Focusing light at sub-wavelength region opens up interesting applications in optical sensing and imaging beyond the diffraction limit. In the past, tapered Au wires with carved gratings have been employed to achieve nanofocusing. The fabrication process however, is expensive and the obtained wires are polycrystalline with high surface roughness. A chemical synthetic method overcoming these hurdles should be an attractive alternative. Here, we report a method to chemically synthesize Au microcrystals (~10 μm) bearing pentagonal bipyramidal morphology with surface corrugations assignable to high index planes. The method is a single step solid state synthesis at a temperature amenable to common substrates. The microcrystals are tapered at both ends forming sharp tips (~55 nm). Individual microcrystals have been used as pick and probe SERS substrates for a dye embedded in a polymer matrix. The unique geometry of the microcrystal also enables light propagation across its length.

Herein, we report on the experimental observation of light propagation and localization capabilities of end-to-end connected silver nanowire (Ag NW) pairs. By exciting the surface plasmon polaritons at one end of Ag NW pair, we observed relatively intense light emission at the junction and weak light emission at the distal end of the pair. To probe the localization of light at nanowire junction, we captured far-field Raman image of an isolated Ag NW pair adsorbed with rhodamine 6 G and observed enhanced Raman scattering at the nanowire junction. Such nanophotonic modules with light propagation and localization capabilities can be harnessed for multiplexed on-chip plasmonics.

The localized interaction between metallic nanostructures and surrounding fluorescent molecules can influence the emission characteristics of the molecule. With this hindsight, herein, by employing a Fourier optical fluorescence microscope, we experimentally show how fluorescence emission from molecules in the vicinity of a chemically prepared silver nanowire-dimer-junction can be directed in one or two channels. Measured forward-to-backward ratio of the fluorescence emission in a single channel was as high as 4.3 dB, and the observed polar and azimuthal angular spread was as narrow as 15° and 60°, respectively. Interestingly, the angle between the two emission channels mimicked the angle between the nanowires, thus, highlighting the prospect of geometrical control of the emitted light. These observations were further corroborated by three-dimensional finite-difference time-domain simulations. The presented results will have implications in momentum-space engineering of molecular fluorescence emission and can be extrapolated to single-emitter studies.

We demonstrate how a silver-nanowire-dimer can be employed to optically excite dual-path surface-plasmon-polaritons and utilize them to perform remote-excitation surface-enhanced Raman scattering (SERS) microscopy. Interestingly, this unique geometry allows us to perform dual-path remote-excitation SERS. Our experiments show that for the same value of excitation-laser powers, dual-path excitation leads to enhanced-SERS signal compared to single-path excitation, which has been corroborated by 3-D finite-difference time-domain simulations. Furthermore, we show that SERS-enhancement can be remotely modulated in this geometry by varying the polarization of excitation-fields. The results discussed herein can be extrapolated to remote-excitation pump-probe spectroscopy and dual-colour optical interrogation.

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