No 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 ...

Waveguide chip-based microscopy reduces the complexity of total internal reflection fluorescence (TIRF) microscopy, and adds features like large field of view illumination, decoupling of illumination and collection path and easy multimodal imaging. However, for the technique to become widespread there is a need of low-loss and affordable waveguides made of high-refractive index material. Here, we develop and report a low-loss silicon nitride (Si 3 N 4 ) waveguide platform for multi-color TIRF microscopy. Single mode conditions at visible wavelengths (488-660 nm) were achieved using shallow rib geometry. To generate uniform excitation over appropriate dimensions waveguide bends were used to filter-out higher modes followed by adiabatic tapering. Si 3 N 4 material is finally shown to be biocompatible for growing and imaging living cells.

Noble metal nanoparticles show specific optical properties due to the excitation of localized surface plasmons that make them attractive candidates for highly sensitive bionanosensors. The underlying physical principle is either an analyte-induced modification of the dielectric properties of the medium surrounding the nanoparticle or an increase of the excitation and emission rates of an optically active analyte by the resonantly enhanced plasmon field. Either way, besides the nanoparticle geometry the dielectric properties of the metal and nanoscale surface roughness play an important role for the sensing performance. As the underlying principles are however not yet well understood, we aim here at an improved understanding by analyzing the optical characteristics of lithographically fabricated nanoparticles with different crystallinity and roughness parameters. We vary these parameters by thermal annealing and apply a thin gold film as a model system to retrieve modifications in the dielectric function. We investigate, on one hand, extinction spectra that reflect the far-field properties of the plasmonic excitation and, on the other hand, surfaceenhanced Raman spectra that serve as a near-field probe. Our results provide improved insight into localized surface plasmons and their application in bionanosensing.

In this article, we investigate the Surface-Enhanced Raman Scattering (SERS) efficiency of methylene blue (MB) molecules deposited on gold nanostripes which, due to their fabrication by electron beam lithography and thermal evaporation, present various degrees of crystallinity and nanoscale surface roughness (NSR). By comparing gold nanostructures with different degrees of roughness and crystallinity, we show that the NSR has a strong effect on the SERS intensity of MB probe molecules. In particular, the NSR features of the lithographic structures significantly enhance the Raman signal of MB molecules, even when the excitation wavelength lies far from the localized surface plasmon resonance (LSPR) of the stripes. These results are in very good agreement with numerical calculations of the SERS gain obtained using the discrete dipole approximation (DDA). The influence of NSR on the optical near-field response of lithographic structures thus appears crucial since they are widely used in the context of nano-optics or/and molecular sensing.