Symmetry and shape on the plasmonic behaviour of nanocavities

Plasmonic nanocavities confine light in deep subwavelength volumes and in recent years have enabled unprecedented control on light-matter interactions. A characteristic example is the nanoparticle-on-mirror geometry, which allows for the fabrication of very robust plasmonic gaps with sub-nanometre accuracy. Due to the extreme field confinement, the size and shape of plasmonic nanocavities dominate their optical response. But so far, the community has mainly focused on idealized spherical nanoparticles, ignoring that during their synthesis nanoparticles actually acquire polyhedral shapes, and that many different geometries can be synthesised these days. Here, we provide a complete description of the plasmonic modes in nanocavities made of three commonly occurring polyhedral nanoparticles (cuboctahedron, rhombicuboctahedron, decahedron). We show that the shape and symmetry of these plasmonic nanocavities dominate both their near- and far-field response, with intricate and rich optical behaviour. Through a recombination technique, the total far-field emission profile is obtained for an emitter placed at various nanocavity positions, which is crucial for understanding how energy couples in and out of the nanocavity. This work paves the way towards controlling light-matter interactions in extreme plasmonic environments for various applications, such as photochemical reactions and non-linear vibrational pumping

Nanoscopy through a plasmonic nanolens.

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.We acknowledge EPSRC grants EP/N016920/1, EP/L027151/1, and NanoDTC EP/L015978/1. OSO acknowledges support of Rubicon fellowship from the Netherlands Organisation for Scientific Research, and RC thanks support from Trinity College Cambridge

Mapping Nanoscale Hotspots with Single-Molecule Emitters Assembled into Plasmonic Nanocavities Using DNA Origami.

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

Strong light-matter coupling in plasmonic nanocavities

Optical properties of a quantum emitter are drastically modified inside a nanometer-sized gap between two plasmonic nanostructures. At such a nanoscale gap, plasmonic resonances confine light far beyond the diffraction limit and form a nanoscopic optical cavity, called a plasmonic nanocavity. This thesis investigates the optical properties of plasmonic nanocavities and highlights their ability to facilitate strong light-matter interaction. We first study plasmonic nanocavities by treating their resonances as leaky modes with complex eigenfrequencies. By varying their gap morphology, several bright and dark gap plasmonic resonances are discovered, which are essential for understanding how a nanocavity optically influences a quantum emitter. Near-field and far-field investigations also reveal intricate multiple-mode interaction with a quantum emitter. Next, this thesis tackles the misconception that fluorescence emission from a quantum emitter is always quenched through non-radiative decay channels when the emitter is placed closer than 10 nm to a plasmonic nanostructure. We demonstrate the suppression of fluorescence quenching in plasmonic nanocavities due to plasmon hybridization, which enhances the excitation and radiation from the emitter. This enhancement is shown to be strong enough to facilitate single-molecule strong coupling, as evident in its dynamic Rabi oscillations. Finally, an innovative sensing scheme is proposed, which combines immunoassay sensing with strong coupling in plasmonic nanocavities. By chemically linking the antibody-antigen-antibody complex with a quantum emitter label, we show that the proposed scheme provides 1500\% sensitivity enhancement compared to plasmonic sensors with conventional labels. This scheme could lead to the development of plasmonic bio-sensing for single molecules and new pathways towards room-temperature quantum sensing.Open Acces

Morphology dependence of nanoparticle-on-mirror geometries: A quasinormal mode analysis

Plasmonic nanoantennas are able to produce extreme enhancements by concentrating electromagnetic fields into sub-wavelength volumes. Recently, one of the most commonly used nanoantennas is the nanoparticle-on-mirror geometry, which allowed for the room temperature strong coupling of a single molecule. Very few studies offer analysis of near-field mode decompositions, and they mainly focus on spherical and/or cylindrically-faceted nanoparticle-on-mirror geometries. Perfectly spherical nanoparticles are not easy to fabricate, with recent publications revealing that a rhombicuboctahedron is a commonly occurring nanoparticle shape – due to the crystalline nature of metallic nanoparticles. In this paper, we perform a quasi-normal mode analysis for the rhombicuboctahedron-on-mirror nanoantenna and map the field distributions of each mode. We examine how the geometry of the cavity defines the near-field distribution and energies of the modes, and we show that in some cases the mode degeneracies break. This has a significant impact on the radiative emission and far-field profile of each mode, which are measured experimentally. Understanding how realistic nanoantenna geometries behave in the near-field and far-field helps us design antennas with specific properties for controlling and sensing quantum emitters in plasmonic systems

Fluorescence enhancement and strong-coupling in faceted plasmonic nanocavities

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