Multiple Surface Plasmon Resonances in Compound Structure with Metallic Nanoparticle and Nanohole Arrays

The optical properties of a compound structure with metallic nanoparticle and nanohole arrays are numerically investigated by the means of finite-difference time domain method. We report on the observation of multi-valleys in the reflection spectra due to the excitation of surface plasmon (SP) resonant modes of the compound structure. Simulation results show that multiple SP resonances consist of surface plasmon polaritons on the gold film, localized surface plasmons on the nanoparticles, and coupling mode between them. These findings are important for applications utilizing multiple surface plasmon resonances.     

High Tunability Multipolar Fano Resonances in Dual-Ring/Disk Cavities

Metallic nanostructures that support multipolar Fano resonances have drawn much attention in recent years. Such structures are applicable especially to enhanced nonlinear optics, where two resonance wavelengths need to be modulated simultaneously. However, how to tune multipolar Fano resonances independently remains a challenge. In the paper, the plasmonic nanostructure consisting of two ring/disk cavities (RDCs) is investigated using the finite element method. The dark multipolar modes of each RDC are excited, and sharp multipolar Fano resonances are induced. The multipolar modes supported by different RDCs can be tuned independently by changing the sizes. The line-widths of such Fano resonances nearly keep below 0.05 eV, and the contrast ratio (CR) of the two quadrupolar Fano dips mostly maintain above 50 %. In addition, the exciting bonding modes of different RDCs make the selective storage of resonance energy available. Such plasmonic nanostructures may find applications in enhanced nonlinear optics or nano-optical elements.     

Localized Surface Plasmon Resonances of Simple Tunable Plasmonic Nanostructures

Plasmonics - We derive and present systematic relationships between the analytical formulas for calculation of the localized surface plasmon resonances (LSPR) of some plasmonic nanostructures which...     

Tuning the Fano Resonance Between Localized and Propagating Surface Plasmon Resonances for Refractive Index Sensing Applications

Localized and propagating surface plasmon resonances are known to show very pronounced interactions if they are simultaneously excited in the same nanostructure. Here, we study the Fano interference that occurs between localized surface plasmon resonance (LSPR) and propagating surface plasmon polariton (SPP) modes by means of phase-sensitive spectroscopic ellipsometry. The sample structures consist of periodic gratings of gold nanodisks on top of a continuous gold layer and a thin dielectric spacer, in which the structural dimensions were tuned in such a way that the dipolar LSPR mode and the propagating SPP modes are excited in the same spectral region. We observe pronounced anti-crossing and strongly asymmetric line shapes when both modes move to each other’s vicinity, accompanied of largely increased phase differences between the respective plasmon resonances. Moreover, we show that the anti-crossing can be exploited to increase the refractive index sensitivity of the localized modes dramatically, which result in largely increased values for the figure-of-merit which reaches values between 24 and 58 for the respective plasmon modes.     

Compact Color Filter and Polarizer of Bilayer Metallic Nanowire Grating Based on Surface Plasmon Resonances

Surface plasmon resonances on bilayer aluminum nanowire gratings are studied in both theory and experiment. It is found that there are two kinds of surface plasmon on the bilayer metallic gating: longitudinal aluminum/dielectric/aluminum slit and lateral aluminum/dielectric interface waveguide mode. The surface plasmon waveguide mode resonance in the slits makes the grating act as a transverse magnetic (TM)-passing polarizer. With the lateral waveguide mode resonance, certain wavelengths of the incident TM light are translated to aluminum/air or aluminum/substrate waveguide light, and the grating acts as a color filter. With both resonances, the bilayer nanowire grating can be a compact-integrated polarizer and color filter.     

Surface Plasmon Resonances of Clustered Nanoparticles

Linear clusters made by tightly connecting two or more metallic nanoparticles have new types of surface plasmon resonances as compared with isolated nanoparticles. These new resonances are sensitive to the size of the junction and to the number of interconnected particles and are described by eigenmodes of a boundary integral equation. This formulation allows effective separation of geometric and shape contribution from electric properties of the constituents. Results for particles covered by a thin shell are also provided highlighting ultrasensitive sensing applications. The present analysis sheds a new light on the interpretation of recent experiments.     

Surface Plasmon Resonances of an Axially Magnetized Plasma Column in the Presence of Collisional Loss

Surface plasmon resonances arising in the course of scattering of an H-polarized plane electromagnetic wave by an axially magnetized plasma column are analyzed. Main attention is paid to the behavior of these resonances in the presence of collisional loss in the magnetoplasma filling the scatterer. The frequencies, Q factors, and amplitude coefficients of the electromagnetic field of multipole surface plasmon resonances of different orders are found, and conditions under which the collisional loss in the plasma completely suppresses a given resonance are determined.     

Investigation of Plasmonic Resonances in Mismatched Gold Nanocone Dimers

The plasmonic properties of two closely adjacent gold nanocones of different sizes have been investigated. The plasmon modes of the first nanocone couple with the plasmon modes of the second one due to which a broad peak and a narrow peak emerges in the extinction spectrum, which can be categorized as bright and dark plasmon modes. The destructive interference of the two modes results in a sharp Fano dip in the spectrum. Several configurations of the conical nanodimer have been considered, which suggests that the plasmon coupling in the nanocone dimer is not only dependent on the interparticle distance and size of the nanoparticles but also on their spatial arrangement. The localized high near-field energy in the nanodimer can be used for surface-enhanced Raman spectroscopy applications.     

Tunable Absorption of Light via Localized Plasmon Resonances on a Metal Surface with Interspaced Ultra-thin Metal Gratings

We studied optical reflection properties of complex metal (Ag) surfaces with close-interspaced ultra-thin metal gratings. Prominent reflection minima were observed corresponding to enhanced absorption of light. Our analysis convinced us that the period-dependent mode is ascribed to Bloch-wave-like resonances of surface plasmon waves at the overall effective metal surfaces, and the ridge-width-dependent mode to Fabry-Pérot-like resonances of localized surface plasmon waves in micro-/nanocavities defined by regions of the grating ridges. The latter resonance mode is shown highly tunable with variation of the grating ridge width. Such structures may be applied in spectrum resolvable photovoltaic devices, bio-sensing, and studying optical properties of cavity-coupled molecules or functional nanomaterials.     

Triple Surface Plasmon Resonances in Zn Nanoparticles in Silica

Theoretical calculations of optical absorption (OA) spectra of Zn nanoparticles (NPs) in silica glass, using a simple model, have been resulted in appearance of three OA bands, all of which are found to blueshift with decrease in NP radius due to quantum confinement effects of free electrons. The intensities of the OA bands are seen to increase with increase in incident light energy. Importantly, all the three OA bands have been found to satisfy, accurately, the surface plasmon resonance (SPR) condition. All these observations clearly indicate that the observed OA bands are originated from the SPR absorptions in Zn NPs, but not due to inter-band absorption.     

Higher Order Tunable Fano Resonances in Multilayer Nanocones

We present a computational study of the plasmonic response of a gold–silica–gold multilayered nanostructure based on truncated nanocones. Symmetry breaking is introduced by rotating the nanostructure and by offsetting the layers. Nanocones with coaxial multilayers show dipole–dipole Fano resonances with resonance frequencies depending on the polarization of the incident light, which can be changed by rotating the nanostructure. By breaking the axial symmetry, plasmonic modes of distinct angular momenta are strongly mixed, which provide a set of unique and higher order tunable Fano resonances. The plasmonic response of the multilayered nanocones is compared to that of multishell nanostructures with the same volume and the former are discovered to render visible high-order dark modes and to provide sharp tunable Fano resonances. In particular, higher order tunable Fano resonances arising in non-coaxial multilayer nanocones can vary the plasmon lines at various spectral regions simultaneously, which makes these nanostructures greatly suitable for plasmon line shaping both in the extinction and near field spectra.     

Localization of Near-Field Resonances in Bowtie Antennae: Influence of Adhesion Layers

The near-field resonances of gold bowtie antennae are numerically modeled. Besides the short-range surface plasmon polariton (SR-SPP) mode along the main axis of the structure, a coupled SPP mode is also found in the gap region (G-SPP). The influence of adhesion layers is considered, which depends on the refractive index and the absorption of the adhesion material and whether it is continuous or etched. A high refractive index causes the peak of the SR-SPP to red-shift. High absorption quenches the intensity of the SR-SPP. The magnitude of influence depends on the overlap of the adhesion layer with the SR-SPP and G-SPP modes. The near-field resonance of the SPP mode on the top surface is also considered. An etched metal adhesion layer changes the near-field localization in the gap and causes the enhancement peaks at different heights within the gap to red-shift from top to bottom. A simple optimization method for the near-field localization by the combination of different top and bottom layers is demonstrated.     

Förster Resonance Energy Transfer Between Molecules in the Vicinity of Graphene-Coated Nanoparticles

The recent demonstration of the plasmonic-enhanced Förster resonance energy transfer (FRET) between two molecules in the vicinity of planar graphene monolayers is further investigated using graphene-coated nanoparticles (GNP). Due to the flexibility of these nanostructures in terms of their geometric (size) and dielectric (e.g., core material) properties, greater tunability of the FRET enhancement can be achieved employing the localized surface plasmons. It is found that while the typical characteristic graphene plasmonic enhancements are manifested from using these GNPs, even higher enhancements can be possible via doping and manipulating the core materials. In addition, the broadband characteristics are further expanded by the closely spaced multipolar plasmon resonances of the GNPs.     

Enhanced Electron Photoemission by Collective Lattice Resonances in Plasmonic Nanoparticle-Array Photodetectors and Solar Cells

We propose to use collective lattice resonances in plasmonic nanoparticle arrays to enhance and tailor photoelectron emission in Schottky barrier photodetectors and solar cells. We show that the interaction between narrow-band lattice resonances (the Rayleigh anomaly) and broader-band individual-particle excitations (localized surface plasmon resonances) leads to stronger local field enhancement. In turn, this causes a significant increase of the photocurrent compared to the case when only individual-particle excitations are present. The results can be used to design new photodetectors with highly selective, tunable spectral response, which are able to detect photons with the energy below the semiconductor bandgap. The findings can also be used to develop solar cells with increased efficiency.     

Highly Sensitive Plasmonic Sensor Based on Fano Resonance from Silver Nanoparticle Heterodimer Array on a Thin Silver Film

We investigate the optical spectrum of a multilayer metallic slab using multiple-scattering formalism. A thin silver film is attached to a periodic array of heterodimers consisting of two vertically spaced silver nanoparticles of different radii. Depending on the radius of nanoparticles, heterodimer array presents a simple nanoscale geometry which gives rise to remarkable plasmonic properties of multipolar resonances. Due to the coherent interference of the localized nanoparticle plasmons (discrete mode) and surface plasmon polaritons of metallic film (continuous mode), the reflection spectrum represents a sharp asymmetric Fano resonance dip, which is strongly sensitive to the refractive index of the surrounding embedded dielectric host. The physical features contribute to a highly efficient plasmonic sensor for refractive index sensing with sensitivity of ～1.5?×?10^?3 RIU/nm.     

Tailoring of Localized Surface Plasmon Resonances of Core-Shell Au@Ag Nanorods by Changing the Thickness of Ag Shell

We report the synthesis and the characterization of core-shell Au@Ag nanorods through reduction by the wet chemical method. UV-visible absorption spectra of core-shell Au@Ag nanorods demonstrate the longitudinal mode of localized surface plasmon resonances (LSPRs) can be tailored from 724 to 786 nm by controlling the thickness of the silver shell, as is assessed by transmission electron microscope (TEM). Furthermore, the tunable and well-controlled LSPRs of core-shell Au@Ag nanorods are also investigated by numerical simulation using the finite difference time domain (FDTD) method, which strongly supports the experimental observations. The growth mechanism for core-shell Au@Ag nanorods is proposed, according to experimental observations and numerical calculations.     

Obtain Quadruple Intense Plasmonic Resonances from Multilayered Gold Nanoshells by Silver Coating: Application in Multiplex Sensing

Four intense and separate localized surface plasmon resonance (LSPR) absorption peaks have been obtained in the gold-dielectric–gold–silver multilayer nanoshells. The silver coating on the gold shell results in a new LSPR peak at about 400 nm corresponding to the $ {{\left| {\omega_{+}^{-}} \right\rangle}_{Ag }} $ mode. The intense local electric field concentrated in the silver shell at the wavelength of 400 nm indicates that this new plasmonic band is coming from the symmetric coupling between the antibonding silver shell plasmon mode and the inner sphere plasmon. Increasing the silver shell thickness also leads to the intensity increasing of the $ {{\left| {\omega_{+}^{-}} \right\rangle}_{Au }} $ mode and blue shift of $ \left| {\omega_{-}^{+}} \right\rangle $ and $ \left| {\omega_{-}^{-}} \right\rangle $ modes. Therefore, quadruple intense plasmonic resonances in the visible region could be achieved in gold-dielectric–gold–silver multilayer nanoshells by tuning the geometrical parameters. And the quadruple intense plasmonic resonances in the visible region provide well potential for multiplex biosensing based on LSPR.     

Plasmon Resonances in a Periodic Square Coaxial Hole Array in a Graphene Sheet

Plasmonics - We present a computational study of the plasmon resonances in a periodic square coaxial hole array in a graphene sheet, which consists of a square hole array and a square strip array....     

Tunable Properties of Surface Plasmon Resonances: The Influence of Core–Shell Thickness and Dielectric Environment

In the present study, we have investigated the extinction spectra of coated sphere (using dipole model) with different core–shell radius, in which the core is TiO₂ and the shell is made up of silver or gold nanoparticles. Nanoparticles exhibit surface plasmon resonance peak; these plasmonic peaks are highly tunable in wavelength range of 300 to 1,100 nm; in fact, the blue and red shifting of resonance peak highly depends on the core–shell thickness. The broadness of resonance peaks are analysed in terms of full width at half maxima (FWHM), and the width of these resonance peaks is also the function of core–shell radius.