New approach to spectroscopy of surface plasmons

We report a novel approach to spectroscopy of surface plasmons based on the simultaneous excitation of surface plasmons by a polychromatic light and the dispersion of light on a diffraction grating coupler of a special design. This approach shows promise for the development of new, miniaturized, spectroscopic surface plasmon resonance (SPR) sensors. Potential of this approach for SPR sensing is demonstrated in a model refractometric experiment.     

Particle plasmons resonant characteristics in arrays of strongly coupled gold nanoparticles

Using the finite difference time domain (FDTD) method, we investigated the optical properties of a periodic array of strongly coupled gold nanoparticles (MNPs). We show the two kinds properties of transmission spectra with both different interparticle spaces and different radii of the particles. It is found that some distinct extra resonant peaks appear in the forbidden band gap and the resonant frequency depends strongly on the space between the particles and the radius of the particles. Based on the localized nature of the field distribution, we also show clearly the presence of local plasmons resonant modes that originate from quadrupole plasmon polaritons.     

Photoemission from multiply excited surface plasmons in Ag nanoparticles

Two-photon photoemission spectroscopy using femtosecond laser pulses is used to investigate the excitation and decay mechanisms of the surface plasmon resonance in Ag nanoparticles grown on graphite. The resonant excitation of this collective excitation leads to a two-orders-of-magnitude-enhanced two-photon photoemission yield from a graphite surface with Ag nanoparticles compared to the yield from pure graphite. From the shape of the photoemission spectra, the polarization dependence of the photoemission yield and the excitation probabilities for different excitation pathways we conclude that excitation with 400-nm femtosecond laser pulses leads to the coherent multiple excitation of the surface plasmon in the Ag nanoparticles. This multiply excited plasmon mode can decay via the coupling to a single-particle excitation leading to the emission of an electron if its final state is located in the continuum. The surface plasmon in metallic nanoparticles is a model system to investigate collective excitations in multiphoton processes. Received: 26 June 2000 / Accepted: 2 September 2000 / Published online: 12 October 2000     

Near-field imaging of pyramid-like nanoparticles at a surface

An exact analytical solution of the self-consistent equation for the local field is used to calculate the near-field optical images of pyramid-like nano-objects placed at a surface of a solid. The diagram method developed previously for near-field image formation is generalized in order to describe layered objects, which are treated as many-body systems. The near-field optical images of triangular and square pyramids are calculated for the illumination configuration as well as those of triangular and square prisms. It is found that the near-field images of nanoparticles having the dielectric constant close to that of the substrate change rapidly and in a complicated manner with the probe–sample distance.     

Numerical simulations on longitudinal surface plasmons of coupled gold nanorods

We analyze numerically the localized surface plasmon resonance (LSPR) frequencies of coupled metal nanorods using discrete dipole approximation (DDA) and finite element method (FEM). The two configurations of identical nanorods are considered: end-to-end and side-by-side. Dependence of LSPR frequencies on the interparticle distance is determined by DDA. The latter for the case of large separations agrees with the result of approximate analytical method developed earlier for single metal nanorods. Distributions of electric near-fields of nanorods as well as enhancement factors at hot spots for both configurations are calculated by FEM.     

Dispersive Fourier-transform spectroscopy of surface plasmons in the infrared frequency range

The optical scheme and the principle of operation of an asymmetric Mach-Zehnder interferometer for dispersive spectroscopy of surface IR plasmons are discussed. A thin-layer sample deposited on a conducting substrate, which guides surface plasmons, is placed in one of the arms of the interferometer. This makes it possible, by applying a complete Fourier transform to the interferogram, to obtain the spectrum of the complex refractive index of surface plasmons and, therefore, of the complex dielectric permittivity of the sample or the substrate material.     

Cavity ring-down spectroscopy of metallic gold nanoparticles

The optical properties of supported gold nanoparticles with sizes of 1.3 nm, 1.6 nm, 2.5 nm, and 2.9 nm have been studies by using cavity ring-down spectroscopy in the photon energy range between 1.8 eV and 3.0 eV. The obtained results show the possibility to obtain optical information of nanoassembled materials with high sensitivity. The experimental findings are compared to calculations using Mie-Drude theory. Whereas the broadening of the surface plasmon resonance with decreasing size is well described by this model, the observed blue-shift of the surface plasmon resonance contradicts the predictions of the Mie-Drude theory. The latter effect can be explained by the presence of a skin region with decreased polarizability typical for coinage metal particles. Furthermore, it is found that the supported gold nanoparticles are robust under ambient conditions, an important issue when using these materials for optical applications.     

The effect of gain and absorption on surface plasmons in metal nanoparticles

The compensation of loss in metal by gain in interfacing dielectric has been demonstrated in a mixture of aggregated silver nanoparticles and rhodamine 6G dye. An increase of the quality factor of surface plasmon (SP) resonance was evidenced by the sixfold enhancement of Rayleigh scattering. The compensation of plasmonic losses with gain enables a host of new applications for metallic nanostructures, including low- or no-loss negative-index metamaterials. We have also predicted and experimentally observed a suppression of SP resonance in metallic nanoparticles embedded in dielectric host with absorption. PACS 61.46.Df; 73.20.Mf; 78.67.Bf     

Plasmon spectroscopy of metallic nanoparticles above flat dielectric substrates

We numerically analyze the spectral properties of localized plasmon resonances in metal nanoparticles when these are above a dielectric substrate. This analysis is performed as a function of the various parameters involved in the problem (relative optical properties, particle-substrate separation, angle of incidence, etc.). It can be shown that from the spectral behavior of the resonance in the far field, information about particle near-field interactions can be obtained.     

表面等离激元与磁表面等离激元

金属微纳结构体系中的表面等离激元以及磁表面等离激元因其独特的光学特性吸引了研究者们的极大兴趣,成为当前的热点研究领域之一.文章对表面等离激元尤其是磁表面等离激元的特点、基本现象、新颖效应及其应用研究前景的最新发展进行了介绍.     

Raman spectroscopy of gold nanoparticles in polycrystalline LiF film

Results of the Raman spectroscopy analysis of a new composite material based on a thin polycrystalline LiF film containing gold nanoparticles are presented. The formation of spherical gold nanoparticles in the film has been confirmed by the X-ray structural analysis and observation of the optical plasmon resonance absorption spectrum with a maximum at 534 nm. The obtained composite layers have been subjected to annealing by ruby laser (λ = 694 nm) in the spectral region on a descending long-wavelength wing of the plasmon absorption band of gold nanoparticles. Raman spectroscopy has been applied for the first time to the investigation of the modification of the shape of gold nanoparticles in LiF during laser annealing. The experimental Raman spectra are compared with calculated modes of in-phase bending vibrations generated in gold nanoparticles.     

Surface plasmons in porous gold films

The surface plasmon resonance effects in porous gold (por-Au) films—nanocomposite porous films containing an ensemble of disordered gold nanoparticles—have been investigated by modulation-polarization spectroscopy. Por-Au films have been obtained by pulsed laser deposition (using a direct particle flow from an erosion torch formed by a YAG:Nd³⁺ laser in argon). The spectral and angular dependences of the polarization difference ρ(λ, θ) of internal-reflection coefficients of s- and p-polarized radiation in the Kretschmann geometry and the spectral dependences of isotropic reflection angles at ρ(θ) = 0 are measured. Two types of surface plasmon resonance are found: one occurs on isolated nanoparticles (dipole and multipole modes), and the other is due to the dipole–dipole interaction of neighboring nanoparticles. The frequency of electron plasma oscillations for the nanoparticle ensemble and the frequencies and decay parameters of resonances are determined. Dispersion relations for the radiative and nonradiative modes are presented. The negative sign of the dispersion branch of nonradiative modes of dipole–dipole interaction is explained by the spatial dispersion of permittivity. The relationships between the formation conditions of the films, their structure, and established resonance parameters (determining the resonant-optical properties of films) are discussed.     

Plasma desorption mass spectroscopy of thiol-passivated gold nanoparticles

Plasma desorption mass spectrometry has been applied to characterization of dodecanthiol-passivated gold nanoparticles. An overview of the experimental set-up and mass analyses for the nanoparticles prepared in different conditions are shown. Mass distributions were found to shift to higher mass regions with increasing reaction temperature and reaction period. The results are consistent with those of transmission electron microscopy observations, UV-visible absorption spectra and also with a reported laser desorption mass spectrometry.     

Channelling surface plasmons

Channel plasmon polaritons (CPPs) propagating along the bottom of subwavelength grooves cut into a metal surface were recently shown to exhibit strong confinement combined with low propagation loss, a feature that makes this guiding configuration very promising for the realisation of ultracompact photonic components. Here, the results of our investigations of CPP guiding by V-grooves cut into gold are presented, demonstrating efficient large-angle bending and splitting of radiation as well as waveguide-ring resonators and Bragg grating filters. Additionally, we present a simple model based on the effective-index method that accounts for the main features of CPP guiding, thus, providing a clear physical picture of this phenomenon. PACS 73.20.Mf; 71.36.+c; 42.82.-m; 07.79.Fc     

局域表面等离激元

局域表面等离激元使得贵金属纳米颗粒具有丰富的光学性质,其应用涵盖能源、生物医学、安全、信息、超材料等诸多领域。文章简要介绍局域表面等离激元的基本性质、贵金属纳米结构中的局域表面等离激元共振耦合以及具有局域表面等离激元特性的贵金属纳米结构的一些重要应用。     

Near-field optical characterization of interacting and non-interacting gold nanoparticles embedded in a silica thin film

By near-field optics, we characterized the local optical properties of clusters of gold nanoparticles randomly distributed under a 50 nm-thick SiO₂ thin film. A local field enhancement is visible above isolated clusters. A few hundred nanometers away from them, we observed a polarization-dependent pattern with elliptical lobes oriented in the incident polarization direction. A simple simulation shows that the observed near-field images can be represented by the sum of the field of an oscillating dipole and the incident field. When the cluster density is larger, the measured near-field images show numerous bright and dark spots. The position of the bright spots does not necessarily coincide with the gold clusters showing the presence of coupling effects between them.     

Resonant surface plasmons of a metal nanosphere treated as propagating surface plasmons

On the assumption that the resonant surface plasmons on a spherical nanoparticle are formed by standing waves of two counter-propagating surface plasmon waves along the surface, by using Mie theory simulation, we find that the dispersions of surface plasmon resonant modes supported by silver nanospheres match with those of the surface plasmons on a semiinfinite medium-silver interface very well. This suggests that the resonant surface plasmons of a metal nanosphere can be treated as a propagating surface plasmon wave.     

Focusing surface plasmons on Er3+ ions through gold planar plasmonic lenses

Gold plasmonic lenses consisting of a planar concentric rings-groove with different periods were milled with a focused gallium ion beam on a gold thin film deposited onto an Er³⁺-doped tellurite glass. The plasmonic lenses were vertically illuminated with an argon ion laser highly focused by means of a 50× objective lens. The focusing mechanism of the plasmonic lenses is explained using a coherent interference model of surface plasmon-polariton (SPP) generation on the circular grating due to the incident field. As a result, phase modulation can be accomplished by the groove gap, similar to a nanoslit array with different widths. This focusing allows a high confinement of SPPs that can excite the Er³⁺ ions of the glass. The Er³⁺ luminescence spectra were measured in the far-field (500?C750?nm wavelength range), where we could verify the excitation yield via the plasmonic lens on the Er³⁺ ions. We analyze the influence of the geometrical parameters on the luminescence spectra. The variation of these parameters results in considerable changes of the luminescence spectra.