Coupled Plasmon Modes Research Articles

Coupling of plasmon and photon modes in a graphene-based multilayer structure.

We propose a graphene-based plasmonic heterostructure in which a new class of plasmon polariton modes can be realized via fundamental coupling of conventional graphene plasmons (GPs) and spoof surface plasmons or radiation modes. We name these electromagnetic modes coupled graphene plasmon polaritons (CGPPs). It is found that the properties of the CGPPs can be tuned by the geometry of the heterostructure, the material used to fill the holes, and the electron density in graphene. Most interestingly, we show that it is possible to achieve CGPPs with about 10 times enhancement of wave localization or about 300 times enhancement of propagation length, compared to usual GPs through varying device parameters. These features can be applied to tunable terahertz and infrared plasmonic devices.

Sui Generis Cerulean Fluorescence from YAG:Ce by Selective Plasmonic Mode Coupling with Silver Nanorods

Two dimensional metal nanostructures such as silver nanorods (AgNRs) exhibit a unique surface plasmon resonance (SPR) pattern involving transverse (TM) and longitudinal modes (LM). Such plasmonic near field is observed to have a profound effect on YAG:Ce nanoparticles placed in optimal proximity resulting in an unprecedented blue emission at TM excitation (383 nm). Unembellished YAG:Ce does not fluoresce under UV light incidence which implies that the phenomenon is exclusively due to modification of Ce3+ energy levels by plasmonic near field generated by AgNRs. Finite difference time domain (FDTD) electrodynamics simulation of near fields generated by exact AgNR and particle hybrids at TM, LM, and emission frequencies establish a cause-effect relationship. Plasmon-induced new emission from industrially important YAG:Ce nanophosphor opens up newer vistas for solid state lighting.

Metamaterial Absorbers for Infrared Detection of Molecular Self-Assembled Monolayers.

The emerging field of plasmonic metamaterials has introduced new degree of freedom to manipulate optical field from nano to macroscopic scale, offering an attractive platform for sensing applications. So far, metamaterial sensor concepts, however, have focused on hot-spot engineering to improve the near-field enhancement, rather than fully exploiting tailored material properties. Here, we present a novel spectroscopic technique based on the metamaterial infrared (IR) absorber allowing for a low-background detection scheme as well as significant plasmonic enhancement. Specifically, we experimentally demonstrate the resonant coupling of plasmonic modes of a metamaterial absorber and IR vibrational modes of a molecular self-assembled monolayer. The metamaterial consisting of an array of Au/MgF2/Au structures exhibits an anomalous absorption at ~3000 cm−1, which spectrally overlaps with C-H stretching vibrational modes. Symmetric/asymmetric C-H stretching modes of a 16-Mercaptohexadecanoic acid monolayer are clearly observed as Fano-like anti-resonance peaks within a broad plasmonic absorption of the metamaterial. Spectral analysis using Fano line-shape fitting reveals the underlying resonant interference in plasmon-molecular coupled systems. Our metamaterial approach achieves the attomole sensitivity with a large signal-to-noise ratio in the far-field measurement, thus may open up new avenues for realizing ultrasensitive IR inspection technologies.

Quantitative Plasmon Mode and Surface-Enhanced Raman Scattering Analyses of Strongly Coupled Plasmonic Nanotrimers with Diverse Geometries.

Here, we quantitatively monitored and analyzed the spectral redistributions of the coupled plasmonic modes of trimeric Au nanostructures with two ∼1 nm interparticle gaps and single-dye-labeled DNA in each gap as a function of varying trimer symmetries. Our precise Mie scattering measurement with the laser-scanning-assisted dark-field microscopy allows for individual visualization of the orientations of the radiation fields of the coupled plasmon modes of the trimers and analyzing the magnitude and direction of the surface-enhanced Raman scattering (SERS) signals from the individual plasmonic trimers. We found that the geometric transition from acute-angled trimer to linear trimer induces the red shift of the longitudinally polarized mode and the blue shift of the axially polarized mode. The finite element method (FEM) calculation results show the distinct "on" and "off" of the plasmonic modes at the two gaps of the trimer. Importantly, the single-molecule-level systematic correlation studies among the near-field, far-field, and surface-enhanced Raman scattering reveal that the SERS signals from the trimers are determined by the largely excited coupled plasmon between the two competing plasmon modes, longitudinal and axial modes. Further, the FEM calculation revealed that even 0.5 nm or smaller discrepancy in the sizes of two gaps of the linear trimer led to >10-fold difference in the SERS signal. Granted that two gap sizes are not likely to be completely the same in actual experiments, one of two gaps plays a more significant role in generating the SERS signal. Overall, this work provides the knowledge and handles for the understanding and systematic control of the magnitude and polarization direction of the both plasmonic response and SERS signal from trimeric nanostructures and sets up the platform for the optical properties and the applications of plasmonically coupled trimers and higher multimeric nanostructures.

STEM-EELS analysis of multipole surface plasmon modes in symmetry-broken AuAg nanowire dimers.

Surface plasmon coupling in nanowires separated by small gaps generates high field enhancements at the position of the gap and is thus of great interest for sensing applications. It is known that the nanowire dimensions and in particular the symmetry of the structures has strong influence on the plasmonic properties of the dimer structure. Here, we report on multipole surface plasmon coupling in symmetry-broken AuAg nanowire dimers. Our dimers, consisting of two nanowires with different lengths and separated by gaps of only 10 to 30 nm, were synthesized by pulsed electrochemical deposition in ion track-etched polymer templates. Electron energy-loss spectroscopy in scanning transmission electron microscopy allows us to resolve up to nine multipole order surface plasmon modes of these dimers spectrally separated from each other. The spectra evidence plasmon coupling between resonances of different multipole order, resulting in the generation of additional plasmonic modes. Since such complex structures require elaborated synthesis techniques, dimer structures with complex composition, morphology and shape are created. We demonstrate that finite element simulations on pure Au dimers can predict the generated resonances in the fabricated structures. The excellent agreement of our experiment on AuAg dimers with finite integration simulations using CST microwave studio manifests great potential to design complex structures for sensing applications.

Light-responsible DNA hydrogel-gold nanoparticle assembly for synergistic cancer therapy.

Assembled AuNPs in a DNA hydrogel (Dgel) showed strongly coupled plasmon modes, and the Dgel vehicle can co-load anticancer drugs such as doxorubicin (Dox) as a light-controlled releasing cargo by DNA intercalations. Upon laser excitation, local heat shock generation was accompanied by the release of Dox. A highly synergistic combination of thermo- and chemotherapy was demonstrated in cellular and animal models. Our Dgel vehicle can be fragmented after the excitation-induced heat generations, which subsequently causes the dispersion of the AuNPs. Our system may be less toxic because it uses small sizes of AuNPs, and the inherently biocompatible scaffold may reduce the long-term toxicity by rapid clearance.

Radial growth of plasmon coupled gold nanowires on colloidal templates

The library of plasmonic nanosystems keeps expanding with novel structures with the potential to provide new solutions to old problems in science and technology. We report the synthesis of a novel plasmonic system based on the growth of gold nanowires radially branching from the surface of silica particles. The nanowires length could be controlled by tuning the molar ratio between metal salt and surface-grafted seeds. Electron microscopy characterization revealed that the obtained one-dimensional nanoparticles are polycrystalline but uniformly distributed on the spherical template. The length of the nanowires in turn determines the optical response of the metallodielectric particles, so that longer wires display red-shifted longitudinal plasmon bands. Accurate theoretical modeling of these complex objects revealed that the densely organized nanowires display intrinsically coupled plasmon modes that can be selectively decoupled upon detachment of the nanowires from the surface of the colloidal silica template.

Effect of Intertip Coupling on the Plasmonic Behavior of Individual Multitipped Gold Nanoflower

We report here, the first experimental realization on the selective excitation of two closely lying tips from the same spherical core of a multitipped gold nanoparticle with flower-like morphology. This gives strong multipeaked resonance in the near-infrared region of the far-field emission spectra showing a clear signature of tip to tip coupling. The cathodoluminescence (CL) technique in a scanning electron microscope (SEM) combined with finite-difference time-domain (FDTD) simulation has helped us to identify the coupled plasmon modes to be originated from the interaction between two closely spaced tips with a narrow angular separation. Our analysis further estimates a range of angular separation between the tips that triggers the onset of the intertip coupling.

Surface Plasmon Mode Coupling in Photonic Crystal Fiber Symmetrically Filled with Ag/Au Alloy Wires

The surface plasmon mode coupling in photonic crystal fiber filled with Ag/Au alloy wires into two symmetrical cladding air holes have been numerically investigated using the finite element method. For the cases of pure noble metal silver and gold, they exhibit similar polarization characteristics. The coupling strongly depends on the complex indices of the core mode and surface plasmon polaritons mode. Ag/Au alloy decreases the refractive index of surface plasmon mode and significantly increases the loss at longer wavelength. What should be paid a special attention is that there is a surge in loss of y-polarized direction in the case of very low Au-doped alloy, loss up to 309.14 dB/cm, and the full width half maximum of 9 nm can be achieved. The results match well with the approximate interpretation.

Substrate Induced Symmetry Breaking in Penta-twinned Gold Nanorod Probed by Free Electron Impact

Cathodoluminescence (CL) spectroscopy and imaging in a high resolution scanning electron microscope (SEM) is used to probe and directly map the localized surface plasmon resonance (LSPR) modes of a penta-twinned gold nanorod deposited on a silicon substrate. Finite-difference time-domain simulation of CL enables us to gain insight into the origin of the plasmon modes. Our experimental results and simulations demonstrate that the substrate plays a very crucial role in the observed plasmonic property of gold nanorod. We have shown that, in the visible domain of the spectrum, the plasmon mode gets split into two distinct peaks due to substrate induced hybridization of in-plane and out-of-plane modes. With increasing the refractive index of the substrate, the intensity of these hybridized modes increases. We provide a detailed analysis on the origin and coupling of various plasmon modes mediated by the substrate.

Coupling of plasmon modes in graphene microstructures

A variety of different graphene plasmonic structures and devices have been proposed and demonstrated experimentally. Plasmon modes in graphene microstructures interact strongly via the depolarization fields. An accurate quantitative description of the coupling between plasmon modes is required for designing and understanding complex plasmonic devices. Drawing inspiration from microphotonics, we present a coupled-mode theory for graphene plasmonics, in which the plasmon eigenmodes of a coupled system are expressed in terms of the plasmon eigenmodes of its uncoupled sub-systems. The coupled-mode theory enables accurate computation of the coupling between the plasmon modes and of the resulting dynamics. We compare theory with experiments performed on the plasmon modes in coupled arrays of graphene strips. In experiments, we tune the coupling by changing the spacing between the graphene strips in the array. Our results show that the coupling parameters obtained from the coupled-mode theory and the plasmon frequency changes resulting from this coupling agree very well with experiments. The work presented here provides a framework for designing and understanding coupled graphene plasmonic structures.

Surface plasmon modes of a nanoegg above a substrate.

An analytical approach describing the surface plasmon modes of a nanoegg (a nanoshell with a nonconcentric core) above a substrate is presented. The bispherical coordinate system is used for mathematical convenience. A general dispersion relation for the coupled modes as function of geometrical characteristics of the system is obtained. In the special case (in the limit of a very large separation between the nanoegg and substrate) dispersion relation of the plasmon modes of an isolated nanoegg is derived. It is shown that the coupled plasmon modes exhibit excellent numerical convergence.

Role of coupling of localized surface plasmon modes in transmission properties of compound structure metamaterials

We have studied the influence of interaction and coupling of localized surface plasmon (LSP) on the transmission peak and resonance frequency of compound structure metamaterials. It is found that the longitudinal interval (L) between metal particle and rectangular holes arrays has an important influence on the interaction and coupling strength of LSP modes of the designed compound structure. Results indicate that the reduction of transmission peak and red-shift of resonance frequency of the designed compound structure can be modulated through increasing the interaction and coupling strength of LSP modes.

Localized Surface Plasmon Resonance Coupling in Self-Assembled Ag Nanoparticles by Using 3-Dimensional FDTD Simulation

In this study, we investigated localized surface plasmon resonance and the related coupling phenomena with respect to various geometric parameters of Ag nanoparticles, including the size and inter-particle distance. The plasmon resonances of Ag nanoparticles were studied using three-dimensional finite difference time domain(FDTD) calculations. From the FDTD calculations, we discovered the existence of a symmetric and an anti-symmetric plasmon coupling modes in the coupled Ag nanoparticles. The dependence of the resonance wavelength with respect to the inter-particle distance was also investigated, revealing that the anti-symmetric mode is more closely correlated with the inter-particle distance of the Ag nanoparticles than the symmetric mode. We also found that higher order resonance modes are appeared in the extinction spectrum for closely spaced Ag nanoparticles. Plasmon resonance calculations for the Ag particles coated with a <TEX>$SiO_2$</TEX> layer showed enhanced plasmon coupling due to the strengthened plasmon resonance, suggesting that the inter-particle distance of the Ag nanoparticles can be estimated by measuring the transmission and absorption spectra with the plasmon resonance of symmetric and anti-symmetric localized surface plasmons.

Slow surface plasmon pulse excitation in metal-insulator-metal plasmonic waveguide with chirped grating.

A scheme for the excitation of slow surface plasmon pulses using photonic interband transition in a metal-insulator-metal (MIM) waveguide is proposed. An investigation the mode transition behavior inside the binary grating confirmed that the proposed concept can be understood in terms of the coupling of symmetric and anti-symmetric plasmonic modes. We observed that, although a binary grating that is optimized for a single frequency can excite slow surface plasmon pulses, it is inadequate for broadband mode conversion. To rectify this, a chirped grating was designed for the demonstration of broadband mode conversion by applying a cascade mode transition with different frequencies.

Optimizing the Refractive Index Sensitivity of Plasmonically Coupled Gold Nanoparticles

The possibility to enhance the local refractive index sensitivity using plasmonic coupling between spherical gold nanoparticles (Au-NPs) has been investigated. A strong and distinct optical coupling between Au-NPs of various sizes was achieved by controlling the interparticle separation using a layer-by-layer assembly of polyelectrolytes. The frequency of the coupled plasmon peak could be tuned by varying either the particle size or the interparticle separation, shown both experimentally and by theoretical simulations. The bulk refractive index (RI) sensitivity for the plasmonic coupling modes was investigated and compared to the RI sensitivity of monolayers of well-separated Au-NPs, and the results clearly demonstrates that the RI sensitivity can be significantly enhanced in plasmonically coupled Au-NPs. The proposed approach is simple and scalable and improves the rather modest RI sensitivity of spherical gold nanoparticles with a factor of 3, providing a new route for fabrication of inexpensive sensors based on plasmonic nanostructures.

Robust multispectral transparency in continuous metal film structures via multiple near-field plasmon coupling by a finite-difference time-domain method

We propose a robust multispectral transparent plasmonic structure and calculate its transparency response by using the three-dimensional finite-difference time-domain (FDTD) method. The proposed structure is composed of a continuous ultrathin metal film sandwiched by double two-dimensional (2D) hexagonal non-close-packed metal-dielectric multilayer core-shell nanoparticle arrays. The top and bottom plasmonic arrays in such a structure, respectively, act as the light input and output couplers to carry out the efficient trapping and release of light. Near-perfect multispectral optical transparency in the visible and near-infrared regions is achieved theoretically. The calculated electric field distribution patterns show that the near-perfect multispectral optical transparency mainly originates from the excitation and hybridization of shell and core plasmon modes, strong near-field coupling of dipole plasmon modes between adjacent nanoparticles as well as the excitation of surface plasmon waves of the metal film. The robust transparency bands can be efficiently tuned in a large range by varying the structural parameters and the surrounding dielectric environment. The proposed structure also shows additional merits such as a deep sub-wavelength size and fully retained electrical and mechanical properties of the natural metal. These features might provide promising applications in highly integrated optoelectronic devices including plasmonic filters, nanoscale multiplexers, and non-linear optics.

Tunable Microstructured Surface-Enhanced Raman Scattering Substrates via Electrohydrodynamic Lithography

Readily fine-tuned structures are an important requirement for the optimization of surface-enhanced Raman scattering (SERS) to obtain the highest enhancements. Here, a lateral modulation of an electric field applied to a dielectric interface enables the rapid replication of nearly any topographic morphology with micrometer resolution by electrohydrodynamic lithography (EHL). Gold-covered periodic EHL-generated arrays yielded the reproducible enhancement of adsorbed SERS-active molecules. Periodic arrays of micropillars with square and circular cross sections give rise to the effective coupling of surface plasmon modes, which generate enhanced SERS signals. The overall enhancement factors depend on the geometry of the gold-coated structures, and intriguingly, a strong correlation is found with the gap-to-width ratio of the square pillar morphology. A numerical simulation of the EHL-based SERS substrates is consistent with this dependence. The EHL surface architectures can be easily tailored at micrometer-to-submicrometer dimensions, allowing the fabrication of reliably engineered and cost-effective highly sensitive SERS substrates.

Enhancement of photonic density of states in finite graphene multilayers

We consider the optical properties of finite systems composed of a series of graphene sheets separated by thin dielectric layers. Because these systems respond as conductors to electric fields in the plane of the graphene sheets and as insulators to perpendicular electric fields, they can be expected to have properties similar to those of hyperbolic metamaterials. We show that under typical experimental conditions graphene/dielectric multilayers have enhanced Purcell factors, and enhanced photonic densities-of-states in both the THz and mid-IR frequency range. These behaviors can be traced to the coupled plasmon modes of the multi-layer graphene system. We show that these results can be obtained with just a few layers of graphene.

Coupled surface plasmon modes of graphene in close proximity to a plasma layer

Coupled surface plasmon modes of graphene on a plasma layer are studied. Three coupled plasmon modes are found, and analytical equations for their dispersion characteristics are given in the long-wavelength limit (q→0). Symmetric and antisymmetric modes are identified; between two strongly coupled modes, the antisymmetric mode is found to be more energetic than the symmetric mode. The great tunability of plasmon characteristics is shown by changing the surrounding dielectric constant, the coupling distance, and the plasma frequency of the substrate.