Multiple surface plasmon resonances and near-infrared field enhancement of gold nanowells.

Arrays of Au nanowells (NWs) were fabricated by electron-beam lithography (EBL) and characterized by surface plasmon resonance (SPR) and surface-enhanced Raman scattering (SERS). It is revealed that these Au NW arrays exhibit multiple SP resonances that can be tuned by adjusting the geometrical characteristics of the NWs. SERS activity of Au NWs was confirmed for a range of excitation wavelengths and a number of model compounds including rhodamine 6G (R6G), phthalazine, and single-stranded oligonucleotides. According to numerical simulations based on the discrete dipole approximation (DDA), SERS enhancement originates from high electromagnetic fields (hot spots) localized both inside and outside individual NWs. In addition, far-field intercoupling effects between NWs have been observed experimentally in arrays with subwavelength pitch sizes. We show that the SERS enhancement factors can also be tuned and optimized by adjusting the geometry of NWs.