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© 1963-2012 IEEE. In this paper, we propose a general boundary integral equation (BIE) approach for solving both exterior (e.g., scattering) and interior (e.g., guided wave propagation) problems involving general plasmonic waveguiding structures with arbitrary cross-sectional geometries and with a continuous translational symmetry in the direction of wave propagation. Contrary to the field-based approach which deals with the electric and magnetic fields, we employ a potential-based formalism instead, involving vector and scalar potentials, and match them at the media interfaces. The proposed approach can handle not only conventional plasmonic waveguides on the order of a few hundred nanometers but also those whose critical sizes are a few nanometers and in which the nonlocal hydrodynamic effects need to be accounted for. The BIEs describing the interaction of light with the plasmonic waveguide are solved by the method of moments (MoM) algorithm. Two illustrative examples, the first one of which deals with the scattering problem, while the second computes the dispersion diagram of a plasmonic cylinder, are considered for both local and nonlocal cases. An excellent agreement is observed between the numerical and theoretical results, with the latter being derived from the generalized Mie theory. ispartof: IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION vol:67 issue:6 pages:3948-3960 status: published

We show strong coupling of a mid-infrared microscale resonator with ultra-localised plasmonic nanocavity modes. This enables new types of mixing between IR absorption and surface-enhanced Raman scattering (SERS), opening up new types of ultrasensitive molecular detection. Nanoscale localized optical cavities are self-assembled by depositing Au nanoparticles onto few μm metal discs with molecular spacers. Coupling between the microscale resonator and nanocavity in this nanoparticle-on-resonator (NPoR) scheme, reveals extreme near-field enhancements resulting in boosted SERS intensities. We anticipate that such near-field enhancements open new horizons in single-molecule photonic circuits and molecular optomechanics.

Subwavelength optical resonators and scatterers are dramatically expanding the toolset of the optical sciences and photonics engineering. By offering the opportunity to control and shape light waves in nanoscale volumes, recent developments using high-refractive-index dielectric scatterers gave rise to efficient flat-optical components such as lenses, polarizers, phase plates, color routers, and nonlinear elements with a subwavelength thickness. In this work, we take a deeper look into the unique interaction of light with rod-shaped amorphous silicon scatterers by tapping into their resonant modes with a localized subwavelength light source-an aperture scanning near-field probe. Our experimental configuration essentially constitutes a dielectric antenna that is locally driven by the aperture probe. We show how leaky transverse electric and magnetic modes can selectively be excited and form specific near-field distribution depending on wavelength and antenna dimensions. The probe's transmittance is furthermore enhanced upon coupling to the Fabry-Perot cavity modes, revealing all-dielectric nanorods as efficient transmitter antennas for the radiation of subwavelength emitters, in addition to constituting an elementary building block for all-dielectric metasurfaces and flat optics.

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.

Plasmonic nanostructures have demonstrated a remarkable ability to control light in ways never observed in nature, as the optical response is closely linked to their flexible geometric design. Due to lack of mirror symmetry, chiral nanostructures allow twisted electric field hotspots to form at the material surface. These hotspots depend strongly on the optical wavelength and nanostructure geometry. Understanding the properties of these chiral hotspots is crucial for their applications; for instance, in enhancing the optical interactions with chiral molecules. Here, the results of an elegant experiment are presented: by designing 35 intermediate geometries, the structure is enantiomorphed from one handedness to the other, passing through an achiral geometry. Nonlinear multiphoton microscopy is used to demonstrate a new kind of double-bisignate circular dichroism due to enantiomorphing, rather than wavelength change. From group theory, a fundamental origin of this plasmonic chiroptical response is proposed. The analysis allows the optimization of plasmonic chiroptical materials. V.K.V. acknowledges support from the Royal Society through the University Research Fellowships. Nuno Braz acknowledges support from the EPSRC grant reference EP/G037256/1. The authors would like to acknowledge the C2 project (C24/15/015) and the PDMK/14/126 project of KU Leuven, the FWO long term stay abroad project grant V405115N, and the Methusalem project funded by the Flemish government. The authors are grateful to Hongjian Tao, Deputy Director of Department of International Cooperation and Exchanges in the Ministry of Education of the People's Republic of China, for facilitating the collaboration between the UK, Chinese, and Belgian teams. All data supporting this study are openly available from the University of Bath data archive at https://doi.org/10.15125/BATH-00483.

Symmetry holds a prominent position in defining the optical response of a nanoantenna. In this work, we harness a mathematical tool, group representation theory, combine it with the eigenmode analysis for a nanoantenna, and illustrate how the symmetry allows or forbids the energetic coupling (i.e., interference) between a nanoantenna’s eigenmodes. We do this especially using a nanobar structure and a symmetric cross structure. Further, the consequence of symmetry-breaking is illustrated by an asymmetric cross-shaped nanostructure, i.e., a strong asymmetric Fano-type resonance line shape due to mode interference is observed with the physics behind elaborated. Both numerical and experimental evidences are provided.

With the development of nanotechnologies, researchers have brought the concept of antenna to the optical regime for manipulation of nano-scaled light matter interactions. Most optical nanoantennas optimize optical function, but are not electrically connected. In order to realize functions that require electrical addressing, optical nanoantennas that are electrically continuous are desirable. In this article, we study the optical response of a type of electrically connected nanoantennas, which we propose to call “dendritic” antennas. While they are connected, they follow similar antenna hybridization trends to unconnected plasmon phased array antennas. The optical resonances supported by this type of nanoantennas are mapped both experimentally and theoretically to unravel their optical response. Photoluminescence measurements indicate a potential Purcell enhancement of more than a factor of 58.

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.

In recent years, nanotechnology and nanoscience have been expanding their toolbox dramatically. Metallic nanoantennas – also known as plasmonic resonators – can be considered as one of these novel tools providing an effective route to couple photons in and out of nanoscale volumes. The higher the level of control over the way a nanoantenna interacts with light, the more effective this tool becomes and the further its applications will reach. Essential to this end is a detailed knowledge of such an antenna’s scattering characteristics. Especially in nanophotonics applications where every photon counts, one immediately benefits from directed photon routing for efficient photon collection. Recent studies have demonstrated that proper antenna designs can result in scattering patterns strongly deviating from the trivial dipole distribution, allowing one to route light in specific directions. [1–4] For instance, left-to-right directionality or unidirectional side scattering (i.e. preferential scattering in a single direction perpendicular to the incident wave) with only a single nanoparticle geometry, namely a V-antenna, was demonstrated by our group. [3, 4] Nevertheless, selectively steering photons at the nanoscale remains a fundamental challenge and most works are, unfortunately, lacking a clear rigorous analysis of the antenna behavior. This constrains the further development of more complex radiative functionality, such as for instance bi-directional scattering where photons of different energy are routed into opposite directions. Here, first, we elucidate the basic principles underlying directional plasmonic nanoantennas. By applying an eigenmode decomposition of full-field 3D simulations (Method of Moments and Finite Difference Time Domain) we are able to rigorously determine the underlying mode interferences that give rise to the experimentally observed directional behaviour. [4] An important result from this analysis is the conclusion that directional behaviour is strongly dependent on the type of antenna feed that is applied, an effect mostly ignored so far. For instance, plane wave excitation is directed in opposite direction from the radiation of a point dipole source. Next, the obtained design principles are applied to construct a bi-directional metallic nanoantenna with a single feed-gap. This means that, for instance, a mix of quantum emitters can be placed in the antenna gap and that the multi-colored emission can be de-multiplexed by beaming different spectral bands in opposite directions. Both the bi-directional scattering of a plane wave and bi-directional emission of local dipole emitters are experimentally verified by means of back-focal-plane microscopy. We demonstrate directional tunability by varying the antenna structural parameters, allowing further optimisation and spectral control. The obtained insight in how directional emission and scattering are generated and how different modes come together to form far-field properties of a nanoantenna device is indispensable to create new nanoscale optical devices for sub-wavelength color routing and self-referenced directional sensing. Moreover, we believe the same concepts are applicable to dielectric nanoantennas. [1] T. Kosako, et al. Nature Photonics (2010) 4(5), 312–315 [2] T. Shegai, et al. Nat. Comm. (2011) 2(481) [3] D. Vercruysse, et al. Nano Lett. (2013) 13 (8), 3843–3849 [4] D. Vercruysse, et al. ACS Nano (2014) 8(8), 8232−8241

The far field of a plasmonic nano dipole antenna topology is studied systematically. The important metals used in plasmonics are considered (silver, aluminum, chromium, gold, and copper), the length of the dipole is varied, and a very wide frequency band is considered. The results concern mainly radiation patterns, gains, and radiation efficiencies.

Scanning second harmonic generation (SHG) microscopy is becoming an important tool for characterizing nanopatterned metal surfaces and mapping plasmonic local field enhancements. Here we study G-shaped and mirror-G-shaped gold nanostructures and test the robustness of the experimental results versus the direction of scanning, the numerical aperture of the objective, the magnification, and the size of the laser spot on the sample. We find that none of these parameters has a significant influence on the experimental results.

In response to the incident light's electric field, the electron density oscillates in the plasmonic hotspots producing an electric current. Associated Ohmic losses raise the temperature of the material within the plasmonic hotspot above the melting point. A nanojet and nanosphere ejection can then be observed precisely from the plasmonic hotspots.

Recently, a great amount of research has been triggered by the prediction, formulated by Pendry and co-authors, that novel and enhanced nonlinear optical phenomena could be observed in metamaterials.[1] This prediction is based on the fact that, in metamaterials, local field enhancements can have a dramatic influence over the optical properties of the material. One of the largest contributions to such local field enhancements is attributable to surface plasmon resonances. Plasmons are collective oscillations of the electrons under the influence of light's electromagnetic field. Plasmons occur naturally on the surfaces of homogeneous metal films, where they usually dissipate quickly and cancel each other's influence. However, upon patterning the metal surface at the nanoscale, plasmons can be manipulated in a manner similar to classical waveguiding, whereby propagation or standing wave patterns can be achieved. In other words, artificial structuring allows for nanoengineering the position and intensity of the local fields. Incidentally, nonlinear optical effects, such as second-, third-, or forth-harmonic generation, scale as the second, third or fourth power of the electromagnetic intensity, respectively. Therefore, it is perfectly reasonable to assume that the large electromagnetic local field enhancements in metamaterials should yield large or previously unobserved nonlinear optical effects.

4–6 ] In such devices, light waves would be used instead of electrons. The possibility arises from the fact that light waves can couple to collective excitations of electrons at the surfaces of metallic nanostructures, a prop-erty referred to as surface plasmon resonance. Because these optically induced resonances occur at the surfaces and interfaces of the nanostructures, they can readily be investigated with a surface- and interface-specifi c optical technique, such as second-harmonic generation (SHG). SHG is a nonlinear optical technique that, within the dipole approximation, is forbidden in materials with a center of symmetry. Consequently, SHG is highly sensitive to regions with broken symmetry, such as surfaces (or interfaces), and it has been successfully applied to the study of plasmonic nano-materials with different geometries.

The interaction between individual plasmonic nanoparticles plays a crucial role in tuning and shaping the surface plasmon resonances of a composite structure. Here, we demonstrate that the detailed character of the coupling between plasmonic structures can be captured by a modified "circuit" model. This approach is generally applicable and, as an example here, is applied to a dolmen-like nanostructure consisting of a vertically placed gold monomer slab and two horizontally placed dimer slabs. By utilizing the full-wave eigenmode expansion method (EEM), we extract the eigenmodes and eigenvalues for these constituting elements and reduce their electromagnetic interaction to the structures' mode interactions. Using the reaction concept, we further summarize the mode interactions within a "coupling" matrix. When the driving voltage source imposed by the incident light is identified, an equivalent circuit model can be constructed. Within this model, hybridization of the plasmonic modes in the constituting nanostructure elements is discussed. The proposed circuit model allows the reuse of powerful circuit analysis techniques in the context of plasmonic structures. As an example, we derive an equivalent of Thévenin's theorem in circuit theory for nanostructures. Applying the equivalent Thévenin's theorem, the well-known Fano resonance is easily explained.

Novel ways to detect the handedness in chiral optical metamaterials by means of the second harmonic generation (SHG) process have recently been proposed. However, the precise origin of the SHG emission has yet to be unambiguously established. In this paper, we present computational simulations of both the electric currents and the electromagnetic fields in chiral planar metamaterials, at the fundamental frequency (FF), and discuss the implications of our results on the characteristics of experimentally measured SHG. In particular, we show that the results of our numerical simulations are in good agreement with the experimental mapping of SHG sources. Thus, the SHG in these metamaterials can be attributed to a strong local enhancement of the electromagnetic fields at the FF, which depends on the particular structure of the patterned metamaterial.

[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.