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Scanning Near-field Optical Microscopy (SNOM) is an essential tool in nano-optics and plasmonics. Among many variants of SNOM, a plasmonic tip is a new type of SNOM tip that is based on a resonant excitation and a superfocusing of a radially polarized conical surface plasmon polariton (SPP). The plasmonic tip is made of a tapered and fully metal-coated M-profiler fiber tips. An M-profile fiber guides the radially polarized fiber mode securely to the tapered region of the tip where it resonantly excites the radially polarized plasmon mode. This resonant excitation process allows us to have higher energy conversion efficiency that is up to 70% for 50 nm gold coating thickness from far-field to near-field than other SNOM tips like aperture tips (0.01% for 100 nm aperture). As the radially polarized plasmon mode further propagates towards the apex, its’ intensity increases anomalously, and its’ phase velocity decreases. Thus, the plasmon gets localized longitudinally as well as transversally due to the SPP nature. This phenomenon is known as a superfocusing of SPP, and in conical structure, it happens only for the fundamental radially polarized mode in the region where the tip radius is smaller than 50 nm. In this study, we introduce the plasmonic tip and explore the plasmon excitation process on a planar gold surface by plasmonic tips and circular aperture SNOM tips to understand the tip emission behavior in near-field. In the experiment, we use ring gratings that are milled on a planar gold surface and place a tip at the center of the structure to excite a planar SPP that propagates toward the grating and gets scattered. By imaging the scattered light through the grating, we study the plasmon excitation pattern and deduce the near-field at the apex. An emission through a small metal aperture (~10 nm) is well explained by Bethe theory that states that the near-field emission resembles that of a dipole. However, for an aperture tip with an aperture as large as 100 nm, we demonstrated that the dipole approximation well describes the excited SPP as long as a linearly polarized single mode is guided within the tip. When the aperture gets larger, the guided light within the tip becomes multimode; thus, the dipole approximation is no longer valid although the tip far-field emission looks like a Gaussian mode. For the plasmonic tip, we showed the emission can be approximated that of an out-of-plane dipole (oscillating perpendicular to the surface) despite the size of the apex. This method allows us also estimate the tilt of a tip with respect to the sample surface and purity of guided mode within the tip, and these information are essential for interpreting the detected signal from the sample. In conclusion, we introduce the plasmonic tip as an efficient SNOM tip due to its resonant excitation of SPP and superfocusing processes, and studied the near-field excitation characteristics in comparison with the conventional aperture tips.
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