Multipolar Plasmon Resonances Research Articles

Local Plasmonic Studies on Individual Core–Shell Gold–Silver and Pure Gold Nano-Bipyramids

Bimetallic systems present new opportunities to tailor the optical properties of nanoparticles. Morphology, structural, and optical properties of gold and of gold–silver nanoparticles have been studied by three-dimensional (3D) scanning transmission electron microscopy (STEM), STEM imaging, and monochromated subnanometer electron beams. The 3D morphology of these nanoparticles consists of bipyramidal prisms with well-defined facets. Furthermore, in the case of the core–shell nanoparticles, from these tomographical studies, we have determined the silver (shell) distribution and their atomic arrangement. Multipolar localized surface plasmon resonances (LSPR) have been studied, at subnanometer level and at high-energy resolution, as a function of their shape, their size (aspect ratio), and the surrounding environment. These results have been interpreted in the framework of the discrete dipole approximation (DDA) simulations. The effect of the silver outer-shell has been elucidated. We observed a significant damping of plasmon excitations due to the difference of dielectric function of these two metals. In addition, we have shown that the combination of the tomographical and plasmonic (experiments and simulations) studies with such high spatial resolution constitutes a very powerful and fundamental tool for understanding and optimizing the photonics properties of nanomaterials.

High-resolution imaging and spectroscopy of multipolar plasmonic resonances in aluminum nanoantennas.

We report on the high resolution imaging of multipolar plasmonic resonances in aluminum nanoantennas using electron energy loss spectroscopy (EELS). Plasmonic resonances ranging from near-infrared to ultraviolet (UV) are measured. The spatial distributions of the multipolar resonant modes are mapped and their energy dispersion is retrieved. The losses in the aluminum antennas are studied through the full width at half-maximum of the resonances, unveiling the weight of both interband and radiative damping mechanisms of the different multipolar resonances. In the blue-UV spectral range, high order resonant modes present a quality factor up to 8, two times higher than low order resonant modes at the same energy. This study demonstrates that near-infrared to ultraviolet tunable multipolar plasmonic resonances in aluminum nanoantennas with relatively high quality factors can be engineered. Aluminum nanoantennas are thus an appealing alternative to gold or silver ones in the visible and can be efficiently used for UV plasmonics.

Optical torque from enhanced scattering by multipolar plasmonic resonance

AbstractWe present a theoretical study of the optical angular momentum transfer from a circularly polarized plane wave to thin metal nanoparticles of different rotational symmetries. While absorption has been regarded as the predominant mechanism of torque generation on the nanoscale, we demonstrate numerically how the contribution from scattering can be enhanced by using multipolar plasmon resonance. The multipolar modes in non-circular particles can convert the angular momentum carried by the scattered field and thereby produce scattering-dominant optical torque, while a circularly symmetric particle cannot. Our results show that the optical torque induced by resonant scattering can contribute to 80% of the total optical torque in gold particles. This scattering-dominant torque generation is extremely mode-specific, and deserves to be distinguished from the absorption-dominant mechanism. Our findings might have applications in optical manipulation on the nanoscale as well as new designs in plasmonics and metamaterials.

Multipole plasmon resonances in self-assembled metal hollow-nanospheres

Recently, multipole plasmonic mode resonances in metal hollow structures, such as dipole, quadrupole, and octupole modes, have been widely investigated by researchers with the aim for potential applications in bio-sensing, fluorescence, nanolasers or nonlinear nano-photonics. Here, in this work, the multipole plasmon resonances in self-assembled metal hollow-nanospheres (HNSs) are theoretically and experimentally demonstrated and the hot spots originating from the higher order mode plasmonic resonance and interparticle coupling effect are proposed to be used for Raman scattering enhancements. Dipole, quadrupole, octupole and hexadecapole mode plasmonic resonances were clearly resolved in the extinction spectra of these Ag HNS arrays showing good agreement with the theoretical simulation results. Strong regular hot spots were obtained around the surface and in the gaps of the Ag HNSs through the higher order mode plasmonic resonances and corresponding interparticle coupling effect between the HNSs. Maximum local field intensity was accomplished by optimizing the size of as well as the coupling distance between the HNSs and then it was applied to SERS sensing. Raman mapping also demonstrated these self-assembled plasmonic cavity arrays to be a stable and uniform SERS-active substrate.

Ultrathin plasmonic metamaterial for spoof localized surface plasmons

The multipolar spoof localized surface plasmons (LSPs) on a planar textured metallic disk are proposed and experimentally demonstrated at microwave frequencies. Based on ultrathin metal film printed on a thin dielectric substrate, the designed plasmonic metamaterial clearly shows multipolar plasmonic resonances, including the dipole, quadrupole, hexapole, octopole, decapole, dodecapole, and quattuordecpole modes. Both numerical simulations and experiments are in good agreement. It is shown that the spoof LSP resonances are sensitive to the disk's geometry and local dielectric environments. Hence, the ultrathin textured metallic disk may be used as plasmonic sensors and find potential applications in the microwave and terahertz frequencies.

Silver Nanoprisms Acting as Multipolar Nanoantennas under a Low-Intensity Infrared Optical Field Exciting Fluorescence from Eu3+

A silver nanoprism (Ag NP) generates a near field due to multipolar surface plasmon resonance (SPR) and lightening rod effects and acts as a multipolar nanoantenna. The ability of Ag NPs to create such an effect even under an infrared (IR) optical field far off of resonance from the SPR frequency is demonstrated through finite difference time domain simulations of exact Ag NPs and hybrids. The conclusive experimental proof of such a near field around Ag NPs under low-intensity (1.5 mW) IR (980 nm) light came when it could excite fluorescence from YVO4/Eu3+ nanoparticles that otherwise do not fluoresce under IR. The results open up new vistas for exclusive plasmonic excitation of fluorescence through metal NP hybrids/ensembles.

Plasmon‐Enhanced Second‐ and Third‐Order Harmonic Generation in Spherical Free‐Electron Nanoclusters with Diffuse Surface

AbstractThe nonlinear scattering of a laser pulse off spherical nanoclusters with free electrons and with a diffuse surface is examined in the collisionless hydrodynamics approximation in the framework of perturbation theory with respect to the laser pulse intensity, as well as of the steady‐state approximation. In a previous publication [S.V. Fomichev and W. Becker, Phys. Rev. A 81, 063201 (2010)] we reported the full nonlinear hydrodynamic model of forced collective electron motion confined to a cluster with diffuse surface and introduced two different perturbation theories corresponding to different laser intensity regimes. In the current paper, in the framework of this hydrodynamic model we focus on the properties of plasmon resonance‐enhanced third‐harmonic generation in a spherical cluster and its dependence on the shape of its diffuse surface whose role increases for nonlinear processes. At the same time, the quadrupole second‐harmonic generation in a spherical cluster is also inspected as a necessary intermediate step. Both cold metal clusters in vacuum or in a dielectric surrounding and hot laser‐heated and laser‐ionized clusters are considered within the same approach for a wide range of the fundamental laser frequency. Nonlinear laser excitation of the dipole plasmon Mie resonance in spherical clusters, as well as of other respective multipole plasmon resonances is investigated analytically and numerically in detail (position, width, and strength) versus the cluster‐surface diffuseness, the outer ionization degree in charged clusters, the electron‐density diffuseness, and their interplay. Under certain conditions, depending on the various cluster parameters, different secondary nonlinear resonances are found. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

A density functional theory study of structural, electronic, optical and magnetic properties of small Ag–Cu nanoalloys

The structures and properties of 13-atom silver and copper bimetallic clusters are systematically investigated by density functional theory (DFT) in the theoretical frame of the generalised gradient approximation (GGA) exchange-collection function. Optical absorption, Raman spectra, vibrational spectra, as well as electronic and magnetic properties are calculated by DFT/GGA and semi-core pseudopotentials. The following lowest-energy structures in the 13-atom Ag–Cu clusters are obtained: cuboctahedron for pure Ag13, icosahedrons for pure Cu13, Ag1Cu12, Ag6Cu7 and Ag12Cu1; and amorphous motifs for Ag m Cu13−m when m = 2–5 and 7–11. Ag2Cu11, Ag7Cu6 and Ag11Cu2 are magic clusters. The Ag2Cu11 cluster exhibits high energetic stability, strong electronic stability, multipole surface plasmon resonance (SPR) mode and small dipole moment. The Ag6Cu7 cluster is a Janus-separated cluster that possesses the strongest electronic stability with a band gap of 0.424 eV and a vertical ionisation potential of 5.8417 eV. The amorphous Ag7Cu6 cluster shows an Ag–Cu alloyed motif. The blue shift of the maximum SPR peak becomes increasingly evident as silver atoms are added. All Raman and vibrational spectra exhibit many significant vibration modes within the wavenumber ranges of 0–270 and 0–306.55 cm−1, respectively. Ferroelectric and ferromagnetic behaviours are observed in the 13-atom Ag–Cu nanoalloys, indicating their new potential applications in nonlinear optical devices.

Asymmetric Silver “Nanocarrot” Structures: Solution Synthesis and Their Asymmetric Plasmonic Resonances

Here we report the wet-chemical synthesis of asymmetric one-dimensional (1D) silver "nanocarrot" structures that exhibit mixed twins and stacking fault domains along the <111> direction. Oriented attachment is the dominant mechanism for anisotropic growth. Multipolar plasmon resonances up to fourth order were measured by optical extinction spectroscopy and electron energy-loss spectroscopy (EELS) and are in agreement with theoretical calculations. Compared with those of symmetric 1D nanostructures of similar length, the dipole modes of the nanocarrots show a clear red shift, and the EELS maps show an asymmetric distribution of the resonant plasmonic fields and a compression of the resonance node spacing toward the tail. In addition, increasing the length of the nanocarrots causes an increase in the intensity and a steady red shift of the longitudinal surface plasmon resonance peaks. The silver nanocarrots also show very high sensitivity to the refractive index of their environment (890 ± 87 nm per refractive index unit).

Controlling the absorption spectrum within a thin amorphous silicon layer by using the size dependent plasmonic behaviour of silver nanoparticles

In this paper, we have used Finite Difference Time Domain method for numerically calculating the absorption spectra within a thin layer of hydrogenated amorphous silicon (a-Si:H) with the front surface regularly patterned with spherical Ag nano particles. We have considered a wide range of particle radius (40 nm≤R≤200 nm) for including nano particles that have dipole dominated extinction spectra as well as the particles that can support multipole plasmon resonances. On performing the size variation analysis, constant surface coverage values (S) have been maintained so that the shading effect by the nano particles array will remain same for all particle sizes. We demonstrate that, for effective contribution to the absorption within the a-Si:H layer, there exists a clear distinction between the smaller size nanoparticles which support dipolar resonance and the larger size nano particles capable of producing higher order plasmon modes in terms of S values. The larger particles require much greater coverage than that of smaller particles for efficient plasmonic enhancement. These observations can have considerable importance in designing plasmonic solar cells or other optoelectronic devices that involve various sized Ag nano particles to enhance the optical absorption within an absorber layer.

Truncated Power Law Analysis of Blinking SERS of Thiacyanine Molecules Adsorbed on Single Silver Nanoaggregates by Excitation at Various Wavelengths

From blinking surface-enhanced Raman scattering (SERS) of anionic thiacyanine adsorbed on single Ag nanoaggregates, the electromagnetic field and the molecular behavior in a nonemissive state were investigated by a truncated power law analysis. The power law that reproduces probability distribution of dark SERS events versus duration time was not truncated often by excitation at long wavelengths; otherwise it was truncated at the long tail. The truncation suggests a high energy barrier from nonemissive to emissive state and a short passage time of molecular random walk to overcome the energy barrier. The energy barrier in blinking SERS likely originates from a nanometer-ordered periodic optical trapping potential well, namely, electromagnetic field around a junction of the Ag nanoaggregate due to coupling of multipolar surface plasmon resonance, which is hardly induced by excitation at long wavelengths. This is consistent with the experimental excitation wavelength dependence of the truncation. At a low concentration of anionic thiacyanine, the power law was truncated at the short tail. The reason may be the short passage time of the molecule on the Ag surface adsorbing a small number of obstacles to reach the junction.

Plasmon resonances of Ag(001) and Ag(111) studied by power density absorption and photoyield

This paper models the surface and bulk plasmon resonances in photoabsorption and photoelectron spectra (PES) of the Ag(001) and the Ag(111) surfaces in the region of 2.8–10eV excited with a p or transverse magnetic linearly polarized laser incident at 45°. Using the recently developed vector potential from electron density-coupled integro-differential equations (VPED-CIDE, [1,2]) model, we calculate the electron escaping probability from the power density absorption, Feibelman's parameter d⊥, the reflectance and the Fermi PE cross section. In the PES experiment the work function is lowered from 4.5 to 2.8eV by adsorption of sodium. In our model, this lowering is introduced by adding a phenomenological term to the DFT-LDA model potential of Chulkov et al. [3]. For both Ag(001) and Ag(111), the calculated observables display two plasmon resonances, the multipole surface at 3.70eV and the bulk at 3.90eV, in fair agreement with the experimental PES of Barman et al. [4,5] and the reflectance. Except for the Fermi PE cross section of Ag(001) which does not display the multipole surface plasmon resonance at 3.70eV. This poor result is probably due to a poor calculation of the conduction band wave functions obtained from the Schrödinger equation using the modified DFT-LDA model potential of Chulkov et al.

Further localization of optical field for flower-like silver particles under laser radiation

We find the surface optical field of submicron silver particles can be further localized by their surface roughening, which leads to a greatly enhanced local field. The flower-like silver particles were prepared and the extinction spectra were measured in our work. A high enhancement factor of 1.3 × 107 was obtained by surface-enhanced Raman spectrum. Besides that, the correlation between the surface plasmon resonance mode and surface local field was also studied by theoretical analysis. The results show the plasmon resonance modes of flower-like silver particles include dipole and multipole plasmon resonances. For the dipole plasmon resonance at long wavelength, the local field near polar regions will be further localized as a result of the rough surface. In contrast, the interaction of local field between surface structures will cause hotspots for the multipole plasmon resonance in the short-wave region. Based on these factors, we demonstrate that the rough-surface-induced field superposition results in the redistribution and enhancement of the optical field around the particles.

Reduced linewidth multipolar plasmon resonances in metal nanorods and related applications

Using dark-field scattering spectroscopy, we study the multipolar plasmon resonances in single crystallized silver nanorods. The lineshapes and homogenous linewidths of the surface plasmon resonances (SPRs) of different orders are analyzed and compared. The high-order resonances are found to sustain asymmetric Fano lineshapes and their linewidths are narrower than the dipolar resonance. A quantitative comparison using the finite element method reveals more than a three times reduction in the linewidth for the third order resonance, as compared with the dipolar one. These narrow linewidths result from the smaller radiative damping of the multipolar SPRs. Benefiting from the reduced damping, multipolar SPRs in nanorods are better candidates for many plasmonic applications, including increased-sensitivity single particle SPR sensors and reduced-threshold nanolasers.

Multipole Surface Plasmon Resonances in Conductively Coupled Metal Nanowire Dimers

We report on the experimental and the theoretical investigation of multipole surface plasmon resonances in metal nanowires conductively connected by small junctions. The influence of a conductive junction on the resonance energies of nanowire dimers was simulated using the finite element method based software CST Microwave Studio and experimentally measured by electron energy-loss spectroscopy in a transmission electron microscope. We extend the analysis of conductively connected structures to higher order multipole modes up to third order, including dark modes. Our results reveal that an increase in junction size does not shift significantly the antibonding modes, but causes a strong blue shift of the bonding modes, leading to an energetic rearrangement of the modes compared to those of a capacitively coupled dimer with similar dimensions.

Strong two-photon fluorescence enhanced jointly by dipolar and quadrupolar modes of a single plasmonic nanostructure

A single gold nano-cylinder presenting multipolar plasmon resonances to enhance two-photon fluorescence is investigated employing three dimensional finite-difference time-domain method. Cylinders of large dimension usually display dipolar and quadrupolar plasmonic resonances. We demonstrate that the dipolar resonance can couple with the incident light resulting in a large localized field enhancement which increases the molecular excitation rate. At the same time, the radiative quadrupolar mode overlaps with the emission band of excited fluorophores to assist the fluorescence emission due to an enhancement in the quantum efficiency. Such dipole-quadrupole jointly enhanced two-photon fluorescence exhibits exceptionally promise in brighter label design.

Fabrication of Nanoengineered Metallic Structures and Their Application to Nonlinear Photochemical Reactions

Abstract We used metallic nanoparticles exhibiting localized surface plasmon resonance and electromagnetic (EM) field enhancement effects induced by plasmon excitation for the photochemical reaction field; this approach makes it possible to increase the probability of an interaction between photons and the materials. EM field enhancement effects induced by gold nanostructures were elucidated using two-photon-induced photoluminescence from nanogap-spaced gold nanoparticles. The photoluminescence was promoted mainly in the surface regions of the gold nanoblocks adjacent to the nanogaps due to the EM field enhancement effect. To elucidate the near-field intensity distribution of the nanogap gold structures, we investigated the photopolymerization of a commercially available negative photoresist assisted by EM field enhancement in the nanogaps separating the gold nanoblocks. Photopolymerization rates were enhanced by several orders of magnitude in the gold nanoblock structures that contained nanogaps narrower than 10 nm. We propose two surface plasmon-assisted nanolithography techniques based on spatially selective photochemical reactions promoted by the EM field enhancement effect. The first method uses ultimate near-field lithography with a local near-field distribution induced in the nanogaps, and the second technique uses the scattering component of multipole plasmon resonance as an exposure mechanism. These lithographic techniques represent a type of application based on plasmon-enhanced chemical reactions.

Laser fields at flat interfaces: II. Plasmon resonances in aluminium photoelectron spectra

Using the model derived in paper I [G. Raseev, Eur. Phys. J. D 66, 167 (2012)], this work presents calculations of the photoelectron spectrum (PES) of low index aluminium surfaces in the 10–30 eV region. The laser is p or transverse magnetic linearly polarized incident on a flat structureless surface and its fields are modeled in I using the vector potential in the temporal gauge. This model uses a tensor and non-local isotropic (TNLI) susceptibility and solves the classical Ampere-Maxwell equation through the use of the vector potential from the electron density-coupled integro-differential equations (VPED-CIDE). The PE cross sections are the squares of the PE transition moments calculated using the VPED-CIDE vector potential function of the penetration coordinate. The PES is obtained in a one step model using either the Fermi golden rule or the Weisskopf-Wigner (WW) expressions. The WW cross section PES compares favorably with the experimental angle and energy resolved photoelectron yield (AERPY) spectrum of Levinson et al. [Phys. Rev. Lett. 43, 952 (1979)], Levinson and Plummer [Phys. Rev. B 24, 628 (1981)] for Al(001) and of Barman et al. [Phys. Rev. B 58, R4285 (1998)], Barman [Curr. Sci. 88, 54 (2005)] for Al(111) surfaces. As in the experiment, our theoretical AERPY displays the multipole surface plasmon resonance at 11.32/12.75 eV for Al(001)/Al(111), mainly due to the surface contribution |⟨ψf|p·A|ψi⟩|2, the bulk plasmon minimum at 15 eV and the two single particle excitation resonances at about 16 and 22 eV. The nature of the plasmon resonances of the PES is analyzed using the reflectance, the electron density induced by the laser and Feibelman’s parameter d⊥ all introduced in paper I.

Silver Nanorice Structures: Oriented Attachment-Dominated Growth, High Environmental Sensitivity, and Real-Space Visualization of Multipolar Resonances

We have synthesized and investigated the anisotropic growth of interesting silver nanorice. Its growth is kinetically controlled at 100 °C, and both oriented attachment and Ostwald ripening are involved, with the former growth mode dominating the anisotropic growth of the nanorice along the ⟨111⟩ direction. This one-directional growth is initiated by an indispensable seed-selection process, in which oxygen plays a critical role in oxidatively etching twinned silver crystals. The inhibition of this process by removing oxygen essentially blocks the nanorice growth. Although increasing reaction temperature to 120 °C accelerates the one-dimensional growth along the ⟨111⟩ direction, further temperature increase to 160 °C makes the oriented attachment dominated one-directional growth disappear; instead, the diffusion-controlled two-dimensional growth leads to the emergence of highly faceted truncated triangular and hexagonal plates mainly bound by low energy faces of {111}. Interestingly, we also found that the longitudinal surface plasmon resonance of the nanorice structures is highly sensitive to the refractive index of surrounding dielectric media, which predicts their promising applications as chemical or biological sensors. Moreover, the multipolar plasmonic resonances in these individual nanorice structures are visualized in real space, using high-resolution electron energy-loss spectroscopy.

Photoelectrochemical Analysis of Allowed and Forbidden Multipole Plasmon Modes of Polydisperse Ag Nanorods

Resonant wavelengths of multipole (i.e., higher-order) plasmon resonance of polydisperse Ag nanorods (NRs) and their dependence on incident angle are analyzed by a photoelectrochemical means coupled with spectroscopy. It also allows us to observe nanostructures of multipole-resonant Ag NRs. Multiple dip formation in extinction spectra is caused by selective oxidation of multipole-resonant NRs on the basis of multipole plasmon-induced charge separation on a TiO2 substrate. The dip wavelengths are the dipole resonant wavelengths of the NRs that are dipole- or multipole-resonant at the irradiation wavelength. Even order plasmon modes (mode index number m = 2, 4) of the NRs are forbidden at perpendicular incidence (θ = 0°), and in the present system, the third-order plasmon mode (m = 3) is also forbidden at a specific incident angle, θ = 37.5°.