Tailoring Electromagnetic Hot Spots toward Visible Frequencies in Ultra-Narrow Gap Al/Al2O3 Bowtie Nanoantennas

Plasmonic bowtie nanoantennas are intriguing nanostructures, capable to achieve very high local electromagnetic (EM) field confinement and enhancement in the hot spots. This effect is strongly dependent on the gap size, which in turn is related to technological limitations. Ultranarrow gap bowtie nanoantennas, operating at visible frequencies, can be of great impact in biosensing applications and in the study of strong light-matter interactions with organic molecules. Here, we present a comprehensive study on the structural and optical properties of aluminum bowties, realized with ultranarrow gap by He+-ion milling lithography, and operating from the near-infrared to the red part of the visible range. Most importantly, this analysis demonstrates that large EM near-field enhancement and different hot spot spatial positions, as a function of nanometer-sized gaps, are constrained by the native aluminum oxide, thus, working as hot spot ruler.