately and efficiently. Several numerical examples are presented to study bistatic radar cross section (RCS) responses under different scenarios. Excellent agreement is achieved by comparing numerical results with those obtained from commercial software packages, while the simulation performance including CPU time and required memory is drastically improved by using the MIDM when computing a general uniaxial material or a relatively larger object. The proposed technique has its merits on simplicity, conciseness and fast computation in comparison to existing numerical methods.

An approach for the calculation of local dielectric properties in nanosized systems on the basis of coupled dipoles method

Despite interest in a description of nanoscale systems properties, there is still a lack of methods that can grasp size dependence and variation across the interface of the static dielectric properties. We propose an approach for the calculation of local nanoscopic electric susceptibility and dielectric permittivity tensors in nanosized systems on the basis of the coupled dipoles method. Requiring only the polarizability of individual molecules, this approach allows one to determine the local dielectric properties of arbitrarily shaped nanoparticles as well as (semi)infinite bodies such as bulk material and infinite films, expanding our ability to predict and govern the behavior of nanoscale systems.