Coupled Dipole Method Research Articles

Revealing the Invisible: Imaging Through Non-Radiating Subspace

This paper presents a novel modification to the subspace optimization method (SOM) for solving inverse scattering problems in diverse background media. By incorporating sequential quadratic programming (SQP), a fast and accurate optimization technique, our approach aims to improve the reconstruction of dielectric objects buried in complex environments. We investigate the influence of non-radiating (NR) subspace reconstruction on imaging quality by analyzing the induced current-exterior field mapping operator's singular values. The scattering formulation of direct problems is performed through numerical methods such as coupled dipole method, finite elements-boundary integral, and electric field integral equations. Radiating and non-radiating objective functions are defined and minimized using SQP to evaluate the effectiveness of the proposed method. Numerical experiments demonstrate significant enhancements in imaging quality, robustness, and computational efficiency compared to existing techniques. This modified SOM offers a promising avenue for precise and reliable reconstruction of electromagnetic scattering objects, with potential applications in various fields including medical imaging, remote sensing, and non-destructive evaluation.

Second‐Order Topological Corner States in Square Lattice Plasmonic Metasurfaces with C4 and Glide Symmetries

Recently, plasmonic metasurfaces have emerged as a platform for topological photonics, exhibiting both advantages of plasmon‐induced tight confinement of local field and topological robustness. Most previous works regarding plasmonic systems are limited to the first‐order topologies and only a few studies dealt with higher‐order topological states in honeycomb lattices. Moreover, second‐order topologies of square lattice plasmonic systems have not yet been studied. This work presents second‐order topological corner states in the square lattices of metallic nanoparticles (NPs) with various symmetries, taking two different C4 and glide symmetries as examples. Their unit cells are obtained from nonprimitive cells, consisting of four equal spheroidal NPs, by expanding (or shrinking), rotating, and resizing. Bulk bands and spectral functions of the unit cells calculated by using the coupled dipole method well agree with COMSOL simulation results, revealing the accuracy of the numerical calculations as well as the experimental realizability of the systems. Second‐order topological corner states and their robustness against structural disorder are numerically shown for three different square lattices. This work will trigger extensive investigations to open a new realm of topological metasurfaces with promising applications.

Emergent mobility edges and intermediate phases in one-dimensional quasiperiodic plasmonic chains

Anderson localization has been widely studied in low-dimensional aperiodic electronic, photonic, and acoustic systems. However, the disorder effect in the plasmonic system, where retardation and long-range couplings interact in complex ways, remains an open question. In this work, we investigate the localization properties of one-dimensional quasiperiodic plasmonic chains using the coupled dipole method and linearized Green's function. Our models, which incorporate nearest-neighbor or long-range dipole interactions, reveal localization transitions, mobility edges, and intermediate phases. It is found that long-range dipole interactions and non-Hermiticity due to retardation both play crucial roles in Anderson localization, yielding the emergence of intermediate phases with varying widths. A link between non-Hermiticity and Anderson transition is established by the mean phase rigidity, revealing strong non-Hermiticity along the phase boundary. The plasmonic model involving long-range interplay and retarded effect presents richer localization phenomena than the electronic counterpart that usually includes only nearest-neighbor coupling, laying a foundation for experimental observations of Anderson localization on plasmonic platforms. Published by the American Physical Society 2024

Electromagnetically Induced Absorption Overcomes the Upper Limit of Light Absorption: Dipole-Dipole Coupling with Phase Retardation in Plasmonic-Dielectric Dimers.

Electromagnetically induced absorption (EIA) by a phase-retarded coupling is theoretically investigated using a dimer composed of a plasmonic and dielectric particle. This phase-retarded coupling originates from the particles interacting with each other through their scattered intermediate fields (in between near and far fields). Our analysis based on the coupled-dipole method and an extended coupled-oscillator model indicates that EIA by the phase-retarded coupling occurs due to constructive interference in the scattered fields of the particles. By employing the finite element method, we demonstrate that the absorption of the plasmonic particle is dramatically enhanced by tuning the interparticle distance and achieving constructive interference. In contrast to EIA by near-field coupling, which has been intensively researched using coupled plasmonic systems, EIA by a phase-retarded coupling enables us to strengthen the absorption of plasmonic systems more significantly. This significant absorption enhancement is expected to be beneficial to advancing various applications, such as energy harvesting and radiative cooling.

Anisotropic Purcell Effect and Quantum Interference in Fractal Aggregates of Nanoparticles

We study theoretically the emergence of an anisotropic Purcell factor in random two-dimensional fractal aggregates of nanoparticles. These nanoparticles can either be metallic nanoparticles made of silver, which exhibit surface plasmon resonances, or high-index dielectric nanoparticles like silicon, which possess optical Mie resonances. To calculate the spontaneous emission rates of a quantum emitter, we utilize the electromagnetic Green’s tensor within the framework of the coupled-dipole method. Our findings reveal that the Purcell factor exhibits spatial variations, with certain regions, referred to as hot spots, displaying high values for dipoles oriented within the plane of the fractal aggregate, while dipoles oriented vertically to the aggregate have values close to unity. This anisotropy in the Purcell factor leads to significant quantum interference effects in the spontaneous emission paths of multi-level quantum emitters. As a consequence of this quantum interference, we demonstrate the occurrence of population trapping in a V-type quantum emitter embedded within a fractal aggregate of nanoparticles which cannot otherwise take place if the emitter is placed in vacuum.

The amplitude of light scattering by a hexagonal column with hollow ends in the Rayleigh-Gans-Debye approximation

The formulas for light scattering amplitude and phase function [or element of scattering matrixf11] of hexagonal column with concave ends in the Rayleigh-Gans-Debye (RGD) approximation are derived. Numerical results for the phase function of light scattering of hexagonal column with pyramidal hollow ends in the RGD approximation and in the method of Purcell-Pennypacker (or discrete dipole method, or coupled dipole method) are compared. The good agreement for hexagonal columns having small phase shifts has been found.

CDPDS: Coupled dipole method-based photonic dispersion solver

We develop and release a photonic band dispersion solver based on the coupled dipole method called CDPDS, which aims to provide an analytical computation of bulk and boundary dispersions and topological phases of a one-dimensional and two-dimensional photonic crystal consisting of an array of particles. The main advantages of CDPDS include (i) a wide coverage of computation that spans the bulk dispersion of a unit cell, boundary dispersion of a supercell comprising one or two types of photonic crystals, and topological phases, (ii) the inclusion of a straightforward graphical user interface that facilitates high accessibility to users who have no expertise in computer programming, and (iii) the addition of built-in options that are useful in examining the photonic dispersions of several widely used systems. The basic principle and computational method incorporated into CDPDS and its performance verification using two distinct photonic crystals are presented in this article. The results indicate that CDPDS will serve as helpful and accessible guidance for computing photonic band dispersions in the fields of conventional and topological photonics. Program summaryProgram Title: CDPDSCPC Library link to program files:https://doi.org/10.17632/865ntt8w7v.1Licensing provisions: LGPLProgramming language: Python 3.9Supplementary material: User manual, tutorial movie, and supplementary material file containing the reproduction of three photonic systems.Nature of problem: An analytical band dispersion analysis of a photonic crystal composed of an array of particles. The photonic dispersion and physics therein are important in investigating periodically arranged photonic systems. Additionally, there is an increasing demand for free and accessible software based on an analytical framework for computing bulk and boundary dispersions, electromagnetic field distributions, and topological phases of one-dimensional and two-dimensional photonic crystals.Solution method: CDPDS provides portable software for fast and efficient computation of bulk and boundary dispersions and topological phases of 1D and 2D photonic crystals using the coupled dipole method incorporated with the Bloch theorem. A particle that consists a photonic crystal is modeled as a point dipole with a prespecified polarizability tensor. CDPDS is highly accessible, especially for users who have no expertise in computer programming.Additional comments including restrictions and unusual features: The coupled dipole method models each sublattice of the photonic crystal of interest to a point dipole. Thus, photonic systems with geometric complexity can only be examined using simplifications.

Generalized coupled dipole method for thermal far-field radiation

We introduce a many-body theory for thermal far-field emission of dipolar dielectric and metallic nanoparticles in the vicinity of a substrate within the framework of fluctuational electrodynamics. Our theoretical model includes the possibility to define the temperatures of each nanoparticle, the substrate temperature, and the temperature of the background thermal radiation, separately. To demonstrate the versatility of our method, we apply it in an exemplary way by discussing the thermal radiation of four particle assemblies of SiC and Ag nanoparticles above a planar SiC and Ag substrate. Furthermore, we use discrete dipole approximation to determine the thermal emission of a spherical nanoparticle in free space and close to a substrate. Finally, the calculation of the thermal far-field radiation of a sharp Si tip close to a SiC substrate using the discrete dipole approximation including the near-field scattering by the tip as well as the thermal emission of the tip and the contribution of the substrate which is partially blocked by the tip serves as another example.

Selective broadband absorption by mode splitting for radiative cooling

A plasmonic-photonic structure based on colloidal lithography was designed for a scalable radiative cooling system and its absorption properties were theoretically investigated. The structure comprises a SiO2 core, which is on top of an Au reflector and partially covered by an indium tin oxide (ITO) shell. This simple and scalable structure possesses a strong selective absorption in the primary atmospheric transparency window (8-13 µm). The strong selective absorption is attributed to a mode splitting of the localized surface plasmon (LSP) of the ITO shell. To understand the mechanisms of the mode splitting, a quantitative analysis was conducted using a coupled-oscillator model and a coupled-dipole method. The analysis revealed that the mode splitting is induced by a strong coupling between the LSP of the ITO shell and a magnetic dipole Mie resonance of the SiO2 core.

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.

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.

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.

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.

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.

Systematics of plexciton formation in linear chains of quantum emitters and metal nanoparticles

We present systematic numerical calculations of plexcitonic properties of a linear chain comprised of quantum emitters - metal nanoparticle dimers interacting in the strong coupling regime. The calculations are based on a recently developed semi-analytical technique based on a multiple-scattering polaritonic-operator formalism in conjunction with an electromagnetic coupled dipole method. Based on this method, we calculate the two main features corresponding to the formation of plexcitonic chain modes: the primary plexcitonic resonance as well as the corresponding Rabi splitting associated with the generation of an avoided-crossing area. We apply an exponential model to the numerical results obtained via the above technique to describe the plexciton redshift induced with increasing chain length. We also identify an asymptotic plexciton frequency at a chain length of approximately 20 dimers. A model Hamiltonian is employed to assess the energy splitting defined by the avoided crossing area of the main plexciton resonance which is found to scale as where N is the number of dimers in the chain.

Unidirectional light scattering by electric dipoles induced in plasmonic nanoparticles

We proposed and demonstrated that unidirectional light scattering, usually realized by utilizing induced electric and magnetic dipole moments, can also be achieved via only electric dipole moments induced in an assembly of plasmonic nanoparticles. The condition for such kinds of unidirectional scattering has been derived in the dipole approximation. Calculated with the coupled dipole method, a contrast between the forward and backward scattering has been demonstrated with nearly five orders of magnitude for a dimer of gold nanospheres. It can be attributed to the destructive interference of scattered far field from the induced electric dipole moments with similar magnitudes and a relative phase of about π. Such properties are general for an assembly of electric dipole moments, and a trimer structure has been demonstrated to realize unidirectional light scattering as well as steering. Our results suggest a simple but yet versatile platform for manipulating light scattering.