Characteristics of light intensity enhancement of a silver nanoprism with rounded corners.

We have fabricated silver nanoprisms of 100-600 nm side length by focussed ion beam lithography and measured the light intensity scattering spectra using dark-field microscopy. Two resonance peaks due to localized surface plasmon excitation were observed in the spectra and their central frequency shown to depend on the prism size. The near-field electromagnetic intensity distribution with TE-polarized light at the vacuum wavelength of 632.8 nm was measured. We have obtained a much lower light intensity enhancement than previously numerically predicated. However, scattering spectra obtained numerically, taking into account roundness of the prism corners, agree well with experimental ones. At the same time, the numerically determined field distribution was different to the near-field intensity obtained experimentally. Our results suggest the particular shape of the corner region of the prism is a key factor for obtaining a large light intensity enhancement and shaping the local field distribution.