Revealing local, enhanced optical field characteristics of Au nanoparticle arrays with 10 nm gap using scattering-type scanning near-field optical microscopy.

Anomalous optical properties displayed by plasmonic structures are commonly attributed to the enhanced, local field within their corrugations. Though theoretical calculations of such field enhancements abound, experimental observations are relatively few, because only few optical microscopic techniques have enough spatial resolution. We used scattering-type scanning near-field optical microscopy to resolve local optical characteristics of a gold nanoparticle array with 10 nm gap between adjacent particles. Subnanometer-resolution measurement of the optical field intensity was achieved by use of etched silicon atomic force microscopy probe tip. The result shows that, with a p-polarized excitation scheme, the induced field is enhanced and the phase undergoes a large change in the gap region. The spatially-resolved signals are attributed to the electromagnetic interaction within an array of vertical dipoles. We show that scattering-type near-field optical microscopy is well-suited to the investigation of field enhancements in plasmon-enhanced sensing and spectroscopy array structures.