Excitation and superfocusing of the radially polarized conical surface plasmon polaritons
With our analytical study, we showed the different characteristics of the modes in the tapered region of the fiber based scanning near-field optical (SNOM) tips and their field evolution during the propagation to understand the functionality of the tips. Compared with other types of fiber based SNOM tips, the plasmonic tip, which is a fully metal coated tapered vortex fiber tip, provides an improvement in the field localization at the tip apex and in the conversion efficiency from the far- to near-fields (~ 70%) owing to the superfocusing effect and the resonant coupling process. The superfocusing effect enables the confinement of fields longitudinally and transversally and the enhancement of field amplitude resulting in a strong and highly confined field at the tip apex. We fabricated plasmonic tips by using the vortex fiber, which has double ring cores, and explored experimentally the near-field excitation and detection characteristics of the plasmonic tips. By selectively exciting only the radially polarized fiber mode and analyzing the far-field emission images of the tip apex, we showed that the fundamental radially polarized plasmonic mode, that has the superfocusing capability, is successfully excited. We also probed the plasmonic tips apex near-field with an annular grating on a planar gold surface. The results showed that the plasmonic tip excites planar SPPs to all radial directions indicating that the field at the tip apex is longitudinally oscillating along the tip axis. Due to this longitudinal field at the apex, the plasmonic tip also detects the longitudinal field in transmission that has been demonstrated by scanning over tightly focused linearly and radially polarized beams. Lastly, we measured the near-field of fluorescent beads with the plasmonic tip and demonstrated that the tip and the longitudinal plasmonic field at its apex act back on the beads improving the absorption of the fluorescent beads and thus the fluorescence emission.
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