Orientation-Preserving Transfer and Directional Light Scattering from Individual Light-Bending Nanoparticles

A nanocup, or semishell, is an asymmetric plasmonic “Janus” nanoparticle with electric and magnetic plasmon modes; the latter scatters light in a direction controlled by nanoparticle orientation, making it the nanoscale analog of a parabolic antenna. Here we report a method for transferring nanocups from their growth substrate to oxide-terminated substrates that precisely preserves their three-dimensional orientation, enabling their use as nanophotonic components. This enables us to selectively excite and probe the electric and magnetic plasmon modes of individual nanocups, showing how the scattered light depends on the direction of incoming light and the orientation of this nanoparticle antenna

Reshaping the Plasmonic Properties of an Individual Nanoparticle

When symmetry is broken in plasmonic nanostructures, new optical properties emerge. Here we controllably reshape an individual Au nanoshell into a reduced-symmetry nanoegg, then a semishell or nanocup by a novel electron-beam-induced ablation method, transforming its plasmonic properties. We follow the changes in the plasmonic response at the single nanostructure level throughout this reshaping process, observing the splitting of plasmon modes and the onset of electroinductive plasmons upon controlled, incremental opening of the outer metallic layer of the nanoparticle

Reshaping the Plasmonic Properties of an Individual Nanoparticle

When symmetry is broken in plasmonic nanostructures, new optical properties emerge. Here we controllably reshape an individual Au nanoshell into a reduced-symmetry nanoegg, then a semishell or nanocup by a novel electron-beam-induced ablation method, transforming its plasmonic properties. We follow the changes in the plasmonic response at the single nanostructure level throughout this reshaping process, observing the splitting of plasmon modes and the onset of electroinductive plasmons upon controlled, incremental opening of the outer metallic layer of the nanoparticle

Heterodimers: Plasmonic Properties of Mismatched Nanoparticle Pairs

Heterodimerstwo closely adjacent metallic nanoparticles differing in size or shapeexemplify a simple nanoscale geometry that gives rise to a remarkably rich set of properties. These include Fano resonances, avoided crossing behavior, and a surprising dependence of the scattering spectrum on the direction of excitation, known as the “optical nanodiode” effect. In a series of studies, we experimentally probe and theoretically analyze these properties in heterodimer nanostructures, where nanoparticle size and plasmon resonance frequency are varied systematically. Polarization-dependent dark-field microspectroscopy on individual heterodimer structures fabricated using a novel electromigration assembly method allows us to examine these properties in detail. These studies expand our understanding of the range of physical effects that can be observed in adjacent metallic nanoparticle pairs

Fano Resonances in Plasmonic Nanoclusters: Geometrical and Chemical Tunability

Clusters of plasmonic nanoparticles and nanostructures support Fano resonances. Here we show that this spectral feature, produced by the interference between bright and dark modes of the nanoparticle cluster, is strongly dependent upon both geometry and local dielectric environment. This permits a highly sensitive tunability of the Fano dip in both wavelength and amplitude by varying cluster dimensions, geometry, and relative size of the individual nanocluster components. Plasmonic nanoclusters show an unprecedented sensitivity to dielectric environment with a local surface plasmon resonance figure of merit of 5.7, the highest yet reported for localized surface plasmon resonance sensing in a finite nanostructure

Nanoshells Made Easy: Improving Au Layer Growth on Nanoparticle Surfaces

The growth of a continuous, uniform Au layer on a dielectric nanoparticle is the critical step in the synthesis of nanoparticles such as nanoshells or nanorice, giving rise to their unique geometry-dependent plasmon resonant properties. Here, we report a novel, streamlined method for Au layer metallization on prepared nanoparticle surfaces using carbon monoxide as the reducing agent. This approach consistently yields plasmonic nanoparticles with highly regular shell layers and is immune to variations in precursor or reagent preparation. Single particle spectroscopy combined with scanning electron microscopy reveal that thinner, more uniform shell layers with correspondingly red-shifted optical resonances are achievable with this approach

A Plasmonic Fano Switch

Plasmonic clusters can support Fano resonances, where the line shape characteristics are controlled by cluster geometry. Here we show that clusters with a hemicircular central disk surrounded by a circular ring of closely spaced, coupled nanodisks yield Fano-like and non-Fano-like spectra for orthogonal incident polarization orientations. When this structure is incorporated into an uniquely broadband, liquid crystal device geometry, the entire Fano resonance spectrum can be switched on and off in a voltage-dependent manner. A reversible transition between the Fano-like and non-Fano-like spectra is induced by relatively low (∼6 V) applied voltages, resulting in a complete on/off switching of the transparency window

Plasmonic Nanoclusters: Near Field Properties of the Fano Resonance Interrogated with SERS

While the far field properties of Fano resonances are well-known, clusters of plasmonic nanoparticles also possess Fano resonances with unique and spatially complex near field properties. Here we examine the near field properties of individual Fano resonant plasmonic clusters using surface-enhanced Raman scattering (SERS) both from molecules distributed randomly on the structure and from dielectric nanoparticles deposited at specific locations within the cluster. Cluster size, geometry, and interparticle spacing all modify the near field properties of the Fano resonance. For molecules, the spatially dependent SERS response obtained from near field calculations correlates well with the relative SERS intensities observed for individual clusters and for specific Stokes modes of a <i>para</i>-mercaptoaniline adsorbate. In all cases, the largest SERS enhancement is found when both the excitation and the Stokes shifted wavelengths overlap the Fano resonances. In contrast, for SERS from carbon nanoparticles we find that the dielectric screening introduced by the nanoparticle can drastically redistribute the field enhancement associated with the Fano resonance and lead to a significantly modified SERS response compared to what would be anticipated from the bare nanocluster

Near-Normal Incidence Dark-Field Microscopy: Applications to Nanoplasmonic Spectroscopy

The spectroscopic characterization of individual nanostructures is of fundamental importance to understanding a broad range of physical and chemical processes. One general and powerful technique that addresses this aim is dark-field microscopy, with which the scattered light from an individual structure can be analyzed with minimal background noise. We present the spectroscopic analysis of individual plasmonic nanostructures using dark-field illumination with incidence nearly normal to the substrate. We show that, compared to large incidence angle approaches, the near-normal incidence approach provides significantly higher signal-to-background ratios and reduced retardation field effects. To demonstrate the utility of this technique, we characterize an individual chemically synthesized gold nanoshell and a lithographically defined heptamer exhibiting a pronounced Fano-like resonance. We show that the line shape of the latter strongly depends on the incidence angle. Near-normal incidence dark-field microscopy can be used to characterize a broad range of molecules and nanostructures and can be adapted to most microscopy setups

Close Encounters between Two Nanoshells

Plasmonic nanoparticle pairs known as “dimers” embody a simple system for generating intense nanoscale fields for surface enhanced spectroscopies and for developing an understanding of coupled plasmons. Individual nanoshell dimers in directly adjacent pairs and touching geometries show dramatically different plasmonic properties. At close distances, hybridized plasmon modes appear whose energies depend extremely sensitively on the presence of a small number of molecules in the interparticle junction. When touching, a new plasmon mode arising from charge transfer oscillations emerges. The extreme modification of the overall optical response due to minute changes in very reduced volumes opens up new approaches for ultrasensitive molecular sensing and spectroscopy