Enhancing local electric fields at plasmonic nanogaps by optimal dielectric coatings

Abstract

Due to the generation of electromagnetic hot spots, metallic nanogaps have attracted great attention in various subjects, especially in plasmonics. For fixed nanogaps, using dielectric coating is an efficient way to narrow the gaps while improving their thermal and chemical stability. Herein, it is numerically demonstrated that the utilization of high refractive index dielectric coatings with zero or little loss, like TiO2, can lead to the generation of remarkable hot spots at the nanogaps. Two mechanisms work synergistically including the improved light coupling effect and a great leap of local electric field intensities at the dielectric-air interface. For higher refractive index dielectric coatings in practice, like graphene, despite a greater electric field leap at the interface, the weakening light coupling effect turns to play a dominant and negative role owing to the lossy dielectric properties. Our results provide a new perspective for the stabilization and functionalization of metallic nanogaps, which should be of significant importance in extended applications of surface enhanced spectroscopies as well as the fabrication, integration and packaging of plasmonic nanogap devices in the near future.