Axis-selective excitation of gold nanoparticle resonances

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. (C) 2014 Optical Society of America