Merging individual metal nanostructures into a superstructure for plasmon mode hybridization and electric-field nanofocusing

Geometry design based plasmon hybridization represents an efficient means for pursuing desired spectral response and near-field enhancement in hybrid structures. Here, we propose to merge individual metal nanostructures into a superstructure to simultaneously realize plasmon mode inheritance and hybridization and electric near-field nanofocusing. As a model demonstration, we combine a metal nanoring supporting two localized surface plasmon resonances (LSPRs) and a metal nanocone sustaining one LSPR and one propagating surface plasmon polaritons (SPPs) mode into a hollow nanocone superstructure, and show numerically that such superstructure exhibits multiple plasmon resonance bands in the visible and near-infrared range and efficient nanofocusing of electric near-fields to the nanocone open apex. We further fabricate the designed superstructure with an ion-track membrane template method that allows for flexible control over relevant structural parameters by varying track etching time. Dark-field scattering measurements on single hollow nanocones confirm the presence of multiple plasmon resonances; surface-enhanced Raman spectroscopy further corroborates that the nanofocusing of electric fields plays a critical role in signal enhancement. We believe that such multi-resonant plasmonic superstructures with efficient near-field nanofocusing capability can find great potential in fundamental nonlinear optics and hot-carrier science studies and practical applications in surface-enhanced spectroscopies and broadband solar light harvesting.