Scanning Near-Field Optical Microscope Study of Ag Nanoprotrusions Fabricated by Nano-oxidation with Atomic Force Microscope

We investigate light transmission of Ag nano protrusions with a scanning near-field optical microscope (SNOM) to study the surface plasmon (SP) in metallic films. A Ag nanoprotrusion whose height is ~20 nm is fabricated by the deposition of Ag on the Ti nano-protrusion template which is made by nano-oxidation with an atomic force microscope (AFM). It is found that through several scans, the tip is coated with Ag from the sample, and this modified tip shows apparent undulation patterns in SNOM imaging, while a fresh tip does not. The undulation pattern in the SNOM image is observed only along the direction of polarization light close to the expected undulation period. This result indicates that local launching of SP is effective with an Ag-coated tip and the SP launching characteristic is consistent with the so-called cos 2? SP distribution. By finite domain time difference (FDTD) simulations, it is confirmed that light transmission is enhanced in the Ag-coated tip compared to the fresh tip. Our results imply that local illumination with a noble-metal-coated cantilever probe tip is desirable for obtaining more detailed features in SNOM images involved with SP. Our results also indicate that a nano protrusion fabricated by AFM-based nano-oxidation can be applied to an optical element using surface plasmon.