Structural determinants of optical cavities in plasmonic nanogaps

Plasmonic nanostructure architectures are known to yield substantial surface-enhanced Raman scattering (SERS) enhancement due to optical confinement effect within an ultra-small volume of <1 nm³. Significant enhancement of electromagnetic field near the nanogap surface gives rise to the emergence of highly localized optical cavities. These optical cavities, formed within the nanogap under illumination of the nanoscale constructs, are pivotal for probing molecular-scale light-molecule interactions. One characteristic route for their formation involves light-induced reconstruction of surface structure within the nanogap, resulting in atomic-level confinement. Despite this, the fundamental mechanisms driving such optical cavity generation remain elusive, hindering a deeper mechanistic understanding. This knowledge gap is further complicated by the optical instability of metallic nanostructures, especially in nanoparticles where high surface-to-volume ratios exacerbate surface atom fluctuations. To better understand these effects, we engineer gold-based nanogaps with controlled dimensional variations and evaluate their optical stability. Through this approach, we identify key structural parameters influencing the robustness of optical responses in Au nanogaps across multiple scales.