We present an experimental and theoretical study of plasmonic modes in high aspect ratio nanostructures in the visible wavelength region and demonstrate their high performance for sensing applications. Ordered and well-defined plasmonic structures with various cross-sectional profiles and heights are obtained using a top-down fabrication process. We show that, compared to cylindrical nanorods, structures with split-ring resonator-like cross sections have great potential for powerful sensing due to a pronounced polarization dependence, strong field enhancement, structural tunability, and improved mechanical stability. The plasmonic structures under study exhibit high sensitivities, up to nearly 600 nm/RIU, and figures of merit above 20.

Via experiments and an intuitive model, this study reports on the dependence of different dipole resonances in gold nanoparticles (NPs) with rectangular, elliptical and diamondlike footprints on the length of the three principal axes and on particle geometry. The length of the two in-plane principal axes of the NPs studied is between 50 and 300 nm, while the particle height is between 10 and 50 nm. The scattering experiments reveal characteristic dependencies of the in-plane dipolar resonances on axis lengths and particle geometry. These experimental findings are discussed according to an intuitive resonance condition based on the electrostatic eigenmode method extended to include terms for retardation (dynamic depolarization) and particle curvature. Focus is laid on the dependence of the retardation term on the relevant depolarization factor, the contributions of static and dynamic depolarization to the resonance wavelength, and to a generalization of the depolarization factors that describe the static depolarization in order to account for the differences in the dipolar surface modes of NPs with different geometries. The result of these studies is a detailed description and improved understanding of the scaling behavior of individual dipolar NP plasmon resonances.

We report on a simple and high-throughput method for the fabrication of gold and silver nanoparticle dimers with extremely small gaps and extending over large areas, using colloidal lithography and shadow thermal-evaporation techniques. The plasmon resonances of the individual dimers with various structural parameters were studied, as well as their coupling phenomena in the nearly touching regime. The method presented enables fabrication of efficient and low-cost plasmonics structures for applications such as surface-enhanced Raman scattering, plasmon-enhanced fluorescence, and biosensing.

Designing the optical response of metallic nanoparticles (NPs) is crucial for applications based on localized surface plasmons. In order to gain insight into the optical properties of metallic NPs and their experimental control, a well-defined, highly flexible excitation of single NPs is needed. In this study, we report on the realization of a measurement setup that allows 3D axis-selective excitation of single NPs with high accuracy, facilitating the study of the resulting scattering cross sections in the visible wavelength range. The experimental setup combines the concepts of dark-field optical microscopy and spectroscopy and objective-type total internal reflection fluorescence microscopy. Its functionality is validated by the study of nanocylinders with elliptical footprints. A retardation-influenced size range of moderate aspect ratios is addressed by choosing axis lengths between 70 and 200 nm. We experimentally separate three different polarization states of dipolar character, one for an electric field along each of the three principal axes, and show that the overall scattering spectrum of such a nanocylinder follows the expected superposition principle. Based on this separation, the scaling of the three resonances with the length of the principal axes is analyzed. It is found that each resonance scales exclusively with the corresponding principal axis in a linear way with slopes of the order of 1 to 2. Reasons for this scaling behavior and for its limited validity are discussed in terms of particle depolarization in the quasi-static limit and retardation.

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