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Accessing the terahertz (THz) spectral domain through surface-enhanced Raman spectroscopy (SERS) is challenging and opens up the study of low-frequency molecular and electronic excitations. Compared to direct THz probing of heterogenous ensembles, the extreme plasmonic confinement of visible light to deep sub-wavelength scales allows the study of hundreds or even single molecules. We show that self-assembled molecular monolayers of a set of simple aromatic thiols confined inside single-particle plasmonic nanocavities can be distinguished by their low-wavenumber spectral peaks below 200 cm-1, after removal of a bosonic inelastic contribution and an exponential background from the spectrum Developing environment-dependent density-functional-theory simulations of the metal-molecule configuration enables the assignment and classification of their THz vibrations as well as the identification of intermolecular coupling effects and of the influence of the gold surface configuration Furthermore, we show dramatically narrower THz SERS spectra from individual molecules at picocavities, which indicates the possibility to study intrinsic vibrational properties beyond inhomogeneous broadening further supporting the key role of local environment.

Strong coupling of molecular vibrations with light creates polariton states, enabling control over many optical and chemical properties. However, the near-field signatures of strong coupling are difficult to map as most cavities are closed systems. Surface-enhanced Raman microscopy of open metallic gratings under vibrational strong coupling enables the observation of spatial polariton localization in the grating near-field, without the need for scanning probe microscopies. The lower polariton is localized at the grating slots, displays a strongly asymmetric lineshape, and gives greater plasmon-vibration coupling strength than measured in the far-field. Within these slots, the local field strength pushes the system into the ultrastrong coupling regime. Models of strong coupling which explicitly include the spatial distribution of emitters can account for these effects. Such gratings form a new system for exploring the rich physics of polaritons and the interplay between their near- and far-field properties through polariton-enhanced Raman scattering (PERS).

Anti-Stokes photoluminescence (PL) is light emission at a higher photon energy than the excitation, with applications in optical cooling, bioimaging, lasing, and quantum optics. Here, we show how plasmonic nano-cavities activate anti-Stokes PL in WSe$_2$ monolayers through resonant excitation of a dark exciton. The tightly confined plasmonic fields excite the out-of-plane transition dipole of the dark exciton, leading to light emission from the bright exciton at higher energy. Through statistical measurements on hundreds of plasmonic cavities, we show that coupling to the dark exciton is key to achieving a near hundred-fold enhancement of the upconverted PL intensity. This is further corroborated by experiments in which the laser excitation wavelength is tuned across the dark exciton. Finally, we show that an asymmetric nanoparticle shape and precise geometry are key for consistent activation of the dark exciton and efficient PL upconversion. Our work introduces a new excitation channel for anti-Stokes PL in WSe$_2$ and paves the way for large-area substrates providing optical cooling, anti-Stokes lasing, and radiative engineering of excitons.

Polarised light is critical for a wide range of applications, but is usually generated by filtering unpolarised light, which leads to significant energy losses and requires additional optics. Herein, the direct emission of linearly-polarised light is achieved from light-emitting diodes (LEDs) made of CsPbI3 perovskite nanoplatelet superlattices. Through use of solvents with different vapour pressures, the self-assembly of perovskite nanoplatelets is achieved to enable fine control over the orientation (either face-up or edge-up) and therefore the transition dipole moment. As a result of the highly-uniform alignment of the nanoplatelets, as well as their strong quantum and dielectric confinement, large exciton fine-structure splitting is achieved at the film level, leading to pure-red LEDs exhibiting a high degree of linear polarisation of 74.4% without any photonic structures. This work unveils the possibilities of perovskite nanoplatelets as a highly promising source of linearly-polarised electroluminescence, opening up the development of next-generation 3D displays and optical communications from this highly versatile, solution-processable system., Comment: 26 pages, 5 figures

Plasmonic nano-antennas can focus light to nanometre length-scales providing intense field enhancements. For the tightest optical confinements (0.5-5 nm) achieved in plasmonic gaps, the gap spacing, refractive index, and facet width play a dominant role in determining the optical properties making tuning through antenna shape challenging. We show here that controlling the surrounding refractive index instead allows both efficient frequency tuning and enhanced in/output-coupling through retardation matching as this allows dark modes to become optically active, improving widespread functionalities.

Plasmonic nanocavities form very robust sub-nanometer gaps between nanometallic structures and confine light in deep subwavelength volumes to enable unprecedented control on light-matter interactions. However, spherical nanoparticles acquire various polyhedral shapes during their synthesis, which has defining impact on controlling many light-matter interactions, such as photocatalytic reactions. Here, we focus on nanocavities made of three polyhedral nanoparticles (cuboctahedron, rhombicuboctahedron, decahedron) that commonly occur during the synthesis of spherical nanoparticles. Their photonic modes have a very intricate and rich optical behaviour, both in the near- and far-field. Through a recombination technique, we obtain the total far-field produced by a molecule placed within these nanocavities, to reveal how energy couples in and out of the system. This work paves the way towards understanding and controlling light-matter interactions, such as photocatalytic reactions and non-linear vibrational pumping, in such extreme environments.

Molecular vibrations and their dynamics are of outstanding importance for electronic and thermal transport in nanoscale devices as well as for molecular catalysis. The vibrational dynamics of <100 molecules are studied through three-colour time-resolved coherent anti-Stokes Raman spectroscopy (trCARS) using plasmonic nanoantennas. This isolates molecular signals from four-wave mixing (FWM), while using exceptionally low nanowatt powers to avoid molecular damage via single-photon lock-in detection. FWM is found to be strongly suppressed in nm-wide plasmonic gaps compared to plasmonic nanoparticles. The ultrafast vibrational decay rates of biphenyl-4-thiol molecules are accelerated ten-fold by a transient rise in local non-equilibrium temperature excited by enhanced, pulsed optical fields within these plasmonic nanocavities. Separating the contributions of vibrational population decay and dephasing carefully explores the vibrational decay channels of these tightly confined molecules. Such extreme plasmonic enhancement within nanogaps opens up prospects for measuring single-molecule vibrationally-coupled dynamics and diverse molecular optomechanics phenomena.

Nanomaterials capable of confining light are desirable for enhancing spectroscopies such as Raman scattering, infrared absorption, and nonlinear optical processes. Plasmonic superlattices have shown the ability to host collective resonances in the mid-infrared, but require stringent fabrication processes to create well-ordered structures. Here, we demonstrate how short-range-ordered Au nanoparticle multilayers on a mirror, self-assembled by a sub-nm molecular spacer, support collective plasmon-polariton resonances in the visible and infrared, continuously tunable beyond 11 $\mu$m by simply varying the nanoparticle size and number of layers. The resulting molecule-plasmon system approaches vibrational strong coupling, and displays giant Fano dip strengths, SEIRA enhancement factors ~10$^6$, light-matter coupling strengths g~100 cm$^{-1}$, Purcell factors ~10$^6$, and mode volume compression factors ~10$^8$. The collective plasmon-polariton mode is highly robust to nanoparticle vacancy disorder and is sustained by the consistent gap size defined by the molecular spacer. Structural disorder efficiently couples light into the gaps between the multilayers and mirror, enabling Raman and infrared sensing of sub-picolitre sample volumes.

The strong coupling of molecules with surface plasmons results in hybrid states which are part molecule, part surface-bound light. Since molecular resonances may acquire the spatial coherence of plasmons, which have mm-scale propagation lengths, strong-coupling with molecular resonances potentially enables long-range molecular energy transfer. Gratings are often used to couple incident light to surface plasmons, by scattering the otherwise non-radiative surface plasmon inside the light-line. We calculate the dispersion relation for surface plasmons strongly coupled to molecular resonances when grating scattering is involved. By treating the molecules as independent oscillators rather than the more typically-considered single collective dipole, we find the full multi-band dispersion relation. This approach offers a natural way to include the dark states in the dispersion. We demonstrate that for a molecular resonance tuned near the crossing point of forward and backward grating-scattered plasmon modes, the interaction between plasmons and molecules gives a five-band dispersion relation, including a bright state not captured in calculations using a single collective dipole. We also show that the role of the grating in breaking the translational invariance of the system appears in the position-dependent coupling between the molecules and the surface plasmon. The presence of the grating is thus not only important for the experimental observation of molecule-surface-plasmon coupling, but also provides an additional design parameter that tunes the system.

Room temperature detection of molecular vibrations in the mid-infrared (MIR, $\lambda$ =3-30$\mu$m) has numerous applications including real-time gas sensing, chemical reactivity, medical imaging, astronomical surveys, and quantum communication [1,2]. However, MIR detection is severely hindered by thermal noise, hence current technologies rely on energy-intensive cooled semiconductor detectors (mercury cadmium telluride, MCT) [3,4,5]. One way to overcome this challenge is to upconvert the low-energy MIR light into high-energy visible wavelengths ($\lambda$ =500-800nm) where detection of single photons is easily achieved using silicon technologies [6,7]. This process suffers from weak cross sections and the mismatch between MIR and visible wavelengths, limiting its efficiency. Here, we exploit molecular emitters possessing both MIR and visible transitions from molecular vibrations and electronic states, coupled through Frank-Condon factors. By assembling molecules into a nanoscale cavity and continuously optically pumping them below the electronic absorption band, we show the transduction of MIR light absorbed by the molecular vibrations. The upconverted signal is observed as enhanced high-energy luminescence. Combining Purcell-enhanced visible luminescence with enhanced rates of vibrational pumping gives transduction efficiencies exceeding 10%. By down-scaling the cavity volume below $1nm^3$, we show MIR detection of single-molecular bonds, inaccessible to any previous detector., Comment: 12 pages, 3 figures

Molecular vibrations couple to visible light only weakly, have small mutual interactions, and hence are often ignored for non-linear optics. Here we show the extreme confinement provided by plasmonic nano- and pico-cavities can sufficiently enhance optomechanical coupling so that intense laser illumination drastically softens the molecular bonds. This optomechanical pumping regime produces strong distortions of the Raman vibrational spectrum related to giant vibrational frequency shifts from an optical spring effect which is hundred-fold larger than in traditional cavities. The theoretical simulations accounting for the multimodal nanocavity response and near-field-induced collective phonon interactions are consistent with the experimentally-observed non-linear behavior exhibited in the Raman spectra of nanoparticle-on-mirror constructs illuminated by ultrafast laser pulses. Further, we show indications that plasmonic picocavities allow us to access the optical spring effect in single molecules with continuous illumination. Driving the collective phonon in the nanocavity paves the way to control reversible bond softening, as well as irreversible chemistry.

Metallic nanoparticle-on-a-mirror (NPoM) cavities enable extreme field confinement in sub-nm gaps, leading to unrivaled performance for nonlinear processes such as surface-enhanced Raman scattering (SERS). So far, prevailing experimental approaches based on NPoMs have been performed by means of free-space light excitation and collection under oblique incidence, since the fundamental radiatively-coupled NPoM mode does not scatter in the normal direction. Retaining this working principle, here we numerically show that plasmonic cavity modes in NPoM configurations can be efficiently excited in an integrated SERS approach through TM guided modes of silicon nitride (SiN) waveguides. Intensity enhancements beyond 10$^{5}$ can be achieved for gap spacings around 1 nm. So as to reduce unwanted SiN Raman background, the output Stokes signals are transferred to transversely placed waveguides, reaching coupling efficiencies of up to 10%. Geometrical parameters such as the gap thickness as well as the radius and position of the nanoparticle provide full control over the main spectral features, thereby enabling us to engineer and drive the optical response of NPoMs for high-performance SERS in Si-based photonic integrated platforms., Comment: 20 pages (13 of main text + 7 of references). 6 figures + Graphical TOC Entry

Recent developments in surface-enhanced Raman scattering (SERS) enable observation of single-bond vibrations in real-time at room temperature. By contrast, mid-infrared (MIR) vibrational spectroscopy is limited to inefficient slow detection. Here we develop a new method for MIR sensing using SERS. This method utilizes nanoparticle-on-foil (NPoF) nanocavities supporting both visible and MIR plasmonic hotspots in the same nano-gap formed by a monolayer of molecules. Molecular SERS signals from individual NPoF nanocavities are modulated in the presence of MIR photons. The strength of this modulation depends on the MIR wavelength, and is maximized at the 6-12$\mu$m absorption bands of SiO$_2$ or polystyrene placed under the foil. Using a single-photon lock-in detection scheme we time-resolve the rise and decay of the signal in a few 100ns. Our observations reveal that the phonon resonances of SiO$_2$ can trap intense MIR surface plasmons within the Reststrahlen band, tuning the visible-wavelength localized plasmons by reversibly perturbing the nanostructure crevices. This suggests new ways to couple nano-scale bond vibrations for optomechanics, with potential to push detection limits down to single-photon and single-molecule regimes., Comment: 16 pages and 6 figures

Lasers are ubiquitous for information storage, processing, communications, sensing, biological research, and medical applications [1]. To decrease their energy and materials usage, a key quest is to miniaturize lasers down to nanocavities [2]. Obtaining the smallest mode volumes demands plasmonic nanocavities, but for these, gain comes from only single or few emitters. Until now, lasing in such devices was unobtainable due to low gain and high cavity losses [3]. Here, we demonstrate a plasmonic nanolaser approaching the single-molecule emitter regime. The lasing transition significantly broadens, and depends on the number of molecules and their individual locations. We show this can be understood by developing a theoretical approach [4] extending previous weak-coupling theories [5]. Our work paves the way for developing nanolaser applications [2, 6, 7] as well as fundamental studies at the limit of few emitters [5, 8, 9].

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 utilise the infrared absorption and Raman activity of molecular vibrations in plasmonic nanocavities to demonstrate frequency upconversion. We convert {\lambda}~10 {\mu}m incoming light to visible via surface-enhanced Raman scattering (SERS) in doubly-resonant antennas that enhance upconversion by >10^10. We show >200% amplification of the SERS antiStokes emission when a MIR pump is tuned to a molecular vibrational frequency, obtaining lowest detectable powers ~1 {\mu}W/{\mu}m^2 at room temperature. These results have potential for low-cost and large-scale infrared detectors and spectroscopic techniques, and bring single-molecule sensing into the infrared

Metal-insulator-metal (MIM) nanogaps in canonical nanoparticle-on-mirror geometry (NPoM) provide deep-subwavelength confinement of light with mode volumes smaller than $V/V_0$ $<$ $10^{-6}$. However, access to these hotspots is limited by the impendence mismatch between the high in-plane $k_{//}$ of trapped light and free-space plane-waves, making the in- and out-coupling of light difficult. Here, by constructing a nanoparticle-on-foil (NPoF) system with thin metal films, we show the mixing of insulator-metal-insulator (IMI) modes and MIM gap modes resulting in MIMI modes. This mixing provides multi-channel access to the plasmonic nanocavity through light incident from both sides of the metal film. The red-tuning and near-field strength of MIMI modes for thinner foils is measured experimentally with white-light scattering and surface-enhanced Raman scattering from individual NPoFs. We discuss further the utility of NPoF systems since the geometry allows tightly confined light to be accessed simply and through different ports., Comment: 16 pages, 5 figures

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

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 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.

The formation of exciton-polaritons allows the transport of energy over hundreds of nanometres at velocities up to 10^6 m s^-1 in organic semiconductors films in the absence of external cavity structures.

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-5nm 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 $\geq4\times10^3$ with high quantum yield ($\geq50$%). 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 $\pm1.5$ nm. Our approach introduces a straightforward non-invasive way to measure and quantify confined optical modes on the nanoscale.

Classical topological phases derived from point degeneracies in photonic bandstructures show intriguing and unique behaviour. Previously identified exceptional points are based on accidental degeneracies and subject to engineering on a case-by-case basis. Here we show that symmetry induced (deterministic) pseudo Weyl points with non-trivial topology and hyper-conic dispersion exist at the centre of the Brillouin zone of chiral cubic systems. We establish the physical implications by means of a $P2_13$ sphere packing, realised as a nano plasmonic system and a photonic crystal.

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 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 8 layer devices with Rabi splittings exceeding 135 meV. We show that such structures improve on prospects for nonlinear exciton functionalities by at least 10^4, while retaining quantum efficiencies above 50%.

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.

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 plasmon-enhanced spectroscopies, and the typical sharp cones are not preferred., Comment: 5 pages, 4 figures