Single Spot Near-field Super-Focusing Planar Plasmonic Lens

Abstract

Ring structure yields a near-field split focal spot when excited with linearly polarized light. A single focal spot can be achieved by manipulating the phase of surface plasmon polaritons (SPPs) with strategically designed geometric modifications to the ring structure. In this work, we propose a planar slit based plasmonic lens for in-plane single-spot super-focusing of a linearly polarized light by manipulating the phase of SPPs with the aid of the unconventional modification in a slit geometry. The single-spot super-focusing is achieved by the introduction of unconventional slits in the symmetric ring structure for compensating the phase mismatch between the SPPs counter propagating from two sides of the ring. To analyze the focusing mechanism and plasmonic properties of the lens, finite-difference time-domain simulations are performed. The diffraction limit is beaten by focusing the light to a sub-wavelength region of approximately one-seventieth of the wavelength of incident light. An array of lens configuration is fabricated to illustrate the feasibility of the design. The proposed structure has potential applications in nanosensing, nanoimaging and nanolithography, wherein closely populated focused spots are desired using linearly polarized excitation.