Nanoscale optical field localization by resonantly focused plasmons

Nanoscale field confinement enabled by plasmonic phenonena [1] has great potential to revolutionize many applications in nanophotonics, including bio-sensing, imaging, and magnetic recording. Various schemes using propagating surface plasmon polariton (SPP) waves have been suggested [2-4], but they experience power losses due to SPP propagation, restricting their practical applications. Here we experimentally demonstrate use of plasmonic resonant phenomena combined with strong field localization to enhance efficiency of confining optical fields. Our approach utilizes a nano-crescent-moon (NCM) shape plasmonic element integrated with a lossless Si waveguide utilized with silicon-on-insulator (SOI) technology, to achieve a sub-diffraction limited focusing with spot size ~25 nm. The metallic NCM effectively converts an incoming propagating waveguide mode into a localized resonant plasmon mode, which is localized in an ultrasmall volume in all 3 dimensions. The novel NCM geometry for efficient field localization simultaneously uses three physical mechanisms: localized surface surface plasmons (LSPs) [5,6], SPP edge localization [4], and TEM field localization [7]. The near-field optical measurements of the fabricated NCM using heterodyne near-field scanning optical microscope (H-NSOM) validate the theoretical predictions showing strong field localization.