Point Electric Dipole Research Articles

Influence of image forces on charge-dipole interaction in two-layered systems.

Interaction between a fixed point electric charge Q and a freely rotating point electric dipole with the magnitude P pinned near a plane interface between two dispersionless insulators with different dielectric permittivities ɛ1 and ɛ2 has been considered. It was shown that, as a result of this interaction and the interaction of the dipole with the polarization charges induced at the interface by the charge Q and the dipole itself, there arise regions where the dipole can possess either one or two equilibrium orientations. The spatial distributions of the electrostatic dipole energy Wtotal under the combined action of the charge Q and the induced interface polarization charges, as well as the equilibrium dipole orientations (orientation maps), the boundaries between the regions with different numbers of dipole orientations, and their evolution with the variation of problem parameters (the charge and dipole magnitudes, the mismatch between ɛ1 and ɛ2, and the charge-interface distance) were calculated. It was shown that there can emerge local minima of Wtotal, which may play the role of traps for dipoles (in particular, excitons in layered structures), and the corresponding requirements for the problem parameters were found. Most results were obtained in analytical form. The model can be applied to various physical systems, for instance, polar molecules, excitons, and trions in layered structures.

A Combined Magnetoelectric Sensor Array and MRI-Based Human Head Model for Biomagnetic FEM Simulation and Sensor Crosstalk Analysis.

Magnetoelectric (ME) magnetic field sensors are novel sensing devices of great interest in the field of biomagnetic measurements. We investigate the influence of magnetic crosstalk and the linearity of the response of ME sensors in different array and excitation configurations. To achieve this aim, we introduce a combined multiscale 3D finite-element method (FEM) model consisting of an array of 15 ME sensors and an MRI-based human head model with three approximated compartments of biological tissues for skin, skull, and white matter. A linearized material model at the small-signal working point is assumed. We apply homogeneous magnetic fields and perform inhomogeneous magnetic field excitation for the ME sensors by placing an electric point dipole source inside the head. Our findings indicate significant magnetic crosstalk between adjacent sensors leading down to a 15.6% lower magnetic response at a close distance of 5 mm and an increasing sensor response with diminishing crosstalk effects at increasing distances up to 5 cm. The outermost sensors in the array exhibit significantly less crosstalk than the sensors located in the center of the array, and the vertically adjacent sensors exhibit a stronger crosstalk effect than the horizontally adjacent ones. Furthermore, we calculate the ratio between the electric and magnetic sensor responses as the sensitivity value and find near-constant sensitivities for each sensor, confirming a linear relationship despite magnetic crosstalk and the potential to simulate excitation sources and sensor responses independently.

(1+1)D QCD with heavy adjoint quarks

In this paper, we determine at weak coupling the non-relativistic $n$-body Schr\"{o}dinger equation that describes the low-lying color singlet bound states of two dimensional adjoint $QCD$ with heavy quarks. In the case of three adjoint quarks, we show that the three-body equation reduces equivalently to the Schr\"{o}dinger equation that describes a point electric dipole in an electric field in a plane angular sector. We conjecture that the three-body problem is solvable. The eigenstates are given in terms of the triconfluent Heun functions. Our conjecture implies that a bound state of three adjoint quarks is described by a particle confined in a two dimensional Cornell potential. We expect the $n$-parton problem also to be solvable in a similar fashion.

Excitation of Chiral Cavity Plasmon Resonances in Film‐Coupled Chiral Au Nanoparticles

AbstractChiral plasmonic nanostructures have attracted increasing attention because of their superchiral near‐fields as well as strong far‐field chiral optical response. Recently, the development of chemical synthesis methods enabled the large‐scale manufacturing of three‐dimensional colloidal chiral plasmonic nanocrystals. Further improving the chiral optical response of such nanostructures will greatly facilitate their practical applications. In this work, it is found both in calculations and experiments that chiral cavity plasmon resonances can be excited in film‐coupled chiral Au helicoid nanoparticles, enabling the significant enhancement of the nanostructure chiral optical response. In addition, it is demonstrated from simulation that the chiral cavity mode can modulate the emission polarization of a point electric dipole placed in the nanocavity formed by the nanoparticle and the Au film, allowing the emission of almost circularly polarized photons by the linear dipole with the emission circular polarization anisotropy factor reaching as high as 93%. The film‐coupled chiral plasmonic nanoparticles therefore provide a promising platform for the construction of advanced chiral optical devices such as on‐chip nonreciprocal nanoscale light sources, chiral plasmonic sensors, chiral metamaterials, plasmonically enabled valleytronic devices, and nanophotonic circuits for future on‐chip communication applications.

Theory and simulations of angular momentum transfer from swift electrons to spherical nanoparticles in scanning transmission electron microscopy

Electron beams in scanning transmission electron microscopy (STEM) exert forces and torques on study samples, with magnitudes that allow the controlled manipulation of nanoparticles (a technique called electron tweezers). Related theoretical research has mostly focused on the study of forces and linear momentum transfers from swift electrons (like those used in STEM) to nanoparticles. However, theoretical research on the rotational aspects of the interaction would benefit not only the development of electron tweezers, but also other fields within electron microscopy such as electron vortices. Starting from a classical-electrodynamics description, we present a theoretical model, alongside an efficient numerical methodology, to calculate the angular momentum transfer from a STEM swift electron to a spherical nanoparticle. We show simulations of angular momentum transfers to aluminum, gold, and bismuth nanoparticles of different sizes. We found that the transferred angular momentum is always perpendicular to the system's plane of symmetry, displaying a constant direction for all the cases considered. In the simulations, the angular momentum transfer increased with the radius of the nanoparticle, but decreased as the speed of the electron or the impact parameter increased. Also, the electric contribution to the angular momentum transfer dominated over the magnetic one, being comparable only for high electron's speeds (greater than 90% of the speed of light). Additionally, for nanoparticles with 1 nm radius of the studied materials, we found validity criteria for the small-particle approximation (in which the nanoparticle is modeled as an electric point dipole). We believe that these findings contribute to the understanding of rotational aspects present in STEM experiments, and might be useful for further developments in electron tweezers and other electron microscopy related techniques.

Enhanced wave localization in multifractal scattering media

In this paper we study the structural, scattering, and wave localization properties of multifractal arrays of electric point dipoles generated from multiplicative random fields with different degrees of multiscale correlations. Specifically, using the rigorous Green's matrix method, we investigate the scattering resonances and wave localization behavior of systems with $N=10^{4}$ dipoles and demonstrate an enhanced localization behavior in highly inhomogeneous multifractal structures compared to homogeneous fractals, or monofractals. We show distinctive spectral properties, such as the absence of level repulsion in the strong multiple scattering regime and power-law statistics of level spacings, which indicate a clear localization transition enhanced in non-homogeneous multifractals. Our findings unveil the importance of multifractal structural correlations in the multiple scattering regime of electric dipole arrays and provide an efficient model for the design of multiscale nanophotonic systems with enhanced light-matter coupling and localization phenomena beyond what is possible with traditional fractal systems.

Near-Field Behaviors of Internal Energy Flows of Free-Space Electromagnetic Waves Induced by Electric Point Dipoles

Both orbital and spin energy fluxes constitute the internal flows decomposed from a Poynting vector. For generic electromagnetic waves propagating through source-free media, these energy fluxes are quadratic in field variables so that their properties are not easily predictable. Notwithstanding, their near-field behaviors play important roles in nanoscale photonics. For time-oscillatory fields, we found two hitherto-overlooked distinctions between the two internal flows. The first is an unequal level between them because the vorticity of an orbital energy flux plays a role comparable to a spin energy flux itself. The second is regarding the electric-magnetic dual symmetry in handling the two internal flows, whence the reactive helicity plays a role as important as the electromagnetic helicity. By helicity conservation, an inter-electric-magnetic transport is possible for the spin angular momentum density, while the electric and magnetic constituents of orbital energy fluxes admit only respective intra-electric and intra-magnetic transports. We have tested the validities of all these claims by exemplarily taking the electromagnetic fields induced by an electric point dipole, either a linear or a circular one. We have thus made new contributions not only in deriving explicit forms of the internal energy flows but also in revealing the magnetic activities hidden under the electromagnetic waves induced by electric point dipoles.

Probing spatial orientability of a Friedmann-Robertson-Walker spatially flat spacetime

One important global topological property of a spacetime manifold is orientability. It is widely believed that spatial orientability can only be tested by global journeys around the Universe to check for orientation-reversing closed paths. Since such global journeys are not feasible, theoretical arguments that combine universality of physical experiments with local arrow of time, $CP$ violation and $CPT$ invariance are usually offered to support the choosing of time- and space-orientable spacetime manifolds. The nonexistence of globally defined spinor fields on a nonorientable spacetime is another theoretical argument for orientability. However, it is conceivable that orientability can be put to test by local physical effects. In this paper, we show that it is possible to locally access spatial orientability of a spatially flat Friedmann-Robertson-Walker spacetime through quantum vacuum electromagnestic fluctuations. We argue that a putative nonorientability of the spatial sections of spatially flat Friedmann-Robertson-Walker spacetime can be ascertained by the study of the stochastic motions of a charged particle or a point electric dipole under quantum vacuum electromagnetic fluctuations. In particular, the stochastic motions of a dipole permit the recognition of a presumed nonorientability of three space in itself.

Fingerprints of Critical Phenomena in a Quantum Paraelectric Ensemble of Nanoconfined Water Molecules.

We have studied the radio frequency dielectric response of a system consisting of separate polar water molecules periodically arranged in nanocages formed by the crystal lattice of the gemstone beryl. Below T = 20-30 K, quantum effects start to dominate the properties of the electric dipolar system as manifested by a crossover between the Curie-Weiss and the Barrett regimes in the temperature-dependent real dielectric permittivity ε'(T). When analyzing in detail the temperature evolution of the reciprocal permittivity (ε')-1 down to T ≈ 0.3 K and comparing it with the data obtained for conventional quantum paraelectrics, like SrTiO3, KTaO3, we discovered clear signatures of a quantum-critical behavior of the interacting water molecular dipoles: Between T = 6 and 14 K, the reciprocal permittivity follows a quadratic temperature dependence and displays a shallow minimum below 3 K. This is the first observation of "dielectric fingerprints" of quantum-critical phenomena in a paraelectric system of coupled point electric dipoles.

Degrees of circular polarization for the electromagnetic waves induced by electric point dipoles

An electric point dipole is the simplest model for emitters, antennas, and other nanostructures. For a dipole sufficiently small in comparison to the wavelength of the radiated electromagnetic wave, its electric-field intensity and Poynting vector are fully understood. Firstly, both decrease in magnitude with increasing radial distance from the dipole location. Secondly, the Poynting vector increases in magnitude with the sinusoid squared of the polar angle measured from the dipolar axis. We will consider here the degree of circular polarization (CP) of the electromagnetic wave induced by an electric point dipole. Resultantly, we learned that the near-perfect CP states form a pair of hollow-cylindrical surfaces. The radius of those surfaces designating near-perfect CP states is approximately one wavelength detached from the dipolar axis. However, these pictures are appreciably altered for the electromagnetic field induced by an array of electric point linear dipoles.

Harnessing superdirectivity in dielectric spherical multilayer antennas

Small form-factor, narrowband, and highly directive antennas are of critical importance in a variety of applications spanning wireless communications, remote sensing, Raman spectroscopy, and single photon emission enhancement. Surprisingly, we show that the classical directivity limit can be appreciably surpassed for electrically small multilayer spherical antennas excited by a point electric dipole even if limiting ourselves to purely dielectric materials. Experimentally feasible designs of superdirective antennas are established by using a stochastic optimization algorithm combined with a rigorous analytic solution.

Excitation of a homogeneous dielectric sphere by a point electric dipole

Electrically small dielectric antennas are of great interest for modern technologies, since they can significantly reduce the physical size of electronic devices for processing and transmitting information. We investigate the influence of the resonance conditions of an electrically small dielectric spherical antenna with a high refractive index on its directivity and analyze the dependence of these resonances on the effectively excited modes of the dielectric sphere.

Analogy approach in solving the problem of a moving electric dipole

Using the principle of permutational duality as well as the analogical approach of both electrical and magnetic dipoles, it is possible to obtain a new relativistic magnetic type vector potential of an arbitrarily moving electric point dipole. The specific cases of the magnetic vector potential in the calculation of fields have been considered, and the obtained results have been compared with the research of other scholars.

Engineering gallium phosphide nanostructures for efficient nonlinear photonics and enhanced spectroscopies

Abstract Optical resonances arising from quasi-bound states in the continuum (QBICs) have been recently identified in nanostructured dielectrics, showing ultrahigh quality factors accompanied by very large electromagnetic field enhancements. In this work, we design a periodic array of gallium phosphide (GaP) elliptical cylinders supporting, concurrently, three spectrally separated QBIC resonances with in-plane magnetic dipole, out-of-plane magnetic dipole, and electric quadrupole characters. We numerically explore this system for second-harmonic generation and degenerate four-wave mixing, demonstrating giant per unit cell conversion efficiencies of up to ∼ 2 W−1 and ∼ 60 W−2, respectively, when considering realistic introduced asymmetries in the metasurface, compatible with current fabrication limitations. We find that this configuration outperforms by up to more than four orders of magnitude the response of low-Q Mie or anapole resonances in individual GaP nanoantennas with engineered nonlinear mode-matching conditions. Benefiting from the straight-oriented electric field of one of the examined high-Q resonances, we further propose a novel nanocavity design for enhanced spectroscopies by slotting the meta-atoms of the periodic array. We discover that the optical cavity sustains high-intensity fields homogeneously distributed inside the slot, delivering its best performance when the elliptical cylinders are cut from end to end forming a gap, which represents a convenient model for experimental investigations. When placing an electric point dipole inside the added aperture, we find that the metasurface offers ultrahigh radiative enhancements, exceeding the previously reported slotted dielectric nanodisk at the anapole excitation by more than two orders of magnitude.

Inquiring electromagnetic quantum fluctuations about the orientability of space

Orientability is an important global topological property of spacetime manifolds. It is often assumed that a test for spatial orientability requires a global journey across the whole 3-space to check for orientation-reversing paths. Since such a global expedition is not feasible, theoretical arguments that combine universality of physical experiments with local arrow of time, CP violation and CPT invariance are usually offered to support the choosing of time- and space-orientable spacetime manifolds. Another theoretical argument also offered to support this choice comes from the impossibility of having globally defined spinor fields on non-orientable spacetime manifolds. In this paper, we argue that it is possible to locally access spatial orientability of Minkowski empty spacetime through physical effects involving quantum vacuum electromagnetic fluctuations. We study the motions of a charged particle and a point electric dipole subject to these electromagnetic fluctuations in Minkowski spacetime with orientable and non-orientable spatial topologies. We derive analytic expressions for a statistical orientability indicator for both of these point-like particles in two inequivalent spatially flat topologies. For the charged particle, we show that it is possible to distinguish the orientable from the non-orientable topology by contrasting the time evolution of the orientability indicators. This result reveals that it is possible to access orientability through electromagnetic quantum vacuum fluctuations. However, the answer to the central question of the paper, namely how to locally probe the orientability of Minkowski 3-space intrinsically, comes about only in the study of the motions of an electric dipole. For this point-like particle, we find that a characteristic inversion pattern exhibited by the curves of the orientability statistical indicator is a signature of non-orientability. This result makes it clear that it is possible to locally unveil spatial non-orientability through the inversion pattern of curves of our orientability indicator for a point electric dipole under quantum vacuum electromagnetic fluctuations. Our findings might open the way to a conceivable experiment involving quantum vacuum electromagnetic fluctuations to locally probe the spatial orientability of Minkowski empty spacetime.

Hyperuniformity and wave localization in pinwheel scattering arrays

We investigate the structural and spectral properties of deterministic aperiodic arrays designed from the statistically isotropic pinwheel tiling. By studying the scaling of the cumulative integral of its structure factor in combination with higher-order structural correlation analysis we conclude that pinwheel arrays belong to the weakly hyperuniformity class. Moreover, by solving the multiple scattering problem for electric point dipoles using the rigorous Green's matrix theory, we demonstrate a clear transition from diffusive transport to localization behavior. This is shown by studying the Thouless number as a function of the scattering strength and the spectral statistics of the scattering resonances. Surprisingly, despite the absence of sharp diffraction peaks, clear spectral gaps are discovered in the density of states of pinwheel arrays that manifest a distinctive long-range order. Furthermore, the level spacing statistics at large optical density exhibits a sharp transition from level repulsion to the Poisson behavior, consistently with the onset of the wave localization regime. Our findings reveal the importance of hyperuniform aperiodic structures with statistically isotropic k-space for the engineering of enhanced light-matter interaction and localization properties.

Time-dependent forces between a swift electron and a small nanoparticle within the dipole approximation

In this paper we calculate the time-dependent forces between a swift electron traveling at constant velocity and a metallic nanoparticle made of either aluminum or gold. We consider that the nanoparticle responds as an electric point dipole and we use classical electrodynamics to calculate the force on both the nanoparticle and the electron. The values for the velocity of the electron and the radius of the nanoparticle were chosen in accordance with electron microscopy observations, and the impact parameter was selected to fulfill the constraints imposed by the dipole approximation. We found that there are times when the force on the nanoparticle is attractive and others when it is repulsive, and show that this is due to the delayed electromagnetic response of the nanoparticle. To establish the limits of validity of our approach, we calculate the total linear momentum transfer to the nanoparticle, and compare it with results obtained, in frequency space, using the full multipole expansion of the fields induced on the nanoparticle, considering the effects of electromagnetic radiation.

A point electric dipole: From basic optical properties to the fluctuation–dissipation theorem

We comprehensively review the deceptively simple concept of dipole scattering in order to uncover and resolve all ambiguities and controversies existing in the literature. First, we consider a point electric dipole in a non-magnetic environment as a singular point in space whose sole ability is to be polarized due to the external electric field. We show that the postulation of the Green’s dyadic of the specific form provides the unified description of the contribution of the dipole into the electromagnetic properties of the whole space. This is the most complete, concise, and unambiguous definition of a point dipole and its polarizability. All optical properties, including the fluctuation–dissipation theorem for a fluctuating dipole, are derived from this definition. Second, we obtain the same results for a small homogeneous sphere by taking a small-size limit of the Lorenz–Mie theory. Third, and most interestingly, we generalize this microscopic description to small particles of arbitrary shape. Both bare (static) and dressed (dynamic) polarizabilities are defined as the double integrals of the corresponding dyadic transition operator over the particle’s volume. While many derivations and some results are novel, all of them follow from or are connected with the existing literature, which we review throughout the paper.

Broadened Angle-Insensitive Near-Perfect Absorber Based on Mie Resonances in Amorphous Silicon Metasurface.

A broadband near-perfect absorber is analyzed by an amorphous silicon (a-Si) hook shaped nanostructure metasurface. The transmission and reflection coefficients of the metasurface are investigated in the point electric and magnetic dipole approximation. By combining square and semicircle nanostructures, the effective polarizabilities of the a-Si metasurface calculated based on discrete dipole approximation (DDA) exhibit broadened peaks of electric dipole (ED) and magnetic dipole (MD) Mie resonances. The optical spectra of the metasurface are simulated with different periods, in which suppressed transmission are shifted spectrally to overlap with each other, leading to broadened enhanced absorption induced by interference of ED and MD Mie resonances. The angle insensitive absorption of the metasurface arrives 95% in simulation and 85% in experiment in spectral range from 564 nm to 584 nm, which provides potential applicability in nano-photonic fields of energy harvesting and energy collection.

Critical behavior for point monopole and dipole electric impurities in uniformly and uniaxially strained graphene

We revisit the problem of bound states in graphene under the influence of point electric monopole and dipole impurity potentials extended to the case in which the membrane of this material is uniformly and uniaxially strained, which leads to a redefinition of the charge and dipole moment, respectively. By considering an anisotropic Fermi velocity, we analytically solve the resulting Dirac equation for each potential. We observe that the effect of the anisotropy is to promote or inhibit the critical behavior known to occur for each kind of impurity, depending on the direction along which strain is applied: both the atomic collapse, for the monopole impurity, and the emergence of cascades of infinitely many bound states with a universal Efimov-like scaling, for the dipole impurity, are phenomena that occur under less or more restrictive conditions due to strain.