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Metallic nanorod antennas can be considered as an analogue to classical half-wave dipole antennas, constituting an important tool for manipulating linear and nonlinear light-matter interactions in nanoscale volumes. Using two-photon luminescence (TPL) scanning laser microscopy, we investigate such optical antennas beyond their fundamental dipole mode. The antenna mode dispersion is extracted from the nonlinear TPL measurement and reveals a TPL process that is dominated by plasmon-induced enhancement of the two-photon absorption in the metal. Additionally, a clear signature of the mode parity is observed in the TPL images. TPL maxima are observed outside the antenna boundaries for even parity modes, whereas they are located inside for odd modes. It is concluded that for even modes the two-photon luminescence emission is strongly mediated by retardation of the excitation field, a consequence of their zero net-dipole moment. This selective excitation of different mode parities is highly relevant for nanoscale enhanced nonlinear optics, as well as plasmonic nanosensor applications and tuning of radiative properties of quantum emitters.7 page(s

Nanoscale topographical control of capillary assembly of nanoparticles

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