Direct imaging of surface plasmon excitation with radially polarized beam

Abstract

When a radially polarized beam is focused onto a metal-dielectric interface, the entire beam is TM polarized with respect to the interface. Consequently surface plasmons can be excited at all directions. These surface plasmons will propagate to the geometric center, constructively interfere with each other and generate a strongly focused evanescent nondiffracting Bessel beam. In this paper, we report the experimental results on the direct imaging of such plasmonic focusing. Radially polarized beam is tightly focused onto a silver-glass interface with a high numerical aperture oil immersion objective lens. The intensity distribution at the back focal plane of the objective lens after reflection is captured with a CCD camera. A dark ring corresponding to surface plasmon resonance excitation by a focused radially polarized beam is observed. A collection mode near field scanning optical microscope is applied to map the two-dimensional intensity distributions at different distances from the sample to verify the non-spreading and decaying natures of the evanescent Bessel beam.