Coinage metal nanostructures support localised surface plasmons, which confine optical fields much tighter than their wavelength (1). This extreme enhancement enables vibrational spectroscopy within small volumes, even down to single molecules (2,3). For many years lateral resolution was believed to be 10 nm (4), however recent experiments resolve the atomic structure of single molecules using tipenhanced Raman spectroscopy (3) and directly sequence RNA strands (5). Atomistic simulations also suggest plasmonic confinement to atomic scales is possible (6). Here we show that light-activated mobilisation of surface atoms in a plasmonic hotspot triggers the formation of additional 'picocavities'bounded by a single gold atom. Their ultra-small light localisation alters which vibrational modes of trapped molecules are observed, due to strong optical field gradients that switch the Raman selection rules. The resulting cascaded ultra-strong plasmonic confinement pumps specific molecular bonds, thereby creating non-thermal vibrational populations, and forms a new type of optomechanical

Strong coupling of monolayer metal dichalcogenide semiconductors with light offers encouraging prospects for realistic exciton devices at room temperature. However, the nature of this coupling depends extremely sensitively on the optical confinement and the orientation of electronic dipoles and fields. Here, we show how plasmon strong coupling can be achieved in compact, robust, and easily assembled gold nano-gap resonators at room temperature. We prove that strong-coupling is impossible with monolayers due to the large exciton coherence size, but resolve clear anti-crossings for greater than 7 layer devices with Rabi splittings exceeding 135 meV. We show that such structures improve on prospects for nonlinear exciton functionalities by at least 104, while retaining quantum efficiencies above 50%, and demonstrate evidence for superlinear light emission.

Coupling noble metal nanoparticles by a 1 nm gap to an underlying gold mirror confines light to extremely small volumes, useful for sensing on the nanoscale. Individually measuring 10 000 of such gold nanoparticles of increasing size dramatically shows the different scaling of their optical scattering (far-field) and surface-enhanced Raman emission (SERS, near-field). Linear red-shifts of the coupled plasmon modes are seen with increasing size, matching theory. The total SERS from the few hundred molecules under each nanoparticle dramatically increases with increasing size. This scaling shows that maximum SERS emission is always produced from the largest nanoparticles, irrespective of tuning to any plasmonic resonances. Changes of particle facet with nanoparticle size result in vastly weaker scaling of the near-field SERS, without much modifying the far-field, and allows simple approaches for optimizing practical sensing.

Fabricating nanocavities in which optically active single quantum emitters are precisely positioned is crucial for building nanophotonic devices. Here we show that self-assembly based on robust DNA-origami constructs can precisely position single molecules laterally within sub-5 nm gaps between plasmonic substrates that support intense optical confinement. By placing single-molecules at the center of a nanocavity, we show modification of the plasmon cavity resonance before and after bleaching the chromophore and obtain enhancements of ≥4 × 103 with high quantum yield (≥50%). By varying the lateral position of the molecule in the gap, we directly map the spatial profile of the local density of optical states with a resolution of ±1.5 nm. Our approach introduces a straightforward noninvasive way to measure and quantify confined optical modes on the nanoscale.

Plasmonic nanocavities with sub-5nm gaps between nanoparticles support multiple resonances possessing ultra-high field confinement and enhancements. Here we systematically compare the two fundamentally different resonant gap modes: transverse waveguide () and antenna modes () which, despite both tightly confining light within the gap, have completely different near-field and far-field radiation patterns. By varying the gap size, both experimentally and theoretically, we show how changing the nanoparticle shape from sphere to cube alters coupling of and modes resulting in strongly hybridized () modes. Through rigorous group representation analysis we identify their composition and coupling. This systematic analysis shows modes with optical field perpendicular to the gap are best to probe the optical properties of cavity-bound emitters, such as single molecules. Effective ways to enhance, confine, couple, and utilize light down to the single-emitter level have been central questions of nanophotonics 1-3. Plasmonic nanocavities made of noble metallic nanostructures have played an important role in addressing this, using collective charge oscillations of surface plasmon polaritons 4,5. Surface plasmons on closely-spaced multiple nanostructures can hybridize with each other to create trapped modes within their gap 6. Such nano-gaps are used to probe optical properties of single-molecules such as their Raman scattering 7-9 , non-linear effects 10 , chiral activity 11 , or rate of emission 12. However, resonant enhancements depend strongly on the morphology of the gap, especially when gaps