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In light of the emergence of nonclassical effects, a paradigm shift in the conventional macroscopic treatment is required to accurately describe the interaction between light and plasmonic structures with deep-nanometer features. Towards this end, several nonlocal response models, supplemented by additional boundary conditions, have been introduced, investigating the collective motion of the free electron gas in metals. The study of the dipole-excited core-shell nanoparticle has been performed, by employing the following models: the hard-wall hydrodynamic model; the quantum hydrodynamic model; and the generalized nonlocal optical response. The analysis is conducted by investigating the near and far field characteristics of the emitter-nanoparticle system, while considering the emitter outside and inside the studied topology. It is shown that the above models predict striking spectral features, strongly deviating from the results obtained via the classical approach, for both simple and noble constitutive metals. ispartof: Nanoscale Advances vol:4 issue:10 ispartof: location:England status: Published online

Determining the chirality of molecules and nanoparticles often relies on circular dichroism and optical rotation: two chiral optical (chiroptical) effects in the linear optical regime. Although these linear effects are weak compared to nonlinear chiroptical effects, they have the advantage of being measured in isotropic liquids - free from the complications of anisotropy. Recently, a nonlinear effect: hyper-Rayleigh scattering optical activity (HRS OA) has been shown to reliably distinguish between the two chiral forms of Ag nanohelices, suspended in isotropic liquids. However, this first demonstration of HRS OA also opened new questions. For instance, at a fundamental level, it is not clear what the role of interactions between nanoparticles is. Moreover, the influence of the ultrafast pulse chirp is unknown. Here, we demonstrate HRS OA from well below two Ag nanohelices in the illumination volume, precluding any interactions. Additionally, we performed the first measurements of HRS depolarization ratios in this system and find a value of ≈1. We also show that HRS is highly robust against the chirp of the ultrafast pulses. An important reason for the strong (down to single nanohelix) sensitivity of our experiments is the large chiroptical interaction at the fundamental frequency; this point is illustrated with two sets of numerical simulations of the electromagnetic near-fields. Our results highlight HRS OA as a highly sensitive experimental method for characterization of chiral solutions/suspensions, in tiny illumination volumes. ispartof: NANOSCALE vol:14 issue:10 pages:3888-3898 ispartof: location:England 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.

Recent progress in nanofabrication has redrawn the boundaries of the applicability of chiroptical (chiral optical) effects. Chirality, often expressed as a twist in biomolecules, is crucial for pharmaceuticals, where it can result in extremely different chemical properties. Because chiroptical effects are typically very weak in molecules, plasmonic nanomaterials are often proposed as a promising platform to significantly enhance these effects. Unfortunately, the ideal plasmonic nanomaterial has conflicting requirements: its chirality should enhance that of the chiral molecules and yet it should have no chiroptical response on its own. Here, we propose a unique reconciliation to satisfy the requirements: a racemic plasmonic nanomaterial, consisting of equal amounts of opposite chiral unit cells. We show how diffraction spectroscopy can be used to unveil the presence of chirality in such racemic nanogratings in the far-field. Our experiments are supported by numerical simulations and yield a circular intensity difference of up to 15%. The physical origin is demonstrated by full wave simulations in combination with a Green's function – group-theory-based analysis. Contributions from Circular Dichroism in the Angular Distribution of Photoelectrons (CDAD) and pseudo/extrinsic chirality are ruled out. Our findings enable the far-field measurement and tuning of racemic nanomaterials, which is crucial for hyper-sensitive chiral molecular characterization.

ispartof: ADVANCED THEORY AND SIMULATIONS vol:2 issue:9 status: published

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.

This research was supported by UK Engineering and Physical Sciences Research Council grants EP/G060649/1 and EP/L027151/1 , and ERC grant LINASS 320503 . F.B. thanks the supports from the Winton Programme for the Physics of Sustainability. Publisher PDF

Since deep nanoscale systems are increasingly studied, accurate macroscopic theories dealing with quantum mechanical effects are in high demand. Concerning the electromagnetic response of nanoplasmonic systems, several hydrodynamic models have been proposed, each introducing an additional boundary condition (ABC) to describe the behavior of the plasmonic electron cloud. Four hydrodynamic approaches with four different boundary conditions are compared: the hard wall hydrodynamic model with the Sauter ABC, the curl-free hydrodynamic model with the Pekar ABC, the shear forces hydrodynamic model with the specular reflection ABC, and the quantum hydrodynamic model with the corresponding ABC. This is done by investigating near- and far-field features of a metallic nanosphere. It is shown that different hydrodynamic models may result in an entirely different prediction of the nanoparticle’s response. These models are validated by using other local and nonlocal response models and experimental results. ispartof: Advanced Theory and Simulations vol:1 issue:12 status: published

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.

A low profile dual-band antenna based on the magneto-electric dipole concept is investigated on-body. An efficient space-utilization configuration of the conductive textiles and substrate layers ensured the compactness of the structure for on-body application. The two U slots in the dipole combine the electric and magnetic resonances to produce a dual band behavior and wideband characteristic in each band, as validated via simulations and measurements. Low backward radiations in both the lower and the upper bands are observed, rendering it insensitive to the coupling to the human body. To validate this, the antenna is first simulated in a bent configuration using a simplified, cylindrically-shaped human phantom and was found to be robust. It was then simulated on a detailed voxel model to assess its bent performance. Besides the slightly degraded reflection performance, the studied specific absorption rate (SAR) is proven to be well below the European limit of 2 W/kg.

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.

Following the impact of a single femtosecond light pulse on nickel nanostripes, material deformations—or “nanobumps”—are created. We have studied the dependence of these nanobumps on the length of nanostripes and verified the link with plasmons. More specifically, local electric currents can melt the nanostructures in the hotspots, where hydrodynamic processes give rise to nanobumps. This process is further confirmed by independently simulating local magnetic fields, since these are produced by the same local electric currents.

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.

In high definition mapping of the plasmonic patterns on the surfaces of nanostructures, the diffraction limit of light remains an important obstacle. Here we demonstrate that this diffraction limit can be completely circumvented. We show that upon illuminating nanostructures made of nickel and palladium, the resulting surface-plasmon pattern is imprinted on the structures themselves; the hotspots (regions of local field enhancement) are decorated with overgrowths, allowing for their subsequent imaging with scanning-probe techniques. The resulting resolution of plasmon pattern imaging is correspondingly improved. ispartof: Physical Review Letters vol:106 issue:22 ispartof: location:United States status: published

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.

While it has been demonstrated that, above its resolution limit, Second Harmonic Generation (SHG) microscopy can map chiral local field enhancements, below that limit, structural defects were found to play a major role. Here we show that, even below the resolution limit, the contributions from chiral local field enhancements to the SHG signal can dominate over those by structural defects. We report highly homogeneous SHG micrographs of star-shaped gold nanostructures, where the SHG circular dichroism effect is clearly visible from virtually every single nanostructure. Most likely, size and geometry determine the dominant contributions to the SHG signal in nanostructured systems. (C) 2011 Optical Society of America ispartof: Optics Express vol:20 issue:1 pages:256-264 ispartof: location:United States status: published

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.