Search

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.

Background Diabetes has severe impacts on the health of patients. The differentiation of mesenchymal stem cells (MSCs) into islet-like cell clusters (ICCs) is an effective protocol for the treatment of diabetes. microRNAs (miRs) regulate multiple cellular processes including cell differentiation. This study sought to identify the mechanism of miR-365-3p in the differentiation of bone marrow MSCs (bMSCs) into ICCs. Methods Initially, the differentiation of bMSCs into ICCs was induced. Then, the miR-365-3p expression pattern in the bMSCs and ICCs was detected. Next, the miR-365-3p expression pattern was silenced in bMSCs to assess the effect on differentiation efficiency and measure the expressions of ICC marker genes during the differentiation of bMSCs into ICCs. The miR-365-3p downstream target genes were predicted and verified. Paired box protein 6 (Pax6) was downregulated in bMSCs with silenced miR-365-3p to evaluate the differentiation of bMSCs into ICCs. Furthermore, the Pax6 downstream pathway was evaluated. Results The differentiation of bMSCs into ICCs was successfully induced. The miR-365-3p expression in bMSCs was higher than that in ICCs. miR-365-3p downregulation in bMSCs facilitated the differentiation of bMSCs into ICCs, as evidenced by elevated releases of insulin and C-peptide in ICCs and elevated expressions of ICC marker genes. Our findings denoted that miR-365-3p targeted Pax6. Inhibition of Pax6 expression annulled the promotion of miR-365-3p downregulation on the differentiation of bMSCs into ICCs. Increased phosphorylation levels of MEK and ERK were identified in ICCs after downregulation of miR-365-3p however they were decreased after downregulation of Pax6. Conclusions This study supported that miR-365-3p inhibited the differentiation of bMSCs into ICCs via targeting Pax6 and inhibiting the MEK/ERK pathway.

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.

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

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 this paper, a new way is introduced to define the concept of recoverable energy for general radiating structures. We also show that this recoverable energy can be maximized, that is, the maximum energy can be extracted from a certain electromagnetic field distribution by minimizing the radiative energy. This maximum recoverable energy obviously never is negative and it is totally coordinate independent. A loop antenna is employed to demonstrate the notions introduced.

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

A wearable UWB antenna is proposed and analyzed in this paper, which is totally fabricated with textile materials, except the feeding connector for testing. By implementing a multilayered structure, the antenna obtains a good matching over the whole UWB band while keeping its low profile. The performance of the antenna is unaffected when worn on the human body due to the shielding by the full ground plane placed under the patch. Besides that, high on-body fidelity and a robust performance are produced when the antenna is studied under several types of deformations.

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.

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.

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.

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.

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.

Clustering is one of the most important and well studied fields of data mining. Many clustering methods have been proposed in the last few decades with different backgrounds. An open problem in clustering field is the lack of a benchmark for contrasting all these algorithms as well as other available ones. In this paper, we use the well-known log marginal likelihood (LML) score and classification accuracy as two criteria of comparison. Experiment results on k-means, expectation maximization (EM) and farthest-first in the t-test show that EM outperforms other two algorithms on most of the benchmark data sets with respect to both criteria. Considering the performance and comprehensibility of EM, it is an ideal model that could be used in many real world applications

Recently, combining Naive-Bayes with the expectation maximization (EM) algorithm for unsupervised learning have received significant attention. AutoClass is a classical Bayesian clustering algorithm that uses Naive-Bayes in combination with EM algorithm to find the probability distribution parameters to best fit the data. In this study, we introduce a robust approach, which is similar to AutoClass, it can arbitrarily impose any Bayesian classifiers in combination with EM algorithm to enhance cluster's performance. This paper focuses on how clustering techniques can benefit from classification. We provide experimental evidence that more accurate than original results of clustering in the t-test on most of the benchmark data sets

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.

[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.

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.