Selective Narrowing of the Bonding Modes of Plasmonic Nanoantennas

Plasmonic bowtie-structured nanocavities are valuable structures for applications due to the large confinement of light at their center and the associated large enhancement of the electric field. Plasmonic bowties suffer, though, from broad spectral lines that limit their interaction with quantum emitters. We report here a significant narrowing of the longitudinal plasmon resonance that couples the two prisms of silver bowties, induced by the deposition of a thin (1 nm) Ti adhesion layer during fabrication. The quality factor of the longitudinal mode of many of these bowties is much larger than that of bowties without an adhesion layer, even while their transverse mode remains unmodified. Cross-sectional electron microscopy analysis reveals that the Ti adhesion layer is fully oxidized. Further, and surprisingly, the TiO₂ adhesion layer is found to cause the bowties to partially embed in the silica substrate, thereby increasing the effective dielectric coefficient of their environment. Analysis of the results shows that the increase in the quality factor might be partially attributed to a reduction in radiative damping. The ability to obtain plasmon excitations of bowties with large quality factors is particularly attractive for multiple applications involving optical coupling to quantum emitters.