Tunable composite nanoparticle for plasmonics

Abstract

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...

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