We have fabricated nanometer-scale gold dipole antennas designed to be resonant at optical frequencies. On resonance, strong field enhancement in the antenna feed gap leads to white-light supercontinuum generation. The antenna length at resonance is considerably shorter than one-half the wavelength of the incident light. This is in contradiction to classical antenna theory but in qualitative accordance with computer simulations that take into account the finite metallic conductivity at optical frequencies. Because optical antennas link propagating radiation and confined/enhanced optical fields, they should find applications in optical characterization, manipulation of nanostructures, and optical information processing.

The optical trapping of Au nanoparticles with dimensions as small as 10 nm in the gap of plasmonic dipole antennas is demonstrated. Single nanoparticle trapping events are recorded in real time by monitoring the Rayleigh scattering spectra of individual plasmonic antennas. Numerical simulations are also performed to interpret the experimental results, indicating the possibility to trap nanoparticles only a few nanometers in size. This work unveils the potential associated with the integration of plasmonic trapping with localized surface plasmon resonance based sensing techniques, in order to deliver analyte to specific, highly sensitive regions ("hot spots").

T he ability of metallic nanostructures to confine and enhance incident radiation offers unique possibilities for manipulating light at the nanoscale. These functionalities are enabled by the excitation of collective electron oscillations known as localized plasmon resonances. 1 When two or more nanostructures are placed next to each other, their plasmons can couple through near-field interactions and can give rise to a new set of hybridized collective plasmonic modes. 2À12 Plasmonic nanoclusters composed of three, 13 four, 8,14,15 seven particles, 9 and even larger aggregates 11,16 can also exhibit interference effects like Fano resonances 17À19 when the near-field coupling between each element is properly controlled. Fano resonances arise from the interference between superradiant and subradiant modes and produce extinction features with characteristic narrow and asymmetric line shapes. Because of their narrower spectral width compared to standard plasmon resonances and large induced field enhancements, Fano resonances have been used for a variety of applications including plasmonic rulers 20À22 and biosensors. 23À25 Despite a large and recent research effort, the design of plasmonic structures exhibiting Fano resonances at specific wavelengths is a challenging task because of their complex nature. A central issue in this design is the spectral engineering of the resonances via controlled hybridization of the available modes. However, this is difficult in systems where higher order modes are excited in the spectral range of interest 12,26,27 or when the modes are very complex and spatially extend over a large part of the nanostructure. 28,29 A small variation of the geometries, like what can occur during the nanofabrication process, can drastically change the resonance line shape and wavelength. This difficulty is particularly challenging when designing Fano resonant structures using spherical or disk shaped nanoparticles where the energies of the individual nanoparticle plasmons are similar and all hybridization and tuning must be accomplished by controlling the interparticle spacings.A more robust approach for Fano resonant systems is to engineer them from metallic nanorods that support highly tunable and polarization sensitive longitudinal

Predetermined and selective placement of nanoparticles onto large-area substrates with nanometre-scale precision is essential to harness the unique properties of nanoparticle assemblies, in particular for functional optical and electro-optical nanodevices. Unfortunately, such high spatial organization is currently beyond the reach of top-down nanofabrication techniques alone. Here, we demonstrate that topographic features comprising lithographed funnelled traps and auxiliary sidewalls on a solid substrate can deterministically direct the capillary assembly of Au nanorods to attain simultaneous control of position, orientation and interparticle distance at the nanometre level. We report up to 100% assembly yield over centimetre-scale substrates. We achieve this by optimizing the three sequential stages of capillary nanoparticle assembly: insertion of nanorods into the traps, resilience against the receding suspension front and drying of the residual solvent. Finally, using electron energy-loss spectroscopy we characterize the spectral response and near-field properties of spatially programmable Au nanorod dimers, highlighting the opportunities for precise tunability of the plasmonic modes in larger assemblies.

An ab initio theory for Fano resonances in plasmonic nanostructures and metamaterials is developed using the Feshbach formalism. It reveals the role played by the electromagnetic modes and material losses in the system, and enables the engineering of Fano resonances in arbitrary geometries. A general formula for the asymmetric resonance in a nonconservative system is derived. The influence of the electromagnetic interactions on the resonance line shape is discussed and it is shown that intrinsic losses drive the resonance contrast, while its width is determined mostly by the coupling strength between the nonradiative mode and the continuum. The analytical model is in perfect agreement with numerical simulations.