Ultrafast third-harmonic spectroscopy of single nanoantennas fabricated using helium-ion beam lithography

Abstract

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.

Helium-ion-beam nanofabrication: extreme processes and applications

“…Due to the high-resolution fabrication capabilities of HIB, for the same design geometry, the quality factor of the fabricated coaxial antennae is higher than that of gallium FIB direct written resonators, as shown in figure 15(A) [104]. Figure 15(B) shows a 6 nm gap on the gold bowtie antennas, which were milled by HIB direct writing to investigate the quantum optical properties in a near-field plasmon nanoantenna with the third harmonic spectroscopy [23,105]. Combined with electron lithography, HIB direct writing can fabricate the smallest size parts at low milling rates.…”