Fabrication of bow-tie antennas with mechanically tunable gap sizes below 5 nm for single-molecule emission and Raman scattering

Raman spectroscopy is a powerful tool to unambiguously identify chemical compounds by their unique vibrational finger-print. The Raman cross-section of a single molecule, however, is extremely small and intense laser sources, long integration times and high concentrations of analytes are typical prerequisites for Raman spectroscopy. A common route to alleviate these drawbacks is the use of surface plasmon polaritons (SPP) to guide and concentrate light in constrained geometries such as nanometer-sized gaps. The resulting local electric field and chemical enhancements are sufficient to perform surface-enhanced Raman spectroscopy (SERS) down to the single-molecule level. Here, we demonstrate advances in nanolithography which enable reproducible fabrication of optical antennas for the visible range with sub-5 nm features on large and wavy substrates. A material-independent process route is described that employs cutting-edge electron-beam lithography (EBL) operated in ultra-silent laboratories to enable undisturbed lithography. Through the use of a reactive ion etching step, free-standing antennas with pillar heights of up to 250 nm have been fabricated. These antennas show a three-fold increase in luminescence intensity and spatial confinement in comparison to antennas in direct contact with the substrate. This underlines their great potential for optical direct-sensing experiments targeting few to single molecules in the feed-gap region through SERS.