Selective excitation of guided surface plasmons on uniform and conicallytapered Au nanowires

We report the precise excitation of guided surface plasmon modes (SPMs) on Au nanowires placed adjacent to a single-mode fibre core (20 mol% Ge, diameter 1.1 µm). The nanowires were produced by pressure-assisted melt-filling of a hollow channel (diameter 510 nm) running parallel to the core with a centre-centre core-wire spacing of 3.6 µm [1, 2]. An unintended byproduct of the fabrication technique was frequent breaks a few µm wide along the nanowires, every 100 µm or so. As a result, when broad-band light was launched into the core, bright red-coloured scattering spots could be seen at the end of each nanowire (Fig. 1(a)). These are caused by radiation from an m = 2 SPM on the wire [2], excited by phase-matched coupling at a particular resonant wavelength. The coupling between core and nanowire is, however, unbalanced because the loss on the nanowire is many orders of magnitude higher than in the glass core, and greater than the coupling constant (estimated value: 3.5 mm −1 ). As a result the supermodes are highly asymmetrical, one of them (with field predominantly in the wire) having extremely high loss and the other (field mainly in the core) very low loss. The scattered light from the spots was collected using a multimode fibre and delivered to an optical spectrum analyzer. The data was then used to work out the attenuation rate of the low-loss supermode (Figs. 1(b)&(c)), which at 7 dB/cm centre-band is remarkably low for a plasmonic device. The attenuation rate can of course be adjusted by varying the geometry of the core and channel.