Enhanced Plasmonic Field and Focusing for Ring-Shaped Nanostructures Via Radial Vector Beam

A ring-shaped plasmonic nanostructure is established for realizing the plasmonic field enhancement and focusing through a radial vector beam incidence. The plasmonic light field of the proposed nanostructure is simulated by finite-difference time-domain (FDTD). The results show that the obvious plasmonic field enhancement and focusing spot can be observed when a ring-shaped Au nanostructure is placed on the gold layer. Especially, the maximum of intensity ratio for single ring-shaped nanostructure can reach up to 41, the minimum of the full width at half maximum (FWHM) of focusing spot closes to 60 nm, and spot size beyond the diffraction limit is as small as 0.19λ0. Moreover, the plasmonic field enhancement and focusing can not be realized in the cases of linear polarization, circular polarization, and angular vector polarization incidences. What’s more, the double ring-shaped nanostructure can further develop the intensity ratio to 109 caused by the interference enhancement of surface plasmons. These research results may pave the way for designing the ultra-strong plasmonic field and plasmonic focusing devices.