Merging the convenience of optical fibers with the extreme confinement of nano-plasmonics, would enable a number of attractive applications including nano-scale imaging, quantum communication, and nano-surgery. Typically, however, interfacing fibers with nano-focused surface plasmons is challenging, due to the difference in their modal profiles and polarizations. Furthermore, fabrication of nanostructures on fibers is often challenging, requiring several complicated steps and nanoscale alignment. Here Minn et al. fabricate and characterize a nano-focusing plasmonic waveguide at the end-face of a commercially available endlessly single mode photonic crystal fiber. Their fabrication technique relies on electron-beam induced metal deposition: the authors identify the exact location where they want to position their nano-plasmonic waveguides and, layer-by-layer, produce a plasmonic nanowire with a tip apex as small as 14 nm radius protruding from the fiber endface. Interestingly, the alignment is facilitated by the presence of the air holes in the photonic crystal fiber cladding, which allows them to position the antenna precisely at the desired location. The authors experimentally couple to the radially polarized mode of the surface plasmon from the linearly polarized fiber mode by “masking” half of the fiber at output; the authors calculate that this results in an overall efficiency of ~2% and a 136-fold field enhancement - comparable to or better than existing near-field probes. These results have the potential to make nano-plasmonics devices with focussing properties significantly more convenient.

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