Helium and Gallium-ion beam milling in combination with a fast and reliable Sketch-and-peel technique is used to fabricate pairs of gold nanoantennas with a superior quality factor and with gap distances down to 6nm. The high fabrication quality compared to standard ion beam milling is proven by polarization-resolved dark-field spectro-microscopy of isolated nanoantennas pairs. We observe a strong coupling of the two antenna arms for both fabrication techniques, however with an outstanding quality factor for the Sketch-and-peel-produced antennas. The presented fabrication technique can pave the way to the production of plasmonic nanostructures on large spatial scales with high milling accuracy.
Metallic nanoantennas are able to spatially localize far-field electromagnetic waves on a few nanometer length scale in the form of surface plasmon excitations 1-3. Standard tools for fabricating bowtie and rod antennas with sub-20 nm feature sizes are Electron Beam Lithography or Ga-based Focused Ion Beam (FIB) Milling. These structures, however, often suffer from surface roughness and hence show only a limited optical polarization contrast and therefore a limited electric field localization. Here, we combine Ga- and He-ion based milling (HIM) for the fabrication of gold bowtie and rod antennas with gap sizes of less than 6 nm combined with a high aspect ratio. Using polarization-sensitive Third-Harmonic (TH) spectroscopy, we compare the nonlinear optical properties of single HIM-antennas with sub-6-nm gaps with those produced by standard Ga-based FIB. We find a pronounced enhancement of the total TH intensity of more than three in comparison to Ga-FIB antennas and a highly improved polarization contrast of the TH intensity of 250:1 for Heion produced antennas 4. These findings combined with Finite-Element Method calculations demonstrate a field enhancement of up to one hundred in the few-nanometer gap of the antenna. This makes He-ion beam milling a highly attractive and promising new tool for the fabrication of plasmonic nanoantennas with few-nanometer feature sizes.
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