Abstract:The interplay between localized surface plasmon (LSP) and surface plasmon-polariton (SPP) is studied in detail in a system composed of a three-dimensional gold particle located at a short distance from a gold thin film. Important frequency shifts of the LSP associated with the particle are observed for spacing distances between 0 and 50 nm. Beyond this distance the LSP and SPP resonances overlap, although some cavity effects between the particle and the film can still be observed. In particular, when the spacing increases the field in the cavity decreases more slowly than one would expect from a simple image dipole interpretation. For short separations the coupling between the particle and the film can produce a dramatic enhancement of the electromagnetic field in the space between them, where the electric field intensity can reach 5000 times that of the illumination field. Several movies show the spectral and time evolutions of the field distribution in the system both in and out of resonance. The character of the different modes excited in the system is studied. They include dipolar and quadrupolar modes, the latter exhibiting essentially a magnetic response.

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We present a numerical study of the tunability properties of a plasmonic subwavelength particle deposited on a metallic slab. The particle is composed of a metallic part, supporting a localized plasmon mode, separated from the slab by a dielectric spacer. It is shown that the position of the resonance wavelength can be modified over a large spectral range by changing either the spacer thickness by a few tens of nanometers or its susceptibility within the range of usual dielectrics. A linear relation is observed between the resonance wavelength and the spacer permittivity. © 2006 Optical Society of America OCIS codes: 130.3120, 220.4830, 240.6680, 260.3910. Plasmon resonances are electromagnetic modes associated with the excitation of collective oscillations of the electronic charge density in metals. They combine both high-field intensities and frequency selectivity and therefore could be used for new photonic devices integrated on very small surfaces. Two types of plasmon resonance exist: surface plasmon polariton (SPP) waves propagating along a metaldielectric interface and localized surface plasmon (LSP) modes, confined to subwavelength metallic objects. 1,2 Individual particles can be assembled into small or complex arrangements such as chains or gratings where complex plasmonic functions can be achieved. Furthermore, the strong optical fields generated in these systems can in turn be used to enhance specific optochemical effects, such as surface enhanced Raman scattering, to realize new integrated devices for chemical and biosensing.3,4 Moreover, the LSP of small metallic particles can be coupled to a metallic film in order to perform complex operations on propagating SPP waves. For example, the utilization of periodically patterned metallic surfaces has already been demonstrated for SPP guiding and manipulation at the nanoscale. 5-8As all these devices are strongly sensitive to the light frequency, it could be interesting to dispose tunable particles to modify their frequency range. In this Letter we present the tunability properties of a single composite particle, as illustrated in Fig. 1. This object can be easily realized experimentally by classical lithography technics. It consists of a two-layer rectangular particle. The upper layer is made of gold, which supports the LSP mode; the lower layer is made of dielectric material. The resonance properties of this composite particle, when coupled to a metallic film, are investigated numerically, and it is shown that varying the dielectric spacer characteristics allows one to easily change the resonance wavelength. Calculations were performed by the Green's tensor method.9,10 Throughout the paper we consider a 50 nm thick gold film deposited on a silica substrate with permittivity ⑀ = 2.25. The gold permittivity data are from Johnson and Christy.11 The structure is illuminated from underneath at normal incidence (electric field propagating along the z direction and polarized along the x direction).Let us consider first what happens when a rectangular meta...

Plasmonic film-nanoparticles coupled systems have had a renewed interest for the past 5 years both for the richness of the provided plasmonic modes and for their high technological potential. Many groups started to investigate the optical properties of film-nanoparticles coupled systems, as to whether the spacer layer thickness is tens of nanometers thick or goes down to a few nanometers or angstroms, even reaching contact. This article reviews the recent breakthroughs in the physical understanding of such coupled systems and the different systems where nanoparticles on top of the spacer layer are either isolated/random or form regular arrays. The potential for applications, especially as perfect absorbers or transmitters is also put into evidence.

We present a numerical study and analytical model of the optical near field diffracted in the vicinity of subwavelength grooves milled in silver surfaces. The Green's tensor approach permits the computation of the phase and amplitude dependence of the diffracted wave as a function of the groove geometry. It is shown that the field diffracted along the interface by the groove is equivalent to replacing the groove by an oscillating dipolar line source. An analytic expression is derived from the Green's function formalism, which reproduces well the asymptotic surface plasmon polariton ͑SPP͒ wave as well as the transient surface wave in the near zone close to the groove. The agreement between this model and the full simulation is very good, showing that the transient "near-zone" regime does not depend on the precise shape of the groove. Finally, it is shown that a composite diffractive evanescent wave model that includes the asymptotic SPP can describe the wavelength evolution in this transient near zone. Such a semianalytical model may be useful for the design and optimization of more elaborate photonic circuits, whose behavior in a large part will be controlled by surface waves.

We validate the nonspherical grafting arrangement of isotropically coated spherical nanoparticles as very recently proposed. We utilize localized surface plasmon resonance enhanced dynamic polarized and depolarized light scattering from Au nanoparticles, the spherical symmetry of which was revealed by single-particle dark-field spectroscopy. The same Au nanospheres are grafted with ligands of different chemistry and length. The wavelength dependent depolarization ratio and the two transport coefficients of these nanoparticles, obtained from the dynamic light scattering experiment, can only be reconciled with the TEM data, the single UV/vis extinction spectrum, and the dark-field spectroscopy experiments if their coating is described as asymmetric. Spatially anisotropic graft distribution on spherical nanoparticles impacts their assembly and understanding its origin will help control the structure and properties of polymer nanocomposites.

Metal-insulator-metal systems exhibit a rich underlying physics leading to a high degree of tunability of their spectral properties. We performed a systematic study on a metal-insulator-nanostructured metal system with a thin 6 nm dielectric spacer and showed how the nanoparticle sizes and excitation conditions lead to the tunability and coupling/decoupling of localized and delocalized plasmonic modes. We also experimentally evidenced a tunable Fano resonance in a broad spectral window 600 to 800 nm resulting from the interference of gap modes with white light broad band transmitted waves at the interface playing the role of the continuum. By varying the incident illumination angle shifts in the resonances give the possibility to couple or decouple the localized and delocalized modes and to induce a strong change of the asymmetric Fano profile. All these results were confirmed with a crossed comparison between experimental and theoretical measurements, confirming the nature of different modes. The high degree of control and tunability of this plasmonically rich system paves the way for designing and engineering of similar systems with numerous applications. In particular, sensing measurements were performed and a figure of merit of 3.8 was recorded ranking this sensor among the highest sensitive in this wavelength range.

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