Emitters placed in an optical cavity experience an environment that changes their coupling to light. In the weak-coupling regime light extraction is enhanced, but more profound effects emerge in the single-molecule strong-coupling regime where mixed light-matter states form1,2. Individual two-level emitters in such cavities become non-linear for single photons, forming key building blocks for quantum information systems as well as ultra-low power switches and lasers3–6. Such cavity quantum electrodynamics has until now been the preserve of low temperatures and complex fabrication, severely compromising their use5,7,8. Here, by scaling the cavity volume below 40 nm3 and using host-guest chemistry to align 1-10 protectively-isolated methylene-blue molecules, we reach the strong-coupling regime at room temperature and in ambient conditions. Dispersion curves from >50 plasmonic nanocavities display characteristic anticrossings, with Rabi frequencies of 300 meV for 10 molecules decreasing to 90 meV for single molecules, matching quantitative models. Statistical analysis of vibrational spectroscopy time-series and dark-field scattering spectra provide evidence of single-molecule strong coupling. This dressing of molecules with light can modify photochemistry, opening up the exploration of complex natural processes such as photosynthesis9 and pathways towards manipulation of chemical bonds10.

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

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.

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.

An emitter in the vicinity of a metal nanostructure is quenched by its decay through non-radiative channels, leading to the belief in a zone of inactivity for emitters placed within <10nm of a plasmonic nanostructure. Here we demonstrate that in tightly-coupled plasmonic resonators forming nanocavities "quenching is quenched" due to plasmon mixing. Unlike isolated nanoparticles, plasmonic nanocavities show mode hybridization which massively enhances emitter excitation and decay via radiative channels. This creates ideal conditions for realizing single-molecule strong-coupling with plasmons, evident in dynamic Rabi-oscillations and experimentally confirmed by laterally dependent emitter placement through DNA-origami.The lifetime of an excited atomic state is determined by the inherent properties of the atom and its environment, first theoretically suggested by Purcell [1] followed by experimental demonstration [2]. Subsequent experiments further verified this by placing atomic emitters within various optical-field-enhancing geometries [3][4][5]. Plasmonic structures have the ability to massively enhance electromagnetic fields, and therefore dramatically alter the excitation rate of an emitter [6]. However, it is well known that placing an emitter close to a plasmonic structure (< 10nm), quenches its fluorescence [7][8][9]. Analysis by Anger et al. [6] showed this is due to the coupling of the emitter to non-radiative higher-order plasmonic modes that dissipate its energy. This 'zone of inactivity' was previously believed to quench all quantum emitters. However, recent advancements have shown that an emitter's emission rate can be enhanced with plasmonic nano-antennas [10][11][12][13][14][15][16][17].Generally a single emitter placed into near-contact with an optical antenna gives larger fluorescence since the antenna efficiently converts far-field radiation into a localized field and vice versa [10,12,13,18]. This was recently demonstrated by Hoang et al. [17] who showed that a quantum dot in a 12nm nano-gap exhibits ultrafast spontaneous emission. What however remains unclear is if this enhanced emission is strong enough to allow for single emitter strong coupling.In this Letter, we demonstrate and explain why quenching is substantially suppressed in plasmonic nanocavities, to such a degree that facilitates lightmatter strong-coupling of single-molecules, even at roomtemperature, as we recently demonstrated experimentally [19]. This is due to: (i) the dramatic increase in the emitter excitation (similar to plasmonic antennas), and (ii) the changed nature of higher-order modes that acquire a radiative component, and therefore increase the quantum yield of the emitter. Modes in plasmonic nanocavities are not a simple superposition of modes from the isolated structures, but instead are hybridplasmonic states [20][21][22][23][24]. Hence, higher-order modes that are dark for an isolated spherical nanoparticle, radiate efficiently for tightly-coupled plasmonic structures [25], significantly reducing the non-radiative...