Gold nanoparticles are separated above a planar gold film by 1.1 nm thick self-assembled molecular monolayers of different conductivities. Incremental replacement of the nonconductive molecules with a chemically equivalent conductive version differing by only one atom produces a strong 50 nm blue-shift of the coupled plasmon. With modeling this gives a conductance of 0.17G0 per biphenyl-4,4′-dithiol molecule and a total conductance across the plasmonic junction of 30G0. Our approach provides a reliable tool quantifying the number of molecules in each plasmonic hotspot, here <200.

Structured metallic tips are increasingly important for optical spectroscopies such as tip-enhanced Raman spectroscopy (TERS), with plasmonic resonances frequently cited as a mechanism for electric field enhancement. We probe the local optical response of sharp and spherical-tipped atomic force microscopy (AFM) tips using a scanning hyperspectral imaging technique to identify plasmonic behaviour. Localised surface plasmon resonances which radiatively couple with far-field light are found only for spherical AFM tips, with little response for sharp AFM tips, in agreement with numerical simulations of the near-field response. The precise tip geometry is thus crucial for plasmonenhanced spectroscopies, and the typical sharp cones are not preferred.Within the last decade nano-optics has benefited from the advent of metallic tip-based near-field enhancement techniques such as TERS and scanning near-field microscopy (SNOM), leading to successes in single molecule detection [1] and spatial mapping of chemical species [2]. Despite their high spatial resolution and scanning capabilities, there remains confusion about the plasmonic response of metallic tips. Tip systems built on AFM probes can exhibit electric field enhancements close to 100 at the apex (Raman enhancements up to 10 8 ) [2], due to a combination of plasmonic localisation and a non-resonant lightning rod effect. The factors determining a tip's ability to enhance the near-field include the experimental excitation/collection geometry, tip sharpness, surface metal morphology, and constituent material.Despite large measured near-field enhancements, the standard sharp AFM tip geometry does not support radiative plasmons. The extended (∼20 µm) size and single curved metal-dielectric interface of an AFM tip supports only weakly confined localised surface plasmons (LSPs) [3] and propagating surface plasmon polaritons (SPPs), which may be localised by adiabatic nanofocussing [4][5][6][7][8][9]. Lack of a dipole moment means that neither LSPs or SPPs strongly couple with radiative light in the same manner as multipolar plasmons in subwavelength nanoparticles [3]. For this reason, the tip near-field is often excited with evanescent waves [10] or via nanofabricated gratings [6] to access the opticallydark SPPs, with resonant scattering of evanescent waves [11][12][13], resonances in the TERS background [14,15] and depolarised scattering images [16] providing evidence for localised plasmon excitation. For Au tips such plasmon resonances are typically found between 600-800 nm.Improvements in enhancement are often found in roughened tips with grains acting as individual nanoantennae for more confined LSPs, however this approach lacks reproducibility [16]. In recent years controlled nanostructuring of the tip apex with a distinct subwavelength-size metallic feature has been explored in order to engineer and tune a plasmonic optical antenna * Email: jjb12@cam.ac.uk; Site: www.np.phy.cam.ac.uk precisely at the apex and better incorporate more localised multipolar plasmons [16...

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We demonstrate a simple, scalable fabrication method for producing large-area arrays of vertically stacked metallic micro-rings, embedded in a deformable polymer sheet. Unusual polarisation-dependent hotspots are found to dominate the reflection images. To understand their origin, the arrays are characterized using point-scanning optical spectroscopy and directly compared to numerical simulations. Individual ring stacks act as microlenses, while polarisation-dependent hotspots arise at the connections between neighbouring stacks, which are comprised of parabolically-arranged parallel gold nanowires. The elastomeric properties of the polymer host opens the door to active control of the optics of this photonic material, through dynamic tuning of the nanowire spacings and array geometry.

Plasmonic interactions between two metallic tips are dynamically studied in a supercontinuum dark-field microscope and the transition between coupled and charge-transfer plasmons is directly observed in the sub-nm regime. Simultaneous measurement of the dc current, applied force, and optical scattering as the tips come together is used to determine the effects of conductive pathways within the plasmonic nano-gap. Critical conductances are experimentally identified for the first time, determining the points at which quantum tunnelling and conductive charge transport begin to influence plasmon coupling. These results advance our understanding of the relationship between conduction and plasmonics, and the fundamental quantum mechanical behaviours of plasmonic coupling.