Quadrupole Plasmon Research Articles

Size dependence of multipolar plasmon resonance frequencies and damping rates in simple metal spherical nanoparticles

Multipolar plasmon oscillation frequencies and corresponding damping rates for nanospheres formed of the simplest free-electron metals are studied. The possibility of controlling plasmon features by choosing the size and dielectric properties of the sphere surroundings is discussed. Optical properties of the studied metals are described within the Drude-Sommerfeld model of the dielectric function with effective parameters acounting for the contribution of conduction electrons and of interband transitions. No approximation is made in respect of the size of a particle; plasmon size characteristics are described rigorously. The results of our experiment on sodium nanodroplets [1] are compared with the oscillation frequency size dependence of dipole and quadrupole plasmon.

Exploration of electronic quadrupole states in atomic clusters by two-photon processes

We analyse particular two-photon processes as possible means to explore electronic quadrupole states in free small deformed atomic clusters. The analysis is done in the time-dependent local density approximation (TDLDA). It is shown that direct two-photon population (DTP) and off-resonant stimulated Raman (ORSR) scattering can be effectively used for excitation of the quadrupole states in high-frequency (quadrupole plasmon) and low-frequency (infrared) regions, respectively. In ORSR, isolated dipole particle–hole states as well as the tail of the dipole plasmon can serve as an intermediate state. A simultaneous study of low- and high-frequency quadrupoles, combining DTP and ORSR, is most effective. Femtosecond pulses with intensities I = 2 × 1010 to 2 × 1011 W cm−2 and pulse durations T = 200–500 fs are found to be optimal. Since the low-lying quadrupole states are dominated by one single electron–hole pair, their energies, being combined with the photoelectron data for hole states, allow us to get the electron spectrum above the Fermi level and thus greatly extend our knowledge on the single-particle spectra of clusters. Besides, the developed schemes allow us to estimate the lifetime of the quadrupole states.

Localized Surface Plasmon Resonance Spectroscopy of Single Silver Triangular Nanoprisms

The plasmonic properties of single silver triangular nanoprisms are investigated using dark-field optical microscopy and spectroscopy. Two distinct localized surface plasmon resonances (LSPR) are observed. These are assigned as in-plane dipolar and quadrupolar plasmon excitations using electrodynamic modeling based on the discrete dipole approximation (DDA). The dipole resonance is found to be very intense, and its peak wavelength is extremely sensitive to the height, edge length, and tip sharpness of the triangular nanoprism. In contrast, the intensity of the quadrupole resonance is much weaker relative to the dipole resonance in the single particle spectra than in the ensemble averaged spectrum. Several parameters relevant to the chemical sensing properties of these nanoprisms have been measured. The dependence of the dipole plasmon resonance on the refractive index of the external medium is found to be as high as 205 nm RIU(-1) and the plasmon line width as narrow as approximately 0.17 eV. These data lead to a sensing figure of merit (FOM), the slope of refractive index sensitivity in eV RIU(-1)/line width (eV), as high as 3.3. In addition, the LSPR shift response to alkanethiol chain length was found to be linear with a slope of 4.4 nm per CH2 unit. This is the highest short-range refractive index sensitivity yet measured for a nanoparticle.

Seeded Growth of Submicron Au Colloids with Quadrupole Plasmon Resonance Modes

A modified seeded growth process has been used for the controlled synthesis of quasispherical, CTAB-stabilized gold nanoparticles from 12 up to 180 nm with narrow size distributions. The UV-visible spectra of the aqueous colloids show distinct bands corresponding to dipole and quadrupole plasmon modes, for diameters above 100 nm, in close agreement with predictions based on Mie theory. The assignment of the modes is demonstrated by calculation of near field enhancement maps based on the boundary element method. Apart from other applications, since absorption is drastically reduced above 600 nm, while scattering is largely increased, these particles open new possibilities for construction of highly efficient photonic structures.

Light scattering from spherical plasmonic nanoantennas: effects of nanoscale roughness

We have investigated the light scattering properties of smooth and roughened nanoshells with dipolar and quadrupolar plasmon resonances tuned to 830 nm. In the dipole resonant case small but measurable variations in the angle dependent light scattering (ADLS) due to the introduction of surface roughness are observed. In the quadrupole case, the distinctive side lobe scattering characteristic of quadrupolar emission is strongly quenched for roughened nanoshells.

Single-Crystalline Gold Microplates: Synthesis, Characterization, and Thermal Stability

Single-crystalline gold microplates of several 10 microm in lateral size, characterized by hexagonal, truncated triangular, and triangular shapes with (111) planes as two basal surfaces, have been synthesized in large quantities through a solution phase process. Significantly, such anisotropic Au nanostructures exhibit remarkable optical properties, in which the dipole plasmon resonance shifting in the NIR region and the quadrupole plasmon resonance at approximately 820 nm were observed. Fragmentation of Au microplates is found when the temperature is higher than 450 degrees C, indicating they are not thermodynamically stable structure at high temperature. Investigations on the Au microplates upon heating suggest that the melting and collapsing start mainly at the edges that should be Au (110) facets. This work is valuable for Au nanostructures applied at elevated temperatures.

Observation of a Quadrupole Plasmon Mode for a Colloidal Solution of Gold Nanoprisms

The synthesis and optical properties of single crystalline gold nanoprisms have been investigated. A three-step mediated seed growth process in an aqueous solution generated gold nanoprisms with a relatively homogeneous size distribution. The purity of these nanostructures has allowed us to observe a weak quadrupole resonance in addition to a strong dipole resonance associated with these novel structures. The experimental optical spectra agree with discrete dipole approximation calculations that have been modeled from the dimensions of gold nanoprisms produced in this synthesis.

Synthesis and Optical Properties of Anisotropic Metal Nanoparticles

In this paper we overview our recent studies of anisotropic noble metal (e.g. gold and silver) nanoparticles, in which a combination of theory and experiment has been used to elucidate the extinction spectra of the particles, as well as information related to their surface enhanced Raman spectroscopy. We used wet-chemical methods to generate several structurally well-defined nanostructures other than solid spheres, including silver nanodisks and triangular nanoprisms, and gold nanoshells and multipods. When solid spheres are transformed into one of these shapes, the surface plasmon resonances in these particles are strongly affected, typically red-shifting and even splitting into distinctive dipole and quadrupole plasmon modes. In parallel, we have developed computational electrodynamics methods based on the discrete dipole approximation (DDA) method to determine the origins of these intriguing optical features. This has resulted in considerable insight concerning the variation of plasmon wavelength with nanoparticle size, shape and dielectric environment, as well as the use of these particles for optical sensing applications.

Linear and Nonlinear Optical Response of Silver Nanoprisms: Local Electric Fields of Dipole and Quadrupole Plasmon Resonances

By means of femtosecond pump and probe spectroscopy, linear and nonlinear optical properties of silver nanoprisms have been investigated, focusing on enhancement of local electric fields due to dipole and quadrupole surface plasmons. Ag nanoprsims were prepared by direct reduction of AgNO3 by the solvent N,N-dimethylformamide in the presence of poly(vinylpyrrolidone). The average edge length and thickness of the nanoprisms (triangles and truncated triangles) were 67 and 35 nm, respectively. In the absorption spectra, dipole and quadrupole plasmon resonance bands have been observed. The values of the imaginary part of nonlinear susceptibility Imχ(3) measured at the dipole and quadrupole plasmon bands are −5.7 × 10-15 and −3.0 × 10-15 esu, respectively. The local electric field factors fQ and fD of quadrupole and dipole plasmon resonances were obtained from the observed dispersion curve of Imχ(3) and absorption spectra, yielding a ratio fQ /fD of ∼0.7. The nonlinear response times of both resonances were found to be ∼2 ps, which is close to the value for spherical nanoparticles, indicating that the relaxation process via electron−phonon interaction is governed by bulk crystal properties.

Orbital magnetism in axially deformed sodium clusters

Low-energy orbital magnetic dipole excitations, known as scissors mode (SM), are studied in alkali metal clusters. Subsequent dynamic and static effects are explored. The treatment is based on a self-consistent microscopic approach using the jellium approximation for the ionic background and the Kohn-Sham mean field for the electrons. The microscopic origin of SM and its main features (structure of the mode in light and medium clusters, separation into low- and high-energy plasmons, coupling high-energy M1 scissors and E2 quadrupole plasmons, contributions of shape isomers, etc) are discussed. The scissors M1 strength acquires large values with increasing cluster size. The mode is responsible for the van Vleck paramagnetism of spin-saturated clusters. Quantum shell effects induce a fragile interplay between Langevin diamagnetism and van Vleck paramagnetism and lead to a remarkable dia-para anisotropy in magnetic susceptibility of particular light clusters. Finally, several routes for observing the SM experimentally are discussed.

Plasmon response of nanoshell dopants in organic films: a simulation study

We examine the effect of an embedding medium refractive index on the plasmon resonant properties of silica core-gold shell nanoshells. The plasmon response is shifted to longer wavelengths with increasing refractive index of the dielectric host matrix, increasing in overall amplitude for nanoparticles in the dipole limit. For nanoshells of constant core-shell ratio, this plasmon shift increases with absolute particle size. We also observe that the plasmon shift is the same for nanoparticles of the same size, independent of core-shell ratio. For larger nanoshells we observe an increase in amplitude of the quadrupole plasmon resonance relative to the dipole plasmon.

The Optical Properties of Metal Nanoparticles: The Influence of Size, Shape, and Dielectric Environment

AbstractFor Abstract see ChemInform Abstract in Full Text.

The Optical Properties of Metal Nanoparticles: The Influence of Size, Shape, and Dielectric Environment

The optical properties of metal nanoparticles have long been of interest in physical chemistry, starting with Faraday's investigations of colloidal gold in the middle 1800s. More recently, new lithographic techniques as well as improvements to classical wet chemistry methods have made it possible to synthesize noble metal nanoparticles with a wide range of sizes, shapes, and dielectric environments. In this feature article, we describe recent progress in the theory of nanoparticle optical properties, particularly methods for solving Maxwell's equations for light scattering from particles of arbitrary shape in a complex environment. Included is a description of the qualitative features of dipole and quadrupole plasmon resonances for spherical particles; a discussion of analytical and numerical methods for calculating extinction and scattering cross-sections, local fields, and other optical properties for nonspherical particles; and a survey of applications to problems of recent interest involving triangula...

Hyper-Rayleigh scattering from silver nanoparticles

We experimentally demonstrate for the first time the existence of distinguishable contributions to hyper-Rayleigh scattering (HRS) intensities from Ag nanoparticles arising from electric–dipole and electric–quadrupole plasmon resonances at the emitted wavelength. We show that these results can be successfully modeled using an electromagnetic theory of HRS which assumes a surface-induced nonlinear susceptibility. In addition, we show that simple angular distribution measurements may be used to determine the relative sizes of the dipole and quadrupole contributions.

Electric Multipole Plasmons in Deformed Sodium Clusters

The random-phase-approximation (RPA) method with separable residual forces (SRPA) is proposed for the description of multipole electric oscillations of valence electrons in deformed alkali metal clusters. Both the deformed mean field and residual interaction are derived self-consistently from the Kohn–Sham functional. SRPA drastically simplifies the computational effort which is urgent if not decisive for deformed systems. The method is applied to the description of dipole, quadrupole, and octupole plasmons in deformed sodium clusters of a moderate size. We demonstrate that, in clusters with the size N>50, Landau damping successfully competes with deformation splitting and even becomes decisive in forming the width and gross structure of the dipole plasmon. Besides, the plasmon is generated by excitations from both ground state and shape isomers. In such clusters familiar experimental estimates for deformation splitting of dipole plasmon are useless.

Photoinduced conversion of silver nanospheres to nanoprisms.

A photoinduced method for converting large quantities of silver nanospheres into triangular nanoprisms is reported. The photo-process has been characterized by time-dependent ultraviolet-visible spectroscopy and transmission electron microscopy, allowing for the observation of several key intermediates in and characteristics of the conversion process. This light-driven process results in a colloid with distinctive optical properties that directly relate to the nanoprism shape of the particles. Theoretical calculations coupled with experimental observations allow for the assignment of the nanoprism plasmon bands and for the first identification of two distinct quadrupole plasmon resonances for a nanoparticle. Unlike the spherical particles they are derived from that Rayleigh light-scatter in the blue, these nanoprisms exhibit scattering in the red, which could be useful in developing multicolor diagnostic labels on the basis not only of nanoparticle composition and size but also of shape.

Scattering of electrons on metal clusters and fullerenes

It is shown that the main contribution to the elastic cross section of fast electrons on metal clusters and fullerenes results from scattering on the frozen cluster potential, which is determined by the electron density distribution of the ground state of the target cluster. The specific shape of the electron distribution in fullerenes and metal clusters manifests itself in the diffraction behaviour of the elastic differential cross section. The analysis of the total elastic cross section dependence upon projectile velocity, the number of atoms in the cluster and its size is provided. The cross section of elastic scattering on a cluster surpasses the sum of the individual scattering cross sections on the equivalent number of isolated atoms. This occurs because of the coherent interaction of the projectile electron with electrons delocalized in the cluster volume. We have demonstrated that collective electron excitations sensitive to the many-electron correlations dominate inelastic scattering. The surface plasmon resonances can be observed in the differential cross section for inelastic scattering. We found a condition for the quadrupole and higher multipole plasmon excitations to contribute relatively little to the electron energy loss spectrum. The results obtained have been compared with experimental data for the electron - fullerene collision. Reasonable agreement between theoretical and experimental results is reported. We have also demonstrated that plasmon excitations provide the main contribution to the total inelastic cross section over a wide energy range. We have calculated the dependence of the total inelastic cross section on collision energy and compared the result obtained with the experimental data available, giving an interpretation for the plateau region in the cross section as caused by plasmon excitations rather than the cluster fragmentation process. We have shown that the single-particle jellium approximation fails to describe this experiment. Our analysis is performed for metal clusters and fullerenes, However, it can also be applied to other polarizable systems, possessing plasmon or giant collective resonances.

Plasmon resonances observed in light scattered by large alkali clusters

Abstract. We studied the possibility of exciting plasmons bylight in arbitrarily large Li, Na, K, Rb and Cs clusters asa function of cluster size. We used the results of the Mietheory to describe the dependency of the total scattering (ex-tinction) cross-sections on the cluster radius. We also appliedthis theory to describe the intensity of the scattered light in di-rectionsorthogonalto the directionof the incident light beam.In the experiment we measured the intensity of light scatteredby growing sodium clusters in directions orthogonalto the di-rection of the incident light beam. We found that the maximaof the measured intensities arose from secondary fields due tothe dipole plasmon and the quadrupole plasmon resonances.PACS: 36.40.+d; 78.20.-eThe possibility of exciting and observing collective oscilla-tions in spherical metal clusters is a subject of growing in-terest [1–11]. In contrast to bulk metal with a flat surface,the curved surface of a spherical metal cluster enables directoptical excitation of collective motions of free electrons. Ex-tensive study of the scattering of light by a sphere of any sizewas made by Mie using classical electrodynamics. The Mietheory only deals with the problem of the continuity of thetangential component of the divergence-free electromagnet-ic field and does not deal with the phenomenon of collectiveelectron oscillations. Nevertheless, peaks in the absorptioncross-sections are usually interpreted as a manifestation ofresonances due to excitation of collective oscillations (plas-mons) (see e.g. [1,2,12,13]).In this paper we focus our attention on the optical proper-ties of metal clusters as a function of their size for differentalkali metals. We do not make any assumption concerningthe size of the cluster in comparison to the wavelength of thelight. We analyze the maxima in the scattering cross-sectionefficiencies for these metal spheres by combining the formal-ism of the classical Mie theory and the concept of plasmon-polariton oscillations. We treat the metal sphere filled withfree conduction-electrons as a cavity with size-dependenteigenfrequencies,l D1

Dipole and quadrupole plasmon resonances in large sodium clusters observed in scattered light

We study the dependence of the optical properties of clusters as a function of cluster size. The Mie theory is used to describe intensities of light scattered by growing clusters in directions orthogonal to the direction of the incident light beam. We assigned the maxima in scattered intensities to secondary fields due to plasmons excited in clusters with appropriate radius. The maxima in measured intensities of scattered light in “perpendicular” and “parallel” polarization geometry are attributed to excitation of dipole plasmon when cluster radius approaches 55 nm and of quadrupole plasmon when cluster size approaches 118 nm, respectively.

Surface Dielectric Response of Collective Plasmon Excitations in C60Fullerite

Abstract The paper begins with a review of available observations from electron-energy-loss spectroscopy (EELS) on the excitation of multipolar plasmons in C60. With these observations in the background, an electrodynamic formalism is developed for computing the response of collective plasma oscillations in a fullerite film subject to an external electric field. This formalism allows us to construct the dielectric response function relevant for applications to EELS in specular-reflection geometry. The only input required by the theory is a set of dynamical multipolar polarizabilities of an isolated molecule, which could be provided by a separate theory but which we simulate here by an empirical model consisting of a dielectric spherical shell. The surface loss function is computed for an fcc (111) film composed of such dielectric shells. Comparison with the few available experimental data indicates that quadrupole and higher-multipolar plasmons in fullerites contribute significantly to the long-range inel...