Influence of surface roughness on the optical properties of plasmonic nanoparticles

Abstract

For plasmonic nanoparticles, we investigate the influence of surface roughness inherent to topdown fabrication on the optical properties, and find that it has a surprisingly small influence on the position and width of the plasmon peaks. Our experimental observation is supported by simulations based on the boundary element method approach. Using a perturbation approach, suitable for metallic nanoparticles with a moderate degree of surface roughness, we demonstrate that the reason for this lies in motional narrowing where the plasmon averages over the random height fluctuations. Surface roughness in large arrays of identical nanoparticles, such as encountered in the context of metamaterials, is thus expected to not constitute a major roadblock. PACS numbers: 73.20.Mf,78.67.Bf,03.50.De Plasmonics bridges the gap between the micrometer length scale of light and the length scale of nanostructures. 1 This is achieved by binding light to coherent charge density oscillations of metallic nanostructures, socalled surface plasmons, which allow to focus electromagnetic radiation down to spots with spatial dimensions of the order of a few nanometers. 2 Coupling of quantum emitters, such as quantum dots or molecules, with plasmonic nanostructures can strongly modify their excitation and emission properties, observable in fluorescence 3 or surface enhanced Raman scattering, 4 and offers a unique means for tailoring light-matter interaction at the nanoscale. This has found widespread applications ranging from (bio)sensors 5 and solar cells 6 to optical and quantum communication technology. 7 Plasmonic nanoparticles are also at the heart of the emerging fields of metamaterials and optical cloaking. 8 Huge advances in fabrication techniques over the last years allow nowadays to fabricate plasmonic nanostructures with well understood and predictable properties. Nevertheless, practically all metallic nanoparticles suffer from size inhomogeneities and nanoscale surface roughness, 9,10 which results in deviations of the plasmonic properties from those of idealized nanoparticles. 11 Particularly top-down approaches for nanoparticle fabrication often involve vacuum deposition of the metal structures, which leads to polycrystalline particles with an apparent surface roughness. 10 Despite the important role of surface roughness, there is still little understanding about the impact of such imperfection on the optical properties. Recent publications report on the control of nanoscale roughness and its strong effect on the nanoparticles farand near-field optical properties. 9,10,12 However, varying surface roughness is often accompanied by varying crystallinity, and therefore the results do not allow for a clear distinction between the contributions of the surface and the bulk.In this paper we provide evidence from experiment, theory, and simulation that a moderate amount of surface roughness has no significant impact on the far-field optical properties of metallic nanoparticles. We interpret this as a kind of motional narrowing, where ...

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