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Controlling absorption and emission of organic molecules is crucial for efficient light-emitting diodes, organic solar cells and single-molecule spectroscopy. Here, a new molecular absorption is activated inside a gold plasmonic nanocavity, and found to break selection rules via spin-orbit coupling. Photoluminescence excitation scans reveal absorption from a normally spin-forbidden singlet to triplet state transition, while drastically enhancing the emission rate by several thousand fold. The experimental results are supported by density functional theory, revealing the manipulation of molecular absorption by nearby metallic gold atoms., Comment: 8 pages, 4 figures

Nonlinear molecular interactions with optical fields produce intriguing optical phenomena and applications ranging from color generation to biomedical imaging and sensing. The nonlinear cross-section of dielectric materials is low and therefore for effective utilisation, the optical fields need to be amplified. Here, we demonstrate that two-photon absorption can be enhanced by 108 inside individual plasmonic nanocavities containing emitters sandwiched between a gold nanoparticle and a gold film. This enhancement results from the high field strengths confined in the nanogap, thus enhancing nonlinear interactions with the emitters. We further investigate the parameters that determine the enhancement including the cavity spectral position and excitation wavelength. Moreover, the Purcell effect drastically reduces the emission lifetime from 520 ns to

Strong-coupling between excitons and confined photonic modes can lead to the formation of new quasi-particles termed exciton-polaritons which can display a range of interesting properties such as super-fluidity, ultrafast transport and Bose-Einstein condensation. Strong-coupling typically occurs when an excitonic material is confided in a dielectric or plasmonic microcavity. Here, we show polaritons can form at room temperature in a range of chemically diverse, organic semiconductor thin films, despite the absence of an external cavity. We find evidence of strong light-matter coupling via angle-dependent peak splittings in the reflectivity spectra of the materials and emission from collective polariton states. We additionally show exciton-polaritons are the primary photoexcitation in these organic materials by directly imaging their ultrafast (5 × 106 m s−1), ultralong (~270 nm) transport. These results open-up new fundamental physics and could enable a new generation of organic optoelectronic and light harvesting devices based on cavity-free exciton-polaritons, Exciton-polaritons are typically formed in organic systems when the molecules are confined between metallic or dielectric mirrors. Here, the authors reveal that interactions between excitons and moderately confined photonic states within the bare organic film can also lead to polariton formation, making them the primary photoexcitation.

Funder: Leverhulme Trust, Funder: Trinity College, University of Cambridge, Funder: Isaac Newton Trust, Transient atomic protrusions in plasmonic nanocavities confine optical fields to sub-1-nm3 picocavities, allowing the optical interrogation of single molecules at room temperature. While picocavity formation is linked to both the local chemical environment and optical irradiation, the role of light in localizing the picocavity formation is unclear. Here, we combine information from thousands of picocavity events and simultaneously compare the transient Raman scattering arising from two incident pump wavelengths. Full analysis of the data set suggests that light suppresses the local effective barrier height for adatom formation and that the initial barrier height is decreased by reduced atomic coordination numbers near facet edges. Modeling the system also resolves the frequency-dependent picocavity field enhancements supported by these atomic scale features.

In the past decade, advances in nanotechnology have led to the development of plasmonic nanocavities that facilitate light–matter strong coupling in ambient conditions. The most robust example is t...

Metal-insulator-metal (MIM) nanogaps in canonical nanoparticle-on-mirror geometry (NPoM) provide deep-subwavelength confinement of light with mode volumes smaller than $V/V_0$ $, Comment: 16 pages, 5 figures

Irreproducibility in surface-enhanced Raman spectroscopy (SERS) due to variability among substrates is a source of recurrent debate within the field. It is regarded as a major hurdle towards the widespread adoption of SERS as a sensing platform. Most of the literature focused on developing substrates for various applications considers reproducibility of lower importance. Here, we address and analyse the sources of this irreproducibility in order to show how these can be minimised. We apply our findings to a simple substrate demonstrating reproducible SERS measurements with relative standard deviations well below 1% between different batches and days. Identifying the sources of irreproducibility and understanding how to reduce these can aid in the transition of SERS from the lab to real world applications.

Plasmonic metafilms have been widely utilized to generate vivid colors, but making them both active and flexible simultaneously remains a great challenge. Here flexible active plasmonic metafilms constructed by printing electrochromic nanoparticles onto ultrathin metal films (, Commercially available printing techniques offer fullyprinted flexible plasmonic metafilms, which show electrically‐tunable omni‐ or bi‐directional color dynamics engineered with structural geometries. Such activelycontrolled directional color dynamics in flexible plasmonics significantly unlocks the full potential of plasmonics for flexible/wearable devices.

Plasmonic nanoconstructs are widely exploited to confine light for applications ranging from quantum emitters to medical imaging and biosensing. However, accessing extreme near-field confinement using the surfaces of metallic nanoparticles often induces permanent structural changes from light, even at low intensities. Here, we report a robust and simple technique to exploit crystal facets and their atomic boundaries to prevent the hopping of atoms along and between facet planes. Avoiding X-ray or electron microscopy techniques that perturb these atomic restructurings, we use elastic and inelastic light scattering to resolve the influence of crystal habit. A clear increase in stability is found for {100} facets with steep inter-facet angles, compared to multiple atomic steps and shallow facet curvature on spherical nanoparticles. Avoiding atomic hopping allows Raman scattering on molecules with low Raman cross-section while circumventing effects of charging and adatom binding, even over long measurement times. These nanoconstructs allow the optical probing of dynamic reconstruction in nanoscale surface science, photocatalysis, and molecular electronics.

Plasmonic nanostructures can focus light far below the diffraction limit, and the nearly thousandfold field enhancements obtained routinely enable few- and single-molecule detection. However, for processes happening on the molecular scale to be tracked with any relevant time resolution, the emission strengths need to be well beyond what current plasmonic devices provide. Here, we develop hybrid nanostructures incorporating both refractive and plasmonic optics, by creating SiO 2 nanospheres fused to plasmonic nanojunctions. Drastic improvements in Raman efficiencies are consistently achieved, with (single-wavelength) emissions reaching 10 7 counts⋅mW −1 ⋅s −1 and 5 × 10 5 counts∙mW −1 ∙s −1 ∙molecule −1 , for enhancement factors >10 11 . We demonstrate that such high efficiencies indeed enable tracking of single gold atoms and molecules with 17-µs time resolution, more than a thousandfold improvement over conventional high-performance plasmonic devices. Moreover, the obtained (integrated) megahertz count rates rival (even exceed) those of luminescent sources such as single-dye molecules and quantum dots, without bleaching or blinking.

In this work, we demonstrate that coordination interactions between Fe\(^{3+}\) and cucurbit[7]uril (CB[7]) can be utilised to build up defined nanoscale spacing layers in metallic nanosystems. We begin by characterising the layer-by-layer deposition of CB[7] and FeCl\(_3\)·6H\(_2\)O coordination layers through the use of a QuartzCrystal Microbalance (QCM) and contact angle measurements. We then apply this layered structure to accurately control the spacing, and thus optical properties, of gold nanoparticles in a Nanoparticle-on-Mirror (NPoM) structure, which is demonstrated via normalising plasmon resonance spectroscopy.

Lipid bilayers assembled on solid substrates have been extensively studied with single-molecule resolution as the constituent molecules diffuse in 2D; however, the out-of-plane motion is typically ignored. Here we present the subnanometer out-of-plane diffusion of nanoparticles attached to hybrid lipid bilayers (HBLs) assembled on metal surfaces. The nanoscale cavity formed between the Au nanoparticle and Au film provides strongly enhanced optical fields capable of locally probing HBLs assembled in the gaps. This allows us to spectroscopically resolve the nanoparticles assembled on bilayers, near edges, and in membrane defects, showing the strong influence of charged lipid rafts. Nanoparticles sitting on the edges of the HBL are observed to flip onto and off of the bilayer, with flip energies of ∼10 meV showing how thermal energies dynamically modify lipid arrangements around a nanoparticle. We further resolve the movement of individual lipid molecules by doping the HBL with low concentrations of Texas Red (TxR) dye-labeled lipids.

In order to optimize surface-enhanced Raman scattering (SERS) of noble metal nanostructures for enabling chemical identification of analyte molecules, careful design of nanoparticle structures must be considered. We spatially map the local SERS enhancements across individual micro-aggregates comprised of monodisperse nanoparticles separated by rigid monodisperse 0.9 nm gaps and show the influence of depositing these onto different underlying substrates. Experiments and simulations show that the gaps between neighbouring nanoparticles dominate the SERS enhancement far more than the gaps between nanoparticles and substrate.

The dynamic restructuring of metal nanoparticle surfaces is known to greatly influence their catalytic, electronic transport, and chemical binding functionalities. Here we show for the first time that non-equilibrium atomic-scale lattice defects can be detected in nanoparticles by purely optical means. These fluctuating states determine interface electronic transport for molecular electronics but because such rearrangements are low energy, measuring their rapid dynamics on single nanostructures by X-rays, electron beams, or tunnelling microscopies, is invasive and damaging. We utilise nano-optics at the sub-5nm scale to reveal rapid (on the millisecond timescale) evolution of defect morphologies on facets of gold nanoparticles on a mirror. Besides dynamic structural information, this highlights fundamental questions about defining bulk plasma frequencies for metals probed at the nanoscale., Dynamic restructuring of metal nanoparticle surfaces greatly influences their catalytic, electronic transport, and chemical binding functionalities. Here, the authors show that non-equilibrium atomic-scale lattice defects can be detected in nanoparticles by using nano-optics at the sub-5nm scale.

Significance Imaging of single to a few molecules has received much recent interest. While superresolution microscopies access subdiffraction resolution, they do not work for plasmonic hot spots due to the loss of positional information that results from plasmonic coupling. Here, we show how to reconstruct the spatial locations of molecules within a plasmonic hot spot with 1-nm precision. We use a plasmonic nanoball lens to demonstrate that plasmonic nanocavities can be used simultaneously as a nanoscopic and spectroscopic tool. This work opens up possibilities for studying the behavior of a few to single molecules in plasmonic nanoresonators, while simultaneously tracking their movements and spectral features. Our plasmonic nanolens is useful for nanosensing, nanochemistry, and biofunctional imaging., Plasmonics now delivers sensors capable of detecting single molecules. The emission enhancements and nanometer-scale optical confinement achieved by these metallic nanostructures vastly increase spectroscopic sensitivity, enabling real-time tracking. However, the interaction of light with such nanostructures typically loses all information about the spatial location of molecules within a plasmonic hot spot. Here, we show that ultrathin plasmonic nanogaps support complete mode sets which strongly influence the far-field emission patterns of embedded emitters and allow the reconstruction of dipole positions with 1-nm precision. Emitters in different locations radiate spots, rings, and askew halo images, arising from interference of 2 radiating antenna modes differently coupling light out of the nanogap, highlighting the imaging potential of these plasmonic “crystal balls.” Emitters at the center are now found to live indefinitely, because they radiate so rapidly.

Quantitative applications of surface enhanced Raman spectroscopy (SERS) often rely on surface partition layers grafted to SERS substrates to collect and trap solvated analytes that would not otherwise adsorb onto metals. Such binding layers drastically broaden the scope of analytes that can be probed. However, excess binding sites introduced by this partition layer also trap analytes outside the plasmonic †hot-spots'. We show that by eliminating these binding sites, limits of detection (LODs) can effectively be lowered by more than an order of magnitude. We highlight the effectiveness of this approach by demonstrating quantitative detection of controlled drugs down to sub-nanomolar concentrations in aqueous media. Such LODs are low enough to screen, for example, urine at clinically relevant levels. These findings provide unique insights into the binding behavior of analytes, which are essential when designing high performance SERS substrates.

Polarized optical dark-field spectroscopy is shown to be a versatile noninvasive probe of plasmonic structures that trap light to the nanoscale. Clear spectral polarization splittings are found to be directly related to the asymmetric morphology of nanocavities formed between faceted gold nanoparticles and an underlying gold substrate. Both experiment and simulation show the influence of geometry on the coupled system, with spectral shifts Δλ = 3 nm from single atoms. Analytical models allow us to identify the split resonances as transverse cavity modes, tightly confined to the nanogap. The direct correlation of resonance splitting with atomistic morphology allows mapping of subnanometre structures, which is crucial for progress in extreme nano-optics involving chemistry, nanophotonics, and quantum devices.

Reproducible confinement of light on the nanoscale is essential for the ability to observe and control chemical reactions at the single-molecule level. Here we reliably form millions of identical nanocavities and show that the light can be further focused down to the subnanometer scale via the creation of picocavities, single-adatom protrusions with angstrom-level resolution. For the first time, we stabilize and analyze these cavities at room temperatures through high-speed surface-enhanced Raman spectroscopy on specifically selected molecular components, collecting and analyzing more than 2 million spectra. Data obtained on these picocavities allows us to deduce structural information on the nanoscale, showing that thiol binding to gold destabilizes the metal surface to optical irradiation. Nitrile moieties are found to stabilize picocavities by 10-fold against their disappearance, typically surviving for >1 s. Such constructs demonstrate the accessibility of single-molecule chemistry under ambient conditions., We acknowledge financial support from EPSRC Grants EP/L027151/1 and EP/N020669/1. C.C. acknowledges support from NPL PO443073. B.d.N. acknowledges support from the Leverhulme Trust and Isaac Newton Trust. R.C. acknowledges financial support from a Junior Research Fellowship of Trinity College.

Emission properties of a quantum emitter can be significantly modified inside nanometre-sized gaps between two plasmonic nanostructures. This forms a nanoscopic optical cavity which allows single-molecule detection and single-molecule strong-coupling at room temperature. However, plasmonic resonances of a plasmonic nanocavity are highly sensitive to the exact gap morphology. In this article, we shed light on the effect of gap morphology on the plasmonic resonances of a faceted nanoparticle-on-mirror (NPoM) nanocavity and their interaction with quantum emitters. We find that with increasing facet width the NPoM nanocavity provides weaker field enhancement and thus less coupling strength to a single quantum emitter since the effective mode volume increases with the facet width. However, if multiple emitters are present, a faceted NPoM nanocavity is capable of accommodating a larger number of emitters, and hence the overall coupling strength is larger due to the collective and coherent energy exchange from all the emitters. Our findings pave the way to more efficient designs of nanocavities for room-temperature light-matter strong-coupling, thus providing a big step forward to a non-cryogenic platform for quantum technologies.

An emitter in the vicinity of a metal nanostructure is quenched by its decay through nonradiative channels, leading to the belief in a zone of inactivity for emitters placed within

Discussions.

Single nanoparticles are shown to develop a localized acoustic resonance, the bouncing mode, when placed on a substrate. If both substrate and nanoparticle are noble metals, plasmonic coupling of the nanoparticle to its image charges in the film induces tight light confinement in the nanogap. This yields ultrastrong ``acoustoplasmonic'' coupling with a figure of merit 7 orders of magnitude higher than conventional acousto-optic modulators. The plasmons thus act as a local vibrational probe of the contact geometry. A simple analytical mechanical model is found to describe the bouncing mode in terms of the nanoscale structure, allowing transient pump-probe spectroscopy to directly measure the contact area for individual nanoparticles.

Under UV illumination, polymer films can undergo chain scission and contract. Using this effect, tightly focused laser light is shown to develop runaway near-field concentration that drills sub-100 nm pits through a thin film. This subwavelength photolithography can be controlled in real time by monitoring laser scatter from the evolving holes, allowing systematic control of the void diameter. Our model shows how interference between the substrate and film together with near-field focusing by the evolving crevice directs this formation and predicts minimum pit sizes in films of 100 nm thickness on gold substrates. The smallest features so far are 60 nm diameter pits using 447 nm light focused onto polystyrene through a ×100 objective (NA = 0.8). Such arrays of pits can be easily used as masks for fabricating more complex nanostructures, such as plasmonic nanostructures and biomicrofluidic devices. This demonstration shows the potential for harnessing near-field feedback in optical direct-writing for nanofabrication.

Plasmonic nanocavities with sub-5-nm 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 (s) and antenna modes (l), 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 s and l modes, resulting in strongly hybridized (j) modes. Through rigorous group representation analysis we identify their composition and coupling. This systematic analysis of the Purcell factors shows that modes with optical field perpendicular to the gap are best to probe the optical properties of cavity-bound emitters, such as single molecules.

The mechanism by which light is emitted from plasmonic metals such as gold and silver has been contentious, particularly at photon energies below direct interband transitions. Using nanoscale plasmonic cavities, blue-pumped light emission is found to directly track dark-field scattering on individual nanoconstructs. By exploiting slow atomic-scale restructuring of the nanocavity facets to spectrally tune the dominant gap plasmons, this correlation can be measured from 600 to 900 nm in gold, silver, and mixed constructs ranging from spherical to cube nanoparticles-on-mirror. We show that prompt electronic Raman scattering is responsible and confirm that “photoluminescence”, which implies phase and energy relaxation, is not the right description. Our model suggests how to maximize light emission from metals.

The first page of this article is displayed as the abstract., Rohit Chikkaraddy opened the discussion of the Introductory Lecture: Regarding quantifying the chemical enhancement, you showed a systematic change in the SERS enhancement for halide substituted molecules due to charge transfer from the metal. Is the extra enhancement due to an inherent increase in the Raman cross-section of the molecule? How do you go about referencing, as the charge transfer changes the vibrational frequency? Richard Van Duyne answered: The extra enhancement is not due to an increase in the Raman cross section, as that is ratioed out in the calculation of the enhancement factor. The charge transfer (CT) process does not transfer a complete electron, it is a fractional degree of CT. Thus the change in vibrational frequency is small. DFT calculations that provide eigenvectors allow one to reference the vibrational modes of the free molecule with those of the adsorbed molecule. Sylwester Gawinkowski asked: You have shown that the enhancement factor curve is redshifted relative to the plasmon resonance band and has a maximum at about 800 nm. This means that the SERS signal should be strongest for excitations in the near infrared spectral region. Why do most SERS reports, particularly related to single molecule SERS, have the excitation in the green or red spectral range and not in the near infrared?

Jurgen Popp opened a general discussion of the paper by Roy Goodacre: What is the advantage of using SERS instead of ordinary Raman spectroscopy?

Discussions.

Trapping light with noble metal nanostructures overcomes the diffraction limit and can confine light to volumes typically on the order of 30 cubic nanometers. We found that individual atomic features inside the gap of a plasmonic nanoassembly can localize light to volumes well below 1 cubic nanometer (“picocavities”), enabling optical experiments on the atomic scale. These atomic features are dynamically formed and disassembled by laser irradiation. Although unstable at room temperature, picocavities can be stabilized at cryogenic temperatures, allowing single atomic cavities to be probed for many minutes. Unlike traditional optomechanical resonators, such extreme optical confinement yields a factor of 106 enhancement of optomechanical coupling between the picocavity field and vibrations of individual molecular bonds. This work sets the basis for developing nanoscale nonlinear quantum optics on the single-molecule level., We acknowledge financial support from EPSRC grants EP/G060649/1 and EP/L027151/1, and ERC grant LINASS 320503. MKS, YZ, AD, RE, and JA acknowledge financial support from Project FIS2013- 41184-P from MINECO and IT756-13 from the Basque Government consolidated groups. FB acknowledges support from the Winton Programme for the Physics of Sustainability. RC acknowledges support from the Dr. Manmohan Singh scholarship from St. John’s College. CC thanks NPL for support. RE acknowledges support from the Fellows Gipuzkoa Program of the Gipuzkoako Foru Aldundia via FEDER funds of the European Union “Una manera de hacer Europa”.

Photon emitters placed in an optical cavity experience an environment that changes how they are coupled to the surrounding light field. In the weak-coupling regime, the extraction of light from the emitter is enhanced. But more profound effects emerge when single-emitter strong coupling occurs: mixed states are produced that are part light, part matter1, 2, forming building blocks for quantum information systems and for ultralow-power switches and lasers3, 4, 5, 6. Such cavity quantum electrodynamics has until now been the preserve of low temperatures and complicated fabrication methods, compromising its use5, 7, 8. Here, by scaling the cavity volume to less than 40 cubic nanometres and using host–guest chemistry to align one to ten protectively isolated methylene-blue molecules, we reach the strong-coupling regime at room temperature and in ambient conditions. Dispersion curves from more than 50 such plasmonic nanocavities display characteristic light–matter mixing, with Rabi frequencies of 300 millielectronvolts for ten methylene-blue molecules, decreasing to 90 millielectronvolts for single molecules—matching quantitative models. Statistical analysis of vibrational spectroscopy time series and dark-field scattering spectra provides 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 the possibility of manipulating chemical bonds10.

The authors thank Javier Aizpurua (CSIC − UPV/EHU/DIPC) for helpful discussions. We acknowledge financial support from EPSRC Grants EP/G060649/1, EP/K028510/1, EP/L027151/1, ERC Grant LINASS 320503. F.B. acknowledges support from the Winton Programme for the Physics of Sustainability. R.C. acknowledges support from the Dr. Manmohan Singh scholarship from St. John’s College. 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. Publisher PDF

The authors acknowledge financial support from UK EPSRC Grants EP/G060649/1, EP/L015978/1, and EP/L027151/1, NanoDTC, and ERC Grant LINASS320503. R.C. acknowledges support from the Dr. Manmohan Singh scholarship from St. John's College.

We develop an analytic circuit model for coupled plasmonic dimers separated by small gaps that provides a complete account of the optical resonance wavelength. Using a suitable equivalent circuit, it shows how partially conducting links can be treated and provides quantitative agreement with both experiment and full electromagnetic simulations. The model highlights how in the conducting regime, the kinetic inductance of the linkers set the spectral blue-shifts of the coupled plasmon., We acknowledge financial support from EPSRC grant EP/G060649/1, EP/I012060/1, EP/L027151/1, EP/K028510/1, ERC grant LINASS 320503. FB acknowledges support from the Winton Programme for the Physics of Sustainability. RC wishes to acknowledge financial support from St. John’s College, Cambridge for Dr. Manmohan Singh Scholarship. CT and JA acknowledge financial support from Project FIS2013- 41184-P from MINECO, ETORTEK 2014-15 of the Basque Department of Industry and IT756-13 from the Basque consolidated groups.

We report on angle-resolved, exciton-polariton photoluminescence measurements from asymmetric terminals of a microsphere-coupled organic waveguide (MOW). The MOW architecture consisted of a SiO2 microsphere coupled with a diaminoanthroquinone mesowire, self-assembled on a glass substrate. The angle-resolved emission measurements were performed using spatially filtered Fourier-plane optical imaging method. The light emanating from the sphere-terminus had two regions of angular emission in the Fourier-plane, of which one had azimuthal angular spread as small as 10°. The emission from wire terminus was uni-directional in nature, with some light emitted beyond the critical angle of glass-air interface. Our results highlight unique directional emission characteristics of a hybrid organic waveguide geometry and may have implications on single-element, exciton-polariton based light-emitting devices and lasers.

[EN] Coherent interconversion of signals between optical and mechanical domains is enabled by optomechanical interactions. Extreme light-matter coupling produced by confining light to nanoscale mode volumes can then access single mid-infrared (MIR) photon sensitivity. Here, we used the infrared absorption and Raman activity of molecular vibrations in plasmonic nanocavities to demonstrate frequency upconversion. We converted approximately 10-micrometer-wavelength incoming light to visible light by surface-enhanced Raman scattering (SERS) in doubly resonant antennas that enhanced upconversion by more than 10(10). We showed 140% amplification of the SERS anti-Stokes emission when an MIR pump was tuned to a molecular vibrational frequency, obtaining lowest detectable powers of 1 to 10 microwatts per square micrometer at room temperature. These results have potential for low-cost and large-scale infrared detectors and spectroscopic techniques., This work was supported by the European Union's Horizon 2020 Research and Innovation Program under grant agreements 829067 (THOR), 861950 (POSEIDON), and 883703 (PICOFORCE) and by the Engineering and Physical Sciences Research Council (EPSRC) (Cambridge NanoDTC grants EP/L015978/1, EP/L027151/1, EP/S022953/1, EP/P029426/1, and EP/R020965/1). X.Z. acknowledges support from KU Leuven Internal Funds C14/19/083, IDN/20/014, KA/20/019, and FWO G090017N. R.C. acknowledges support from Trinity College, University of Cambridge. Z.K.B. and E.R. acknowledge funding from the EPSRC (EP/R013012/1, EP/L027151/1) and ERC project 757850 BioNet. We are grateful to the UK Materials and Molecular Modelling Hub, which is partially funded by EPSRC (EP/P020194/1), for computational resources.

[EN] Plasmonic self-assembled nanocavities are ideal platforms for extreme light localization as they deliver mode volumes of 10(5). Plasmon interference in these hybrid microresonator nanocavities produces surface-enhanced Raman scattering (SERS) signals many-fold larger than in the bare plasmonic constructs. These now allow remote access to molecules inside the ultrathin gaps, avoiding direct irradiation and thus preventing molecular damage. Combining subnanometer gaps with micrometer-scale resonators places a high computational demand on simulations, so a generalized boundary element method (BEM) solver is developed which requires 100-fold less computational resources to characterize these systems. Our results on extreme near-field enhancement open new potential for single- molecule photonic circuits, mid-infrared detectors, and remote spectroscopy., We acknowledge support from the European Research Council (ERC) under the Horizon 2020 Research and Innovation Programme THOR (829067), POSEIDON (861950) and PICOFORCE (883703). We acknowledge funding from the EPSRC (Cambridge NanoDTC EP/L015978/1, EP/L027151/1, EP/S022953/1, EP/P029426/1, and EP/R020965/1). R.C. acknowledges support from Trinity College, University of Cambridge. A.D. acknowledges support from the Royal Society University Research Fellowship URF/R1/180097 and the Royal Society Research Fellows Enhancement Award RGF/EA/181038.

Plasmonic nano-cavities have the ability to confine light in sub-wavelength gaps. The smallest possible plasmonic nano-cavities are achieved with the nanoparticle on mirror (NPoM) configuration [1], where a flat metal substrate is separated from a nanoparticle with a thin molecular spacer, typically of 1–1.5nm thick (Fig.1a). The massive field enhancement facilitates the strong-coupling of a single molecule with a plasmon mode at room temperature, as we recently demonstrated [2]. Here, we discuss the complex behaviour of plasmonic nano-cavities and the dissociation between the near-field interacting with the molecule and the far-field spectra measured experimentally.