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A spiral plasmonic lens can focus circular polarization of a given handedness while simultaneously defocus the circular polarization of the opposite chirality, which may be used as a miniature circular polarization analyzer. In this letter, we experimentally investigated the plasmonic focusing properties of the spiral lens using a collection mode near-field scanning optical microscope. A single Archimedes' spiral slot with a single turn was etched through gold thin film as a spiral plasmonic lens. The plasmonic field at the focus of a spiral lens strongly depends on the spin of the incident photon. Circular polarization extinction ratio better than 50 is obtainable with a device size as small as only 4 times of surface plasmon wavelength.

Optimal plasmonic focusing can be achieved through matching the rotational symmetry of the plasmonic lens to the polarization symmetry of a radially polarized illumination. In this letter, we report the experimental confirmation of the focusing properties and field enhancement effect of plasmonic lens made of multiple concentric annular rings using a collection mode near field scanning optical microscope. Surface plasmons excited at all azimuthal directions propagate toward the geometric center and constructively interfere at the focus to create a strongly enhanced evanescent optical "needle" field that is substantially polarized vertically to the plasmonic lens surface. The field enhancement factor can be improved through adding more rings while maintaining the plasmonic focal spot size. Strategy for optimizing the field enhancement factor is studied with both analytical and numerical methods.Surface plasmons are collective oscillations of free electrons that can be excited by transverse-magnetic (TM) polarized light at dielectric/metal interface. The surface plasmon waves are associated with shorter effective wavelengths and strong field enhancement effects, making them very attractive for a variety of applications. Because of its shorter effective wavelength, surface plasmon wave can be focused into a highly confined spot with size beyond the diffraction limit, finding applications in many areas such as subwavelength optics, 1,2 super-resolution imaging, 3-5 nanolithography, 6 high harmonic generation, 7 waveguiding, 8 near-field imaging and sensing. 9 The challenges these applications face are the manipulation of the spot size, shape, and strength of the surface plasmon field through designing appropriate optical excitation geometry and plasmonic structures. Plasmon focusing with linearly polarized illumination usually resulted in a minimum longitudinal field at the geometric focus due to destructive interference between counter-propagating surface plasmon waves and an inhomogeneous plasmon focal spot owing to the symmetry mismatch between the incident polarization and the plasmonic structures.