Collective Resonances in Gold Nanoparticle Arrays

Abstract

We present experimental evidence of sharp spectral features in the optical response of 2D arrays of gold nanorods. A simple coupled dipole model is used to describe the main features of the observed spectral line shape. The resonance involves an interplay between the excitation of plasmons localized on the particles and diffraction resulting from the scattering by the periodic arrangement of these particles. We investigate this interplay by varying the particle size, aspect ratio, and interparticle spacing, and observe the effect on the position, width, and intensity of the sharp spectral feature. DOI: 10.1103/PhysRevLett.101.143902 PACS numbers: 42.25.Fx, 73.20.Mf Nanoparticles of noble metals have been the subject of many detailed studies due to their unique optical properties [1], in particular because they can support localized surface plasmon resonances (LSPR) [2]. The LSPR associated with metal nanoparticles exhibit a high degree of optical field confinement [3,4], together with a high sensitivity to their local environment. Where two or more metallic nanostructures are in close proximity, the possibility exists for interaction between the modes of the individual nanostructures to form new hybrid modes [5]. The case of two interacting particles has been extensively studied (for example, see [6]). For multiple nanostructures there is also the possibility of coherent interaction arising from multiple scattering. Light that is scattered so as to propagate in the plane of the particles will undergo multiple scattering by the regularly spaced particles. A geometric resonance arises when the wavelength of the scattered light is commensurate with the periodicity of the array, which, when it occurs in the same spectral range as the LSPR, may lead to a dramatic modification of the measured optical extinction. It appears that this effect was first predicted by Carron et al. (see, for example,[7]) and Markel [8,9] and more recently followed up by Zou et al. [10]. Further theoretical or computational work by these groups has extended our understanding [8][9][10][11][12][13], and a tutorial review linking these concepts to those associated with hole arrays has recently been given [14].Experiments to confirm the existence of these sharp diffractive features in the optical response of metallic nanoparticle arrays have met with only limited success. Haynes et al. [18] performed detailed studies of arrays of gold and silver nanoparticles, but the effect was not as pronounced as expected from the modeling. In each case, failure to observe the sharp spectral features appears to be due to one or more of the following factors: lack of an homogeneous environment, an angle spread of the illumination, an inappropriate choice of the particle volume and aspect ratio. Félidj et al. [19] reported sharp features in a system consisting of a regular array of gold nanorods supported on a thin indium tin oxide (ITO) layer. However, the presence of the ITO layer complicates the analysis and makes the underlying physics harder to unrav...

Surface modes in plasmonic crystals induced by diffractive coupling of nanoantennas

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“…An efficient diffractive coupling between nanoantennas is obtained when the array is embedded in a homogeneous dielectric environment. [13][14][15] Therefore, we put an amorphous quartz upperstrate on top of the antenna array. Good optical contact between substrate and upperstrate was obtained with a thin layer of refractive index matching liquid.…”

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“…21 These samples are most commonly manufactured on a substrate of high refractive index compared to the upper medium ͑typically air or water for biosensing applications͒. It has been suggested from experimental data 10 and theoretical modeling 22 that such an asymmetric configuration is incompatible with the existence of delocalized surface modes and therefore prevents the observation of efficient narrowing of the localized surface-plasmon line shapes. This is in contrast to other studies of fluorescence in particle arrays.…”

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