Quadrupole Plasmon Modes Research Articles

Enhanced photoelectron emission from dark plasmon mode in gold nanoring

By the advantage of light concentration in nanoscale and strong light absorption, LSPRs have presented benefits for ultrafast electron pulse generation, promoting high spatio-temporal electron microscopy and spectroscopy. However, previous studies generally pay more attention to the bright mode of LSPRs; the dark mode, which corresponds to longer dephasing time and less radiative decay that is potential to deliver a leap forward in plasmon-enhanced photoemission, is still neglected. Here, we report on the investigation of quadrupole plasmon mode (dark mode) induced photoemission electron from a gold nanoring with photoemission electron microscopy. The result shows that the photoemission electron yield is around 11,200 for the dark mode supported by nanoring under s-polarized light irradiation, which is higher than the one under p-polarized light irradiation (around 8600). The higher photoemission electron yield is attributed to stronger absorption of the s-pol light excited mode. In addition, the result shows that the dephasing time of s polarization excitation mode is longer than that of p polarization excitation. The experiment results are well reproduced by FDTD simulation. Our investigation may lay the foundation for dark plasmon mode enhanced photoemission and offers an additional benefit for the progress of plasmon-based femtosecond on-chip electronics, high spatial-temporal electron microscopy and spectroscopy.

Plasmons in a square of two-dimensional electrons

Microwave absorption spectra of a single square of two-dimensional electrons (2DESs) have been investigated using an optical detection technique. Fundamental dipole and harmonic quadrupole plasmon modes have been identified and compared to those in the disk geometry. In the square-shaped 2DES, a strong interaction is discovered between the neighboring plasmon modes, whereas no such hybridization is observed in the disk-shaped geometry. We establish a rigid theoretical platform to analytically describe the magneto-optical response of confined two-dimensional systems. The developed theory provides a proper description of the obtained experimental results.

In-Plane Electrical Connectivity and Near-Field Concentration of Isolated Graphene Resonators Realized by Ion Beams.

Graphene plasmons provide great opportunities in light-matter interactions benefiting from the extreme confinement and electrical tunability. Structured graphene cavities possess enhanced confinements in 3D and steerable plasmon resonances, potential in applications for sensing and emission control at the nanoscale. Besides graphene boundaries obtained by mask lithography, graphene defects engineered by ion beams have shown efficient plasmon reflections. In this paper, near-field responses of structured graphene achieved by ion beam direct-writing are investigated. Graphene nanoresonators are fabricated easily and precisely with a spatial resolution better than 30 nm. Breathing modes are observed in graphene disks. The amorphous carbons around weaken the response of edge modes in the resonators, but meanwhile render the isolated resonators in-plane electrical connections, where near-fields are proved gate-tunable. The realization of gate-tunable near-fields of graphene 2D resonators opens up tunable near-field couplings with matters. Moreover, graphene nonconcentric rings with engineered near-field confinement distributions are demonstrated, where the quadrupole plasmon modes are excited. Near-field mappings reveal concentrations at the scale of 3.8×10-4λ02 within certain zones which can be engineered. The realization of electrically tunable graphene nanoresonators by ion beam direct-writing is promising for active manipulation of emission and sensing at the nanoscale.

Spatial evolution of the near-field distribution on planar gold nanoparticles with the excitation wavelength across dipole and quadrupole modes

We investigate the superposition properties of the dipole and quadrupole plasmon modes in the near field both experimentally, by using photoemission electron microscopy (PEEM), and theoretically. In particular, the asymmetric near-field distributions on gold (Au) nanodisks and nanoblocks under oblique incidence with different polarizations are investigated in detail. The results of PEEM measurements show that the evolutions of the asymmetric near-field distributions are different between the excitation with s-polarized and p-polarized light. The experimental results can be reproduced very well by numerical simulations and interpreted as the superposition of the dipole and quadrupole modes with the help of analytic calculations. Moreover, we hypothesize that the electrons collected by PEEM are mainly from the plasmonic hot spots located at the plane in the interface between the Au particles and the substrate in the PEEM experiments.

Exploring Coupled Plasmonic Nanostructures in the Near Field by Photoemission Electron Microscopy.

The extraordinary optical properties of coupled plasmonic nanostructures make these materials potentially useful in many applications; thus, they have received enormous attention in basic and applied research. Coupled plasmon modes have been characterized predominantly using far-field spectroscopy. In near-field spectroscopy, the spectral response of local field enhancement in coupled plasmonic nanostructures remains largely unexplored, especially experimentally. Here, we investigate the coupled gold dolmen nanostructures in the near field using photoemission electron microscopy, with wavelength-tunable femtosecond laser pulses as an excitation source. The spatial evolution of near-field mapping of an individual dolmen structure with the excitation wavelength was successfully obtained. In the near field, we spatially resolved an anti-bonding mode and a bonding mode as the result of plasmon hybridization. Additionally, the quadrupole plasmon mode that could be involved in the formation of a Fano resonance was also revealed by spatially resolved near-field spectra, but it only contributed little to the total near-field enhancement. On the basis of these findings, we obtained a better understanding of the near-field properties of coupled plasmonic nanostructures, where the plasmon hybridization and the plasmonic Fano resonance were mixed.

Effects of morphology, diameter and periodic distance of the Ag nanoparticle periodic arrays on the enhancement of the plasmonic field absorption in the CdSe quantum dots

In this work, the numerical calculations of plasmonic field absorption of Ag nanoparticles (Ag NPs) periodic arrays in the CdSe quantum dot (QD) film are investigated by the three-dimensional finite difference time domain (FDTD). Diameter (D), periodic distance (P), and morphology effects of Ag NPs are investigated on the improvement of the plasmonic field absorption in CdSe QD film. Results show that plasmonic field absorption in CdSe QD film is enhanced with reduction of D of Ag NPs until 5nm and reduces thereafter. It is observed that with raising D of Ag NPs, optimum plasmonic field absorption in CdSe QD film is shifted toward the higher P. Moreover, with varying morphology of Ag NPs from spherical to cylindrical, cubic, ringing and pyramid, the plasmonic field absorption is considerably enhanced in CdSe QD film and position of quadrupole plasmon mode (QPPM) is shifted toward further wavelength. For cylindrical Ag NPs, the QPPM intensity increased with raising height (H) until 15nm and reduces thereafter.

Gold Nanorod Arrays: Excitation of Transverse Plasmon Modes and Surface-Enhanced Raman Applications

We describe a method for the fabrication of gold nanorod arrays and their characterization. The initial requirements were for a robust substrate with strong surface enhancement of Raman scattering with minimal surface contamination. The rods with length up to 20 μm were electrochemically deposited in alumina templates bonded to gold-coated silicon wafers. After the removal of the template, surface-bound cyanide from the deposition process was removed by mild electropolishing. Reflectance and cathodoluminescence measurements revealed transverse dipole and quadrupole plasmon modes. Surface enhancement of 106 was determined by taking Raman spectra of self-assembled monolayers of 4-aminothiophenol. The enhancement factor (EF) was found to be independent of nanorod aspect ratio because only the length of the nanorods was varied while their diameter was kept constant, and because of the large length of nanorods only the transverse plasmons were excited.

Tunable Fano resonances and plasmonic hybridization of gold triangle–rod dimer nanostructure**Project supported by the National Natural Science Foundation of China (Grant No. 61275153) and the Natural Science Foundation of Zhejiang Provice, China (Grant No. LY12A04002).

A gold dimer structure consisting of a notched triangle nanoslice and a rectangle nanorod is proposed to produce distinct Fano resonance. Owing to the coupling between the dipole plasmon mode of the nanorod and the dipole or quadrupole plasmon mode of the nanoslice, the extinction spectrum with a deep Fano dip is formed and can be well fitted by the Fano interference model for different geometry parameters. In addition, Fano resonance of the gold dimer nanostructure also intensely depends on the polarization direction of incident light. Moreover, Fano resonance of the triangle–rod trimer is also analyzed by adding another nanorod into the former dimer and exhibits the splitting of plasmonic resonant peak in high order coupling modes. The plasmonic hybridizations in these nanostructures have been analyzed for revealing the physical origin of the Fano resonance.

Influence of localized surface plasmon in a lens-shaped metal cluster on the decay dynamics of a point-dipole emitter

Influence of localized surface plasmon in a lens-shaped metal nanoparticle on the decay dynamics of a point-dipole emitter was studied by means of numerical simulations. Dependencies of radiative and nonradiative decay rates on the distance between the particle and the dipole were calculated for dipole and quadrupole plasmon modes. The presence of the etching cone changes the nonradiative dynamics of a dipole located under a metal cluster and can be a convenient tool to reduce the surface influence on emitters. The cone also selectively enhances the nonradiative decay rate for the emitter located under the vertex enabling preferential plasmon coupling in ensembles of emitters.

Quadrupole plasmon excitations in confined one-dimensional systems

A new mode of electronic collective excitations (quadrupole plasmon) in confined one-dimensional electronic systems is predicted herein by an eigen-equation method. The eigen equation based on the time-dependent density-functional theory is presented for calculating the collective excitations in confined systems. With this method, all modes of collective excitations in the 1D systems may be found out. These modes include dipole plasmons and quadrupole plasmons. The dipole plasmon mode corresponds to the antisymmetric oscillation of the induced charge, and can be shown as a resonance of the dipole response. In the quadrupole plasmon modes, the induced-charge distribution is symmetric, and the dipole response vanishes. The motion of the electrons in the quadrupole modes is similar to the vibration of atoms in the breathing mode of phonons. This type of plasmons can be shown as a resonance of the quadrupole response, and has to be excited by a nonuniform field.

Strong dipole-quadrupole coupling and Fano resonance in H-like metallic nanostructures.

Under certain conditions of the incident light polarization direction a Fano resonance arises in small gold nanorods arranged in a H-like configuration. This stems from the coupling between a bright dipole plasmon mode excited in the horizontal rod and dark quadrupole plasmon modes in both vertical rods. We investigate these surface plasmon modes, and analyze the dependence of the Fano resonance on the geometry parameters such as rod size and interparticle separation, and refractive index of embedding medium. To describe the degree of this energy transfer, we introduce a new parameter: the Fano resonance efficiency. We calculate absorption cross-sections for visible and NIR spectrum in each element of the structure, and near-field distributions at different wavelengths. We show that Fano resonance in small H-like structures exhibits high sensitivity with respect to the refractive index of the host medium, outperforming the values for larger plasmonic structures based on nanorods already investigated.

Fano resonance properties of gold nanocrescent arrays.

The Fano resonance induced by symmetry breaking could improve the sensitivity of localized surface plasmon resonance sensors. In this work, the spectra of gold nanocrescent arrays are measured and confirmed by simulation results through the finite element method (FEM). The Fano resonance presented in the spectra could be modulated by the symmetry breaking with different waist widths, which are understood through plasmonic hybridization theory with the help of surface charge distribution. Our results indicate the Fano lineshape is generated by the coherent coupling of the quadrupole plasmon mode QH of nanohole and the antibonding plasmon mode D(AB) of nanocrescent. Finally, the high figure of merit (FoM=1.6-3.5) of the Q mode in the visible region illustrates this nanocrescent Fano sensor is of great value in the biological and chemical scientific fields.

Quadrupole plasmon excitations in finite-size atomic chain systems

The existence and the nature of a new mode of electronic collective excitations (quadrupole plasmons) in confined one-dimensional electronic systems, used to mimic finite-size linear metal atomic chains, have been predicted by an eigen-equation method. The eigen-equation based on the time-dependent density-functional theory is presented for calculating the collective excitations in confined systems. With this method, all modes of collective excitations in the 1D systems can be found out. These modes include dipole plasmons and quadrupole plasmons. The dipole plasmon mode corresponds to the antisymmetric oscillation of induced charge, and can be shown as a resonance of the dipole response. In the quadrupole plasmon modes, the induced charge distribution is symmetric, and the dipole response vanishes. The motion of the electrons in the quadrupole modes is similar to the vibration of atoms in the breathing mode of phonons. This type of plasmon can be shown as a resonance of the quadrupole response, and has to be excited by a non-uniform field.

Palladium bridged gold nanocylinder dimer: plasmonic properties and hydrogen sensitivity

Plasmonic nanodimers facilitate electromagnetic hotspots at their gap junction. By loading these gap junctions with nanomaterials, the plasmonic properties of nanodimer can be varied. In this study, we bridged the gap junction of gold (Au) nanocylinder dimer with palladium (Pd), and numerically evaluated the plasmonic properties of the designed nanostructure. We simulated the far-field extinction spectra of Pd bridged Au nanocylinder dimer, and identified the dipole and quadrupole plasmon modes at 839 and 578 nm, respectively. By varying the geometrical parameters of the Pd bridge, we revealed the ability to tune the dipolar plasmon resonance of the bridged dimer. Further, we exploited the hydrogen sensitivity of Pd bridge to harness the bridged-Au dimer as nanoplasmonic hydrogen sensor. Such nano-optical detection platforms have minimal spatial footprint and can be further harnessed for chip-based plasmonic sensing.

Enhancement of excitonic emission in semiconductor heterostructures due to resonant coupling to multipole plasmon modes in a gold particle

We apply an exact electrodynamic theory to analyze experimental data on the emission enhancement in epitaxial semiconductor structures, induced by resonant interaction of emitting excitons with plasmons localized in a gold nanoparticle. Near-field scanning optical microscopy was employed to investigate the effect of the gold particle positioned near the surface of a rough InGaN film or a CdSe/ZnSe quantum dot heterostructure fabricated by molecular-beam epitaxy. It is shown that the frequently used quasistatic approximation is unable to explain the experimental data due to the inability to take into consideration the contribution of higher-order multipole plasmon resonances. In particular, it follows from the performed calculations that the observed effect of the emission enhancement owes mainly to the resonant interaction with the quadrupole and octupole plasmon modes in the spherical gold particle. The design of optimized metal-semiconductor structures is proposed.

Optical Properties of Tipless Gold Nanopyramids

Controlling the size and shape of plasmonic nanoparticles (NPs) is a convenient approach to tune their optical properties for a broad range of applications. For example, metal NPs can function as discrete refractive index elements in optical sensing devices[1] as well as contrast agents in biological imaging.[2] An emerging area in plasmonics that has received less attention, however, is the tailoring of specific structural features of a NP (e.g., sharp asperities) to manipulate their optical properties. Systems of interest that could display signatures of such nanoscale geometrical changes are large (>100 nm) anisotropic NPs because they can support tunable dipole, quadrupole, and higher-order plasmon modes.[3] Although synthetic methods can access different shapes by changing the morphology of the seed (hard) or molecular (soft) templates[4] or by introducing metal ions and other molecules into the growth solution,[5] the overall sizes are usually too small to access higher-order plasmons. One important shape is Au nanorods, which can be grown to various lengths by keeping the width constant and increasing the length[6] or to various widths by preferential overgrowth on their sides.[7] In general, fabrication methods offer a more systematic approach to control different aspects of large particle shapes.[8] Also, the shapes of fabricated NPs tend to have a higher degree of monodispersity because their structures are governed by hard templates (e.g., anodized alumina membranes).[3e, 9]

Visualization of Near-Field Enhancements of Gold Triangles by Nonlinear Photopolymerization

We report in this paper the near-field distribution in the case of gold triangle arrays by means of two-photon polymerization for a dipole and a quadrupole plasmon mode. In order to link the finite difference in the time domain (FDTD) simulations of the triangle array and the experimental results, extinction spectra for both cases in air and SU-8 environments are shown. In case of the 40-nm thick gold triangles with 85-nm side-length, we show that the calculated and experimentally obtained near-field for the excited dipole mode has the same distribution along the polarization of the exciting laser beam. In case of bigger triangles of 540-nm side-length a quadrupole mode is excited, which leads to a rotation of the near-field distribution by 90° referred to the polarization of the beam. This effect is also shown in the FDTD simulations.

Optical Near-Field Mapping of Plasmonic Nanoprisms

The optical local-field enhancement on nanometer length scales provides the basis for plasmonic metal nanostructures to serve as molecular sensors and as nanophotonic devices. However, particle morphology and the associated surface plasmon resonance alone do not uniquely reflect the important details of the local field distribution. Here, we use interferometric homodyne tip-scattering near-field microscopy for plasmonic near-field imaging of crystalline triangular silver nanoprisms. Strong spatial field variation on lengths scales as short as 20 nm are observed sensitively depending on structural details and environment. The poles of the dipole and quadrupole plasmon modes, as identified by phase-sensitive probing and calculations performed in the discrete dipole approximation (DDA), reflect the particle symmetry. Together with the observation that the largest enhancement is not necessarily found to be associated with the tips of the nanoprisms, our results provide critical information for the selection of particle geometries as building blocks for plasmonic device applications.

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.

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.