Studies of metamaterial structures

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Subradiant entanglement in plasmonic nanocavities

Plasmonic nanocavities are known for their extreme field enhancement and sub-wavelength light confinement in gaps of just a few nanometers. Pairing this with the ability to host quantum emitters, they form highly promising platforms for controlling and engineering quantum states at room temperature. Here, we show how sub-radiant entangled states emerge between two or more quantum emitters within plasmonic nanocavities. We develop a theoretical description that directly links quantum variables to experimentally measurable quantities, such as the extinction cross-section. We show that the lossy nature of plasmonic nanocavities aids the emergence of sub-radiant entangled states between quantum emitters, persisting for 100 times longer than the plasmonic excitation. This work paves the way towards designing and engineering quantum entangled states in ambient conditions with plasmonic nanocavities, for potential applications as rapid quantum memories, quantum communications and quantum sensors

Analytic theory of optical nanoplasmonic metamaterials

Recent advances in nano-fabrication techniques allow for the manufacture of optical metamaterials, bringing their unique and extra-ordinary properties to the visible regime and beyond. However, an analytical description of optical nano-plasmonic metamaterials is challenging due to the characteristic optical behaviour of metals. Here we present an analytical theory that allows to bring established microwave metamaterials models to optical wavelengths. This method is implemented for nano-scaled plasmonic wire-mesh and tri-helical metamaterials, and we obtain an accurate prediction for their dispersive behaviour at optical and near-IR wavelengths

Optical nano-woodpiles: large-area metallic photonic crystals and metamaterials.

Metallic woodpile photonic crystals and metamaterials operating across the visible spectrum are extremely difficult to construct over large areas, because of the intricate three-dimensional nanostructures and sub-50 nm features demanded. Previous routes use electron-beam lithography or direct laser writing but widespread application is restricted by their expense and low throughput. Scalable approaches including soft lithography, colloidal self-assembly, and interference holography, produce structures limited in feature size, material durability, or geometry. By multiply stacking gold nanowire flexible gratings, we demonstrate a scalable high-fidelity approach for fabricating flexible metallic woodpile photonic crystals, with features down to 10 nm produced in bulk and at low cost. Control of stacking sequence, asymmetry, and orientation elicits great control, with visible-wavelength band-gap reflections exceeding 60%, and with strong induced chirality. Such flexible and stretchable architectures can produce metamaterials with refractive index near zero, and are easily tuned across the IR and visible ranges.We acknowledge financial support from EPSRC grant EP/G060649/1, EP/I012060/1, EP/L027151/1, ERC grants LINASS 320503 and 3DIMAGE 291522, EU FP7 280478, and the Leverhulme Trust and Rolls-Royce plc.This is the final version of the article, originally published in Scientific Reports 5, Article number: 8313. DOI: 10.1038/srep08313

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

Anomalous Spectral Shift of Near- and Far-Field Plasmonic Resonances in Nanogaps.

The near-field and far-field spectral response of plasmonic systems are often assumed to be identical, due to the lack of methods that can directly compare and correlate both responses under similar environmental conditions. We develop a widely tunable optical technique to probe the near-field resonances within individual plasmonic nanostructures that can be directly compared to the corresponding far-field response. In tightly coupled nanoparticle-on-mirror constructs with nanometer-sized gaps we find >40 meV blue-shifts of the near-field compared to the dark-field scattering peak, which agrees with full electromagnetic simulations. Using a transformation optics approach, we show such shifts arise from the different spectral interference between different gap modes in the near- and far-field. The control and tuning of near-field and far-field responses demonstrated here is of paramount importance in the design of optical nanostructures for field-enhanced spectroscopy, as well as to control near-field activity monitored through the far-field of nano-optical devices.We acknowledge financial support from EPSRC grants EP/G060649/1, EP/L027151/1, EP/G037221/1, EPSRC NanoDTC, and ERC grant LINASS 320503. J.A. acknowledges support from project FIS2013-41184-P from Spanish MINECO and project NANOGUNE'14 from the Dept. of Industry of the Basque Country. F.B. acknowledges support from the Winton Programme for the Physics of Sustainability. R.C. acknowledges financial support from St. John's College, Cambridge for Dr. Manmohan Singh Scholarship. P.A. acknowledges funding from the Helmholtz Association for the Young Investigator group VH-NG-928 within the Initiative and Networking fund. We thank Laurynas Pukenas and Steve Evans (University of Leeds, UK) for support with the ellipsometry measurementsThis is the final version of the article. It first appeared from the American Chemical Society via https://doi.org/10.1021/acsphotonics.5b0070