Chains Of Plasmonic Nanoparticles Research Articles

Opto-mechanically generated resonant field enhancement

A link between the resonant cumulative field enhancement experienced by a chain of plasmonic nanoparticles in a light field and the orientation of the chain with respect to the field is obtained. We calculate analytically the optical torque and the equilibrium configuration and we show how stable orientations are triggered by the geometric resonance conditions. Analytical predictions are checked using numerical calculations based on the coupled dipoles method (CDA) for the particular case of a chain of silver nanoparticles. The reported resonance driven optical torque allows for a tuning of the orientation of the chain depending on radiation’s wavelength.

1-D Periodic Green’s Function for Leaky and Complex Waves Using the Ewald Method

The Ewald method for evaluating the free-space 1-D periodic Green’s function is extended to leaky waves by allowing for complex wavenumbers. It is shown that care must be taken when choosing the path of integration in the complex plane in the derivation of the Ewald method in order to obtain solutions that correspond to physical leaky-wave solutions. The use of different paths results in an evaluation of the Ewald method using an analytical continuation of the exponential integral function previously used when the wavenumber is real. The extension of the Ewald method to complex wavenumbers allows for the treatment of practical periodic leaky-wave antennas and periodic guiding structures with losses, as well as metamaterials, including 1-D chains of plasmonic nanoparticles.

Dark spots along slowly scaling chains of plasmonic nanoparticles.

We numerically investigate the optical response of slowly scaling linear chains of mismatched silver nanoparticles. Hybridized plasmon chain resonances manifest unusual local field distributions around the nanoparticles that result from symmetry breaking of the geometry. Importantly, we find localization patterns characterized by bright hot-spots alternated by what we term dark spots. A dark spot is associated to dark plasmons that have collinear and antiparallel dipole moments along the chain. As a result, the field amplification in the dark interjunction gap is extinguished for incident polarization parallel to the chain axis. Despite the strong plasmonic coupling, the nanoparticles on the sides of this dark gap experience a dramatic asymmetric field amplification with amplitude gain contrast > 2×102. Remarkably, also for polarization orthogonal to the axis, gap hot-spots form on resonance.

Overcoming the adverse effects of substrate on the waveguiding properties of plasmonic nanoparticle chains

We have studied numerically the propagation of surface plasmon polaritons (SPPs) in linear periodic chains of plasmonic nanoparticles of different shapes. The chains are deposited on top of a thick dielectric substrate. While in many commonly considered cases the substrate tends to suppress the SPP propagation, we have found that this adverse effect is practically absent in the case when the nanoparticles have the shape of oblate spheroids with sufficiently small aspect ratio (e.g., nanodisks) whose axes of symmetry coincide with the axis of the chain.

New approach for extraordinary transmission through an array of subwavelength apertures using thin ENNZ metamaterial liners.

Extraordinary transmission (ET) through a periodic array of subwavelength apertures on a perfect metallic screen has been studied extensively in recent years, and has largely been attributed to diffraction effects, for which the periodicity of the apertures, rather than their dimensions, dominates the response. The transmission properties of the apertures at resonance, on the other hand, are not typically considered 'extraordinary' because they may be explained using more conventional aperture-theoretical mechanisms. This work describes a novel approach for achieving ET in which subwavelength apertures are made to resonate by lining them using thin, epsilon-negative and near-zero (ENNZ) metamaterials. The use of ENNZ metamaterials has recently proven successful in miniaturizing circular waveguides by strongly reducing their natural cutoff frequencies, and the theory is adapted here for the design of subwavelength apertures in a metallic screen. We present simulations and proof-of-concept measurements at microwave frequencies that demonstrate ET for apertures measuring one-quarter of a wavelength in diameter and suggest the potential for even more dramatic miniaturization simply by engineering the ENNZ metamaterial dispersion. The results exhibit a fano-like profile whose frequency varies with the properties of the metamaterial liner, but is independent of period. It is suggested that similar behaviour can be obtained at optical frequencies, where ENNZ metamaterials may be realized using appropriately arranged chains of plasmonic nanoparticles.

Topological edge plasmon modes between diatomic chains of plasmonic nanoparticles.

We study the topological edge plasmon modes between two "diatomic" chains of identical plasmonic nanoparticles. Zak phase for longitudinal plasmon modes in each chain is calculated analytically by solutions of macroscopic Maxwell's equations for particles in quasi-static dipole approximation. This approximation provides a direct analogy with the Su-Schrieffer-Heeger model such that the eigenvalue is mapped to the frequency dependent inverse-polarizability of the nanoparticles. The edge state frequency is found to be the same as the single-particle resonance frequency, which is insensitive to the separation distances within a unit cell. Finally, full electrodynamic simulations with realistic parameters suggest that the edge plasmon mode can be realized through near-field optical spectroscopy.

Topological Majorana States in Zigzag Chains of Plasmonic Nanoparticles

We propose a simple realization of topological edge states in zigzag chains of plasmonic nanoparticles, mimicking the Kitaev model of Majorana fermions. We demonstrate the one-to-one correspondence between the coupled dipole equations in the zigzag plasmonic chain and the Bogoliubov-de-Gennes equations for the quantum wire on top of superconductor and support the analytical theory by the full-wave electromagnetic simulations. We reveal that localized plasmons can be excited selectively at both edges of the zigzag chain of plasmonic nanoparticles depending on the incident plane wave polarization.

Longitudinal chirality, enhanced nonreciprocity, and nanoscale planar one-way plasmonic guiding

When a linear chain of plasmonic nanoparticles is exposed to a transverse dc magnetic field, the chain modes are elliptically polarized in a single plane parallel to the chain axis; hence, a new chain mode of longitudinal plasmon rotation is created. If, in addition, the chain geometry possesses longitudinal rotation, e.g., by using ellipsoidal particles that rotate in the same plane as the plasmon rotation, strong nonreciprocity is created. The structure possesses a new kind of chirality---longitudinal chirality---and supports one-way guiding. Since all particles rotate in the same plane, the geometry is planar and can be fabricated by printing leaflike patches on a single plane. Furthermore, the magnetic field is significantly weaker than in previously reported one-way guiding structures. These properties are examined for ideal (lossless) and lossy chains.

Coupling and guided propagation along parallel chains of plasmonic nanoparticles

We derive a dynamic closed-form dispersion relation for the analysis of the entire spectrum of guided wave propagation along coupled parallel linear arrays of plasmonic nanoparticles, operating as optical ‘two-line’ waveguides. Compared to linear arrays of nanoparticles, our results suggest that these waveguides may support more confined beams with comparable or even longer propagation lengths, operating analogously to transmission-line segments at lower frequencies. Our formulation fully takes into account the entire dynamic interaction among the infinite number of nanoparticles composing the parallel arrays, considering also the realistic presence of losses and the frequency dispersion of the involved plasmonic materials, providing physical insights into the guidance properties that characterize this geometry.

Magnetized Spiral Chains of Plasmonic Ellipsoids for One-Way Optical Waveguides

When a linear chain of plasmonic nanoparticles is subject to longitudinal magnetic field, it exhibits optical Faraday rotation. If the magnetized nanoparticles are plasmonic ellipsoids arranged as a spiral chain, the interplay between the Faraday rotation and the geometrical spiral rotation (structural chirality) can strongly enhance nonreciprocity. This interplay forms a waveguide that permits one-way propagation only, within four disjoint frequency bands, two bands for each direction.

Slow waves on magnetic metamaterials and on chains of plasmonic nanoparticles: Driven solutions in the presence of retardation

Slow waves on chains or lattices of resonant elements offer a unique tool for guiding and manipulating the electromagnetic radiation on a subwavelength scale. Applications range from radio waves to optics with two major classes of structures being used: (i) metamaterials made of coupled ring resonators supporting magnetoinductive waves and (ii) plasmonic crystals made of nanoparticles supporting waves of near-field coupling. We derive dispersion equations of both types of slow waves for the case when the interelement coupling is governed by retardation effects, and show how closely they are related. The current distribution is found from Kirchhoff’s equation by inverting the impedance matrix. In contrast to previous treatments power conservation is demonstrated in a form relevant to a finite structure: the input power is shown to be equal to the radiated power plus the powers absorbed in the Ohmic resistance of the elements and the terminal impedance. The relations between frequency and wave number are determined for a 500-element line for two excitations using three different methods. Our approach of retrieval of the dispersion from driven solutions of finite lines is relevant for practical applications and may be used in the design of metamaterials and plasmonic crystals with desired properties.

Guided propagation along quadrupolar chains of plasmonic nanoparticles

The employment of periodic arrays of plasmonic nanoparticles has been proposed by several groups for enhanced transmission or absorption and for realizing optical nanowaveguides. Generally, due to their small transverse dimensions, such linear arrays have been operated near their dipolar resonance. However, it has been recently shown that nanoscale plasmonic particles may also support higher-order resonances, which provide some advantages in different applications. Here we derive a full-wave analytical closed-form dispersion equation for the guided and leaky modes supported by linear chains of nanoparticles near a quadrupolar resonance. We show that, despite the vanishing bandwidth of the individual quadrupolar resonance in each of the nanoparticles composing the chain, the overall bandwidth of quadrupolar chain guidance is relatively large due to strong coupling, even considering realistic losses and frequency dispersion of optical materials. Applications for low-damping optical nanotransmission lines and leaky-wave nanoantennas are suggested.