Excitation of short-range surface-plasmon polaritons in a gold nanowire enhanced step-index fiber

Abstract

Surface plasmon polaritons (SPPs) enable extreme device miniaturization via deeply sub-wavelength electromagnetic field confinements [1]. Conventional coupling techniques typically require phase matching or mode-matching, which is challenging at sub-wavelength dimensions. Here, we introduce and experimentally demonstrate a novel near-field hybrid fiber [2, 3] nanoprobe for the efficient excitation of short-range (SR) SPPs on a metallic nanowire which has a sub-10 nm nanotip at its end. A device schematic is shown in Fig. 1(a): a gold nanowire (NW) (radius: ∼300 nm, length: 80 μm) is incorporated into a (few-mode) step-index fiber with a central nanochannel, and protrudes from the endface forming a nanotip (Fig. 1(a) inset). Finite element calculations (Fig. 1(b)) show that a doughnut-shape mode is transmitted, but only a radially polarized (RP) mode induces an accumulation of energy at the apex of the nanotip via a hybrid mode, which contains energy on the wire surface. The experimental characterization relies on launching broadband azimuthally polarized (AP) [3] or RP (500–750 nm) light into the sample and analysing the far-field scattered and transmitted power. Figure 1(c) shows a typical side-scattered image (perpendicular to the fiber axis), showing the SR-SPPs excited on the tip. For an RP input mode the scattered intensity is maximal, and for an azimuthally polarized (AP) input it is mininal, with the transmitted power being independent on the input polarization. We analysed the scattered power as a function of the angle of a linear polarizer (LP) inserted between sample and camera (Fig. 1(e)) for a RP input, and observed a clear maximum when the LP is parallel to the fiber axis, confirming a longitudinally polarization state of the light emitted by the nanotip, characteristic of a plasmonic mode on a nanotip. This hybrid plasmonic fiber device can both deliver and collect broadband subwavelength radiation to and from the apex of a nanotip; in collection mode, light would efficiently couple from the plasmonic mode into the dielectric core. This device will find applications in near-field microscopy and nanophotonics, and can potentially offer significantly improved resolution compared to current SNOM tips [5].