Plasmonic heptamer-arranged nanoholes in a gold film on the end-facet of a photonic crystal fiber.

We use the end-facet of a solid-core polarization-maintaining photonic crystal fiber (PM-PCF) as a platform on which to fabricate resonant plasmonic nanostructures. Solid-core PM-PCFs can be excited in a polarization-aligned single mode by supercontinuum light, so they are well-suited to the wavelength-interrogation of resonant plasmonic nanostructures, especially supporting complex spectra over a broad spectral range. The nanostructures implemented consist of an array of heptamer-arranged nanoholes formed in a thin Au film. The nanoholes were milled with a focused ion beam, with the array polarization-aligned in situ to cover the solid core of the PM-PCF. Transmittance spectra, measured using a supercontinuum source coupled to the input of the PM-PCF, reveal a rich set of Fano resonances associated with localized and propagating surface plasmons. The measured spectra are compared to computations in order to identify the resonant modes. The spectra redshift as the medium covering the nanoholes changes from air to oil, anticipating application to sensing.