Plasmonic Lens Made of Multiple Concentric Metallic Rings under Radially Polarized Illumination

Abstract

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.

Single-layer metal nanolenses with tight foci in far-field

“…Figure 4 shows electric energy densities in focal regions of both lenses. A characteristic separation of foci from the metal surface can be seen, which is very different from plasmonic needles observed in other works [12].…”

“…At the same time, ways of fabricating extremely smooth patterned metal layers for plasmonics were sought [11]. In a recent paper by Chen et al [12], a plasmonic lens made of concentric silver rings with on-axis stop and illuminated with radially polarized light is experimentally proven to concentrate evanescent fields into a narrow needle. The needle results from propagation of generated plasmons toward the lens axis and their constructive interference, which results in strong enhancement of polaritons over the on-axis stop.…”

“…Diameters of focal spots in the far-field depend on the use of linearly [17,18] or radially polarized [12,19] illumination. Use of a central aperture is beneficial for generating collimated beams from incident, e.g.…”

Enhancing resonances of optical nanoantennas by circular gratings

“…to receive locally light. Note this is different from the previous study on propagating SPPs in large circular metallic plates [12][13][14].…”

Light manipulation with encoded plasmonic nanostructures

“…Therefore, it is possible to fully control the SPP A fundamental function of a plasmonic device is focusing in-plane SPPs into a tight focal spot. This has been successfully realized by using various curved nanoslits that were fabricated on metal film [71][72][73][74][75][76]. The curved nanoslit enables focusing SPPs well into one focal spot.…”