Dipole Plasmon Resonance Research Articles

Spectral Degeneracy Breaking of the Plasmon Resonance of Single Metal Nanoparticles by Nanoscale Near-Field Photopolymerization

We report on controlled nanoscale photopolymerization triggered by enhanced near fields of silver nanoparticles excited close to their dipolar plasmon resonance. By anisotropic polymerization, symmetry of the refractive index of the surrounding medium was broken: C infinity v symmetry turned to C2v symmetry. This allowed for spectral degeneracy breaking in particles plasmon resonance whose apparent peak became continuously tunable with the incident polarization. From the spectral peak, we deduced the refractive-index ellipsoid fabricated around the particles. In addition to this control of optical properties of metal nanoparticles, this method opens new routes for nanoscale photochemistry and provides a new way of quantification of the magnitude of near fields of localized surface plasmons.

Midinfrared metamaterials fabricated by nanoimprint lithography

A metamaterial comprising an ordered array of four metallic L-shaped components designed to operate in the mid-IR frequency regime has been fabricated and characterized. The fourfold rotational symmetry of the unit cell should suppress the undesirable bianisotropy observed for split-ring resonators. Nanoimprint lithography was used to demonstrate scalability for mass production. A dipole plasmon resonance with a negative permittivity and a magnetic resonance with a negative permeability were observed at wavelengths of 3.7 and 5.25μm, respectively, in agreement with theoretical predictions.

Optical metamaterials at near and mid-IR range fabricated by nanoimprint lithography

Two types of optical metamaterials operating at near-IR and mid-IR frequencies, respectively, have been designed, fabricated by nanoimprint lithography (NIL), and characterized by laser spectroscopic ellipsometry. The structure for the near-IR range was a metal/dielectric/metal stack "fishnet" structure that demonstrated negative permittivity and permeability in the same frequency region and hence exhibited a negative refractive index at a wavelength near 1.7 um. In the mid-IR range, the metamaterial was an ordered array of four-fold symmetric L-shaped resonators (LSRs) that showed both a dipole plasmon resonance resulting in negative permittivity and a magnetic resonance with negative permeability near wavelengths of 3.7 um and 5.25 um, respectively. The optical properties of both metamaterials are in agreement with theoretical predictions. This work demonstrates the feasibility of designing various optical negative-index metamaterials and fabricating them using the nanoimprint lithography as a low-cost, high-throughput fabrication approach.

Multiple ionization of C60 in collisions with 2.33 MeV/u O-ions and giant plasmon excitation

Single and multiple ionization of C60 in collisions with fast (v=9.7a.u.) Oq+ ions have been studied. Relative cross sections for production of C601+ to C604+ have been measured. The intensity ratios of double-to-single ionization agree very well with a model based on giant dipole plasmon resonance (GDPR). Almost linear increasing trend of the yields of single and double ionizations with projectile charge state is well reproduced by the single and double plasmon excitation mechanisms. The observed charge state independence of triple and quadruple ionization is in sharp contrast to the GDPR model.

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.

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.

Spectral tuning of plasmon-enhanced silicon quantum dot luminescence

In the presence of nanoscale silver island arrays, silicon quantum dots exhibit up to sevenfold luminescence enhancements at emission frequencies that correspond to the collective dipole plasmon resonance frequency of the Ag island array. Using electron-beam lithography to alter the pitch and particle diameter, this wavelength-selective enhancement can be varied as the metal array resonance wavelength is tuned from 600to900nm. The luminescence intensity enhancement upon coupling is attributed to an increase in the radiative decay rate of the silicon quantum dots.

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.

Preparation and characterization of metal (Au)– and bimetallic alloys (AuNi)–gelatin nanocomposites

Hybrid inorganic/organic materials consisting of Au and AuNi nanoparticles and gelatin have been synthesized by soft chemical routes (reduction by N 2H 4 aqueous solution at 277 and 373 K). The average particle size, obtained by transmission electron microscopy, ranged between 4 and 20 nm. For Au–gelatin sample, at 277 K, the gelatin 3D network was maintained and Au nanoparticles present close-packed linear assemblies along gelatin filaments forming a spider web-like network. Because of inter-particle coupling and gelatin adsorption, Au arrays exhibit dipolar plasmon resonances that shift and broaden by hundreds of nanometers. For the first time, AuNi nanoparticles could also be formed at 373 K. At this temperature gelatin was solubilized but AuNi particles were still stabilized by the protein chains without array formation. AuNi fcc phases were observed by X-ray diffraction. A blue-shift of the plasmon resonance was observed for AuNi–gelatin nanocomposites when compared to Au–gelatin. SQUID magnetometer measurements indicate that AuNi–gelatin samples are ferromagnetic at low temperature. At 2 K hysteresis loops were observed with coercivity ( H c) values between 450 and 650 Oe and squareness values between 0.15 and 0.37. These new nanocomposites therefore appear very promising for the design of nanodevices.

Optical properties of a spherical two-dimensional electron gas in a uniform magneticfield

Using the random phase approximation, we calculate the plasmon frequencies of anelectron gas on a two-dimensional spherical surface in the presence of a weak magneticfield. We show that the magnetic field results in a coupling between electronic states withdifferent angular momentum numbers. This coupling results in a blue-shift of the dipolarplasmon resonance with increasing magnetic field. We also investigate how the plasmonenergies vary as a function of the number of electrons and radius of the sphere.

Confined Plasmons in Nanofabricated Single Silver Particle Pairs: Experimental Observations of Strong Interparticle Interactions

We report on the optical properties of single isolated silver nanodisks and pairs of disks fabricated by electron beam lithography. By systematically varying the disk size and surface separation and recording elastic scattering spectra in different polarization configurations, we found evidence for extremely strong interparticle interactions. The dipolar surface plasmon resonance for polarization parallel to the dimer axis exhibited a red shift as the interdimer separation was decreased; as expected from previous work, an extremely strong shift was observed. The scattering spectra of single particles and pairs separated by more than one particle radius can be well described by the coupled dipole approximation (CDA), where the particles are approximated as point dipoles using a modified dipole polarizability for oblate spheroids. For smaller particle separations (d < 20 nm), the simple dipole model severely underestimates the particle interaction, indicating the importance of multipolar fields and finite-size effects. The discrete dipole approximation (DDA), which is a finite-element method, describes the experimental results well even at d < 20 nm, including particles that have metallic bridges.

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.

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

AbstractFor Abstract see ChemInform Abstract in Full Text.

Structural Tunability of the Plasmon Resonances in Metallic Nanoshells

Using the time-dependent LDA method, we investigate the tunability of the optical polarizability of small metallic nanoshells. We show that the energies of the two dipolar plasmon resonances vary with the ratio of shell thickness to particle radius in a manner similar to what has been predicted using classical Mie scattering and semiclassical models for uniform metallic shell structures.

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...

Plasmonresonances in large noble-metal clusters

We investigate the optical properties of spherical gold andsilver clusters with diameters of 20 nm and larger. The lightscattering spectra of individual clusters are measured usingdark-field microscopy, thus avoiding inhomogeneous broadeningeffects. The dipolar plasmon resonances of the clusters arefound to have nearly Lorentzian line shapes. With increasingsize we observe polaritonic red-shifts of the plasmon line andincreased radiation damping for both gold and silver clusters.Apart from some cluster-to-cluster variations of the plasmonlines, agreement with Mie theory is reasonably good for thegold clusters. However, it is less satisfactory for the silverclusters, possibly due to cluster faceting or chemicaleffects.

Synthesis and Characterization of Truncated Triangular Silver Nanoplates

Truncated triangular silver nanoplates have been synthesized in large quantities using a solution phase method in the presence of cetyltrimethylammonium bromide micelles. The obtained particles have an average edge size of 68 nm, thickness of 24 nm, and the degree of truncation of 0.35. They are single crystals, with a (111) lattice plane as the basal plane. The aqueous solution of these particles displays a clear red color due to a strong in-plane dipole plasmon resonance peaked at 552 nm. These particles can self-assemble into chainlike nanostructures through either partial or close stacking of neighborhood nanoplates.

Synthesis of Silver Nanoprisms in DMF

Polygonal (mainly triangular) silver nanoprisms were synthesized by boiling AgNO3 in N,N-dimethyl formamide, in the presence of poly(vinylpyrrolidone). Although during the synthesis, a mixture of nanoprisms and nanospheroids is formed, the latter can be removed through careful centrifugation. The UV−visible spectra of the nanoprisms display an intense in-plane dipolar plasmon resonance band, as well as weak bands for in-plane and out-of-plane quadrupolar resonances. The nanoprisms are also stable in other solvents, such as ethanol and water, and solvent exchange leads to strong shifts of the in-plane dipole plasmon band.

The effect of a dielectric core and embedding medium on the polarizability of metallic nanoshells

Using the time dependent density functional method we investigate the effects of a dielectric core and a dielectric embedding medium on the optical properties of metallic nanoshells. The polarizability is shown to be strongly influenced by the presence of the dielectric which induces additional screening charges that need to be included self-consistently. We show that the energies of the dipolar plasmon resonances depend strongly on the dielectric constant of the core or embedding medium. The results compare very well with classical Mie scattering theory calculations.

Quadrupolar vibrational mode of silver clusters from plasmon-assisted Raman scattering

Absorption and low-frequency Raman-scattering experiments have been performed on thin films consisting of small silver clusters embedded in a porous alumina matrix. When the Raman excitation wavelength is close to the maximum (\ensuremath{\approx}420 nm) of the Mie band (dipolar surface plasmon resonance) the Raman spectra exhibit a strong band located around 10 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$, the maximum of which depends on the cluster diameter D at the maximum of the cluster-size distribution in the sample according to the approximate law ${\ensuremath{\omega}}_{\mathrm{vib}}\ensuremath{\propto}{D}^{\ensuremath{-}1}.$ The Raman band corresponds to the excitation of the quadrupolar vibration mode of the clusters, via the plasmon-phonon interaction. Moreover, the maximum of the Raman band shifts towards lower frequencies when the excitation light is shifted to the red. This feature, as well as the rather large Mie-band width, is shown to reflect the ellipsoidal shape distribution of part of the embedded clusters.