Subwavelength beaming using depth-tuned annular nanostructures

摘要

A subwavelength aperture surrounded by periodic grooves can give rise to enhanced transmission and directional beaming and the groove depths can be tuned to control the focusing length, but the one-dimensional structure is still restricted to certain applications where a symmetric focus spot is needed. In this paper, we study the nano-beaming effect using a central circular hole surrounded by annular grooves with different depth tuning or depth. We found two plasmonic modes at wavelengths 440 and 560 nm and there is no mode shift due to a different amount of depth tuning. Of the modes, the former is suitable for collimation and the other for focusing. Particularly good subwavelength collimation is obtained, e.g. FWHM = 0.85λalong the lateral direction and FWHM = 1.1X along the transverse direction at ) =440 nm, over the propagation distance z= 1.75 ism (= 4y.). Hence, we propose a new plasmonic lens using depth-tuned annular structures.