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...

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.

In the past decade, advances in nanotechnology have led to the development of plasmonic nanocavities which facilitate light-matter strong coupling in ambient conditions. The most robust example is the nanoparticle-on-mirror (NPoM) structure whose geometry is controlled with subnanometer precision. The excited plasmons in such nanocavities are extremely sensitive to the exact morphology of the nanocavity, giving rise to unexpected optical behaviors. So far, most theoretical and experimental studies on such nanocavities have been based solely on their scattering and absorption properties. However, these methods do not provide a complete optical description of a NPoM. Here, the NPoM is treated as an open non-conservative system supporting a set of photonic quasinormal modes (QNMs). By investigating the morphology-dependent 1 arXiv:1910.02273v1 [physics.optics] 5 Oct 2019 optical properties of nanocavities, we propose a simple yet comprehensive nomenclature based on spherical harmonics and report spectrally overlapping bright and dark nanogap eigenmodes. The near-field and far-field optical properties of NPoMs are explored and reveal intricate multi-modal interactions. Introduction Metallic nanostructures have the ability to confine light below the diffraction limit via the collective excitation of conduction electrons, called localized surface plasmons. Through recent advances in nanofabrication techniques, gaps of just 1-2 nm between nanostructures have been achieved. 1,2 At such extreme nanogaps, the plasmonic modes of two nanostructures hybridize to allow an unprecedented light confinement, 3,4 making coupled nanostructures an ideal platform for field-enhanced spectroscopy, 5,6 photocatalysis 7 and nano-optoelectronics. 8One such nanostructure is the nanoparticle-on-mirror (NPoM) geometry where a nanoparticle is separated from an underlying metal film by a molecular mono-layer. 9,10 This geometry (which resembles the prototypical dimer but is more reliable and robust to fabricate) has attracted considerable interest since it enables light-matter strong-coupling of a single molecule at room temperature, 11 and it has many potential applications, including biosensing 12 and quantum information. 13,14 A wide range of theoretical and experimental studies have been conducted to investigate the optical properties of NPoM nanocavities. 12,15,16 Several studies examine resonances of NPoMs 17-21 and their influence on optical emission of single molecules in the nanogaps. [22][23][24] However, most studies on the nanocavities have so far described their optical response via a scattering method and infer their resonances from resulting far-field spectral peaks. Although

Plasmon-polaritons are among the most promising candidates for next generation optical sensors due to their ability to support extremely confined electromagnetic fields and empower strong coupling of light and matter. Here we propose quantum plasmonic arXiv:1908.03543v1 [physics.optics] 9 Aug 2019 an enhanced sensitivity that is no longer dependent on the concentration of antibodyantigen-antibody complexes -down to the single-analyte limit. The quantum plasmonic immunoassay scheme thus not only leads to the development of plasmonic bio-sensing for single molecules but also opens up new pathways towards room-temperature quantum sensing enabled by biomolecular inspired protocols linked with quantum nanoplasmonics.

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 SiO2nanospheres fused to plasmonic nanojunctions. Drastic improvements in Raman efficiencies are consistently achieved, with (single-wavelength) emissions reaching 107counts⋅mW−1⋅s−1and 5 × 105counts∙mW−1∙s−1∙molecule−1, for enhancement factors >1011. 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.

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.

Abstract. 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.

Strong light-matter interaction is at the heart of modern quantum technological applications and is the basis for a wide range of rich optical phenomena. Coupling a single quantum emitter strongly with electromagnetic fields provides an unprecedented control over its quantum states and enables high-fidelity quantum operations. However, single-emitter strong coupling is exceptionally fragile and has been realized mostly at cryogenic temperatures. Recent experiments have, however, demonstrated that single-emitter strong coupling can be realized at room temperature by using plasmonic nanocavities that confine optical fields via surface plasmons strongly on metal surfaces and facilitate sub-picosecond light-matter interaction. Here, we outline recent theoretical developments and experimental demonstrations of roomtemperature strong coupling in the plasmonic platform, from emitter ensembles down to the single emitter limit, before placing a focus on selective studies that explore and provide insight into applications of plexcitonic strong coupling including sensing of single biological molecules, qubit entanglement generation, and reconfigurable single-photon sources and provide an outline of research directions in quantum sensing, quantum information processing, and ultrafast spectroscopy.