Optimal plasmonic focusing with radial polarization

Surface plasmon is collective oscillation of free electrons at metal/dielectric interface. As a wave phenomenon, surface plasmon can be focused using appropriate excitation geometry and metallic structures. The strong spatial confinement and high field enhancement make plasmonic lenses very attractive for near-field optical imaging and sensing in biological applications. In this paper, we show that optimal plasmonic focusing can be achieved through a combination of radially polarized illumination and axially symmetric dielectric/metal plasmonic lens structures. As examples, plasmonic lens with planar interface, conical shape and annular rings under radial polarization illumination are studied. The focusing properties and field enhancement effect of these plasmonic lenses are numerically studied with a finite-element-method model. The numerical simulation results show that the field distribution with a full-width-half-maximum of as small as 10 nm and intensity enhancement factor of five orders of magnitude can be achieved with 632.8 nm optical excitation.