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In this communication, we propose a robust broadband anomalous refractor (BAR), which minimizes chromatic aberration. The design is based on engineering the dispersion curve of spoof surface plasmon polaritons (SSPPs) and manipulating the wavefront of the transmitted waves. We begin by deriving deflection conditions such that the discrete scatterers of the refractor need to have linear dispersion curves in the operating frequency band. The rich dispersion properties of the SSPP make it a good candidate for meeting the broadband deflection condition. Next, we design symmetric metallic blade structures (MBSs) to efficiently transform free-space waves into SSPPs and to realize the broadband linear dispersion relationship. The BAR is comprised of these MBSs with different strip heights. A prototype is designed, fabricated, and measured. Both the simulated and experimental results demonstrate excellent fixed-direction deflection in the 6.9–9.1 GHz band for the normal incidence case. Furthermore, the BAR still performs well when its outer profile is deformed to some extent, which is desirable in practical applications. This alternative approach to designing achromatic devices at microwave frequencies can be readily extended to higher frequencies.

This work illustrates the effect of nano-antenna shape on the efficiency of a complete nano-rectenna system for one specific geometry: the bowtie. The bowtie geometry is very well known for its wider band characteristics compared to the straight dipole. Based on full wave electromagnetic field solvers, the radiation efficiency and maximum matching efficiency for nano-bowties with different lengths and base angles are calculated. The results demonstrate that by adopting the bowtie topology, the radiation efficiency for an aluminum dipole can be improved from 51% to 61%, while the relative total efficiency can be improved from 46% to 57%, compared to the common straight nano-dipole mainly used in literature.

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.

Although many commercially available electromagnetic tools are conveniently used in RF and microwave applications, only a few of them provide the capability to analyze the optical response of nanometric radiators and scatterers. The assessment of their performance in the visible to near ultraviolet part of the electromagnetic (EM) spectrum becomes more and more important, considering the exponential rise of nanoscale systems. Since the accuracy of these numerical tools has not been fully investigated in literature, in this paper we essentially demonstrate a comparative study of the most widely used EM field solvers in the area of nano-plasmonics: COMSOL, CST and Lumerical. This is done through the investigation of the near and far field characteristics of basic canonical nanoparticles such as spheres, shells, cubes and cuboids, varying their sizes and constituting materials. The benchmarking results clearly show that at this moment not all EM field solvers offer the same accuracy. ispartof: Optics Express vol:25 issue:22 pages:26760-26780 ispartof: location:United States status: published

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

On the basis of the relationship between charge dimensionality and singular field behavior, it is proven that in a volumetric description of a volume current carrying topology, half rooftops of different binary hierarchical level are allowed without introducing numerical difficulties. This opens the possibility to use a very efficient multilevel hierarchical meshing scheme in a volumetric method-of-moments (V-MoM) algorithm. The new meshing scheme is validated by numerical calculations and experiments. It paves the way toward a much more efficient use of MoM in the description of arbitrarily shaped nanostructures at infrared and optical frequencies.

We have studied both theoretically and experimentally symmetric and asymmetric planar metallic Split Ring Resonators. We demonstrate that introducing structural asymmetry makes it possible to excite several higher order modes of both even (l = 2) and odd (l = 3, 5) order, which are otherwise inaccessible for a normally incident plane wave in symmetric structures. Experimentally we observe that the even mode resonances of asymmetric resonators have a quality factor 5.8 times higher than the higher order odd resonances. ispartof: Optics Express vol:21 issue:13 pages:15464-15474 ispartof: location:United States status: published

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.

Circularly polarized light imparts a sense of rotation on the electron density in ring-shaped gold nanostructures. As a consequence, the near-field enhancement becomes homogeneous on the surface of the nanostructures, thereby increasing the opportunity for interaction with molecules. This type of nanostructured samples can find a broad range of applications in chemical processes where the interaction between molecules and local field enhancements play an important role. © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. ispartof: Advanced Materials vol:24 issue:35 pages:OP208-OP215 ispartof: location:Germany status: published

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

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.

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.

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.

We experimentally and by means of FDTD simulations and eigenmode analysis demonstrate unidirectional side scattering of a plane wave and unidirectional emission from photo luminescent molecules mediated by a V-shaped single-element metallic nanoantenna.