Fig. 1. Schematic of the device fabrication. a) A tapered fibre is manufactured from a singlemoded optical fibre; d is the taper tip diameter. b) First focused ion beam (FIB) milling.α is defined as the angle between the taper axis and the normal n to the cut. α represents the angle at which the light propagation constant along the newly formed surface matches the surface plasmon polariton (SPP) propagation constant. c) Chrome and gold coating. d) A second FIB milling generates the sub-wavelength window: h represents the distance between the cut and the coated fibre tip. E) Detail of the final device.

Fig. 2. Dependence of the resonance angle on the taper diameter for fiber tapers coated with gold. The inset shows a detail for taper diameters between 1 and 2μm.

Fig. 3. Samples realized by focused ion beam (FIB) milling. The bright and the dark areas represent gold coated and bare silica regions, respectively.

Fig. 4. Schematic of the set-up used to measure the prototype transmission efficiency: a supercontinuum source is coupled through a xyz stage to a modal filter, necessary to have single moded propagation along all the range of wavelengths considered. The filter is spliced to the sample and the transmitted light is collected by an OSA using a multimodal fiber.

Fig. 5. Spectra of the supercontinuum source with the microwire mode filter (a), and with the sample in dry (b) and humid (c) environments. Spectra (b) and (c) have been normalised with respect to (a) and are reported in red and blue in (d).

Fig. 6. (a) Geometrical structure of an equivalent waveguide used for simulations. Light is emitted only through the white (uncovered) section. (b) Simulated transmission from the uncovered taper window because of simple geometrical considerations.