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The 193 nm deep UV immersion lithography is leveraged to fabricate highly dense and uniform arrays of Au-capped Si nanopillars on a 300 mm wafer level, and the substrates are applied in surface enhanced Raman spectroscopy for reliable molecule detection. Due to the sub-10 nm gap sizes and ultra-high array density with the lattice constant less than 100 nm, our nanopillar based substrates outperform the current commercial products in terms of the signal intensity, reproducibility and fabrication scale. crosscheck: This document is CrossCheck deposited related_data: Supplementary Information copyright_licence: The Royal Society of Chemistry has an exclusive publication licence for this journal copyright_licence: The accepted version of this article will be made freely available after a 12 month embargo period history: Received 29 July 2014; Accepted 27 August 2014; Accepted Manuscript published 2 September 2014; Advance Article published 18 September 2014; Version of Record published 9 October 2014 ispartof: Nanoscale vol:6 issue:21 pages:12391-12396 ispartof: location:England status: published

In the investigation of the sensing performance based on localized surface plasmon resonance (LSPR), the bulk sensitivity, which is the spectral shift per refractive index unit upon the change of the surrounding index to spatial infinite, is often measured and considered as the indication of the sensing ability. To the contrary, in biosensing applications only the refractive index in a confined region close to the metal nanostructure surface is altered due to the attachment of the recognition and target molecules. The correlation between the bulk and surface sensitivity is nevertheless ambiguous, especially in strongly coupled plasmonic systems, and rarely discussed in the literature. In this paper, we examine the bulk and surface sensing performance of periodic Au nanodisk arrays on quartz substrates. By means of diffractive coupling of LSPR in the periodic arrays, the bulk sensitivity and the figure of merit (FoM, as high as ~30) could be varied by engineering the coupling strength and adjusting the size of the Au nanodisk and the array pitch. The surface sensing performance is also explored by sequential atomic layer deposition of Al2O3, and the electric field decay from the Au nanodisk surface could thus be extracted using an exponential function. It is demonstrated that in these substrates the surface and bulk sensitivity have an opposite dependence on the coupling strength. In spite of the high bulk sensitivity and FoM in systems with strong coupling, a low surface sensitivity is demonstrated due to the large electric field decay length. Therefore, detailed and careful design of the coupling strength and the surface electric field to match the size of the target biomolecule is very critical for the best sensing performance. Moreover, we provide a more reliable biosensor design protocol based on the surface sensing performance.

We present the experimental observation of spectral lines of distinctly different shapes in the optical extinction cross-section of metallic nanorod antennas under near-normal plane wave illumination. Surface plasmon resonances of odd mode parity present Fano interference in the scattering cross-section, resulting in asymmetric spectral lines. Contrarily, modes with even parity appear as symmetric Lorentzian lines. Finite element simulations are used to verify the experimental results. The emergence of either constructive or destructive mode interference is explained with a semianalytical 1D line current model. This simple model directly explains the mode-parity dependence of the Fano-like interference. Plasmonic nanorods are widely used as half-wave optical dipole antennas. Our findings offer a perspective and theoretical framework for operating these antennas at higher-order modes. © 2014 American Chemical Society., J.A.S.-G. and R.P.-D. acknowledge the Spanish “Ministerio de Economia y Competitividad ́ ”, through the Consolider-Ingenio project EMET (CSD2008-00066) and NANOPLAS+ (FIS2012-31070), and the “Comunidad de Madrid” (grant MICROSERES II P2009/TIC-1476), for financial support. R.P.-D. also acknowledge support from the European Social Fund and CSIC through a JAE-Pre grant.

We study the influence of artificial pinning centers on the vortex critical velocity in Al thin films deposited on top of a periodic array of Permalloy (FeNi) square rings. We demonstrate that the field dependence of the flux flow velocity strongly depends on the particular magnetic state of the rings. In particular, we find that, even when the rings are in a flux closure state, i.e. with little stray field, the vortex critical velocity shows a non-monotonic magnetic field dependence. This behaviour is in sharp contrast with the results obtained in a reference plain film, with no rings underneath. A comparison with the intrinsic strong pinning Nb films previously studied, suggests an interpretation in terms of a channel-like motion of vortices, here induced by the artificial pinning structure. (C) 2010 Elsevier B.V. All rights reserved. ispartof: Physica c-superconductivity and its applications vol:470 issue:19 pages:904-906 ispartof: location:GREECE, Rhodes status: published

The design of many promising, newly emerging classes of photonic metamaterials and subwavelength confinement structures requires detailed knowledge and understanding of the electromagnetic near-field interactions between their building blocks. While the electric field distributions and, respectively, the electric interactions of different nanostructures can be routinely measured, for example, by scattering near-field microscopy, only recently experimental methods for imaging the magnetic field distributions became available. In this paper, we provide direct experimental maps of the lateral magnetic near-field distributions of variously shaped plasmonic nanoantennas by using hollow-pyramid aperture scanning near-field optical microscopy (SNOM). We study both simple plasmonic nanoresonators, such as bars, disks, rings and more complex antennas. For the studied structures, the magnetic near-field distributions of the complex resonators have been found to be a superposition of the magnetic near-fields of the individual constituting elements. These experimental results, explained and validated by numerical simulations, open new possibilities for engineering and characterization of complex plasmonic antennas with increased functionality.

Plasmonic resonators are nanosized metallic antennas that convert electromagnetic waves at optical frequencies into localized fields, providing an effective route to couple photons in and out of nanoscale volumes. This unique ability makes these nanostructures excellent tools to study and manipulate light-matter interaction at the nanoscale. The strong coupling of a plasmonic resonator to light, resulting in optical cross-sections of more than 10 times the particle's physical size, is driven by collective oscillations of the conduction electrons in the metal - the so-called surface plasmon resonances.

Unidirectional side scattering of light by a single-element plasmonic nanoantenna is demonstrated using full-field simulations and back focal plane measurements. We show that the phase and amplitude matching that occurs at the Fano interference between two localized surface plasmon modes in a V-shaped nanoparticle lies at the origin of this effect. A detailed analysis of the V-antenna modeled as a system of two coherent point-dipole sources elucidates the mechanisms that give rise to a tunable experimental directivity as large as 15 dB. The understanding of Fano-based directional scattering opens a way to develop new directional optical antennas for subwavelength color routing and self-referenced directional sensing. In addition, the directionality of these nanoantennas can increase the detection efficiency of fluorescence and surface enhanced Raman scattering. ispartof: Nano Letters vol:13 issue:8 pages:3843-3849 ispartof: location:United States status: published

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

As-prepared and heat-treated oxyfluoride glasses, co-doped with Ag nanoclusters/nanoparticles, are prepared at 0.15 at. % Ag concentration. The as-prepared glass shows an absorption band in the UV/violet attributed to the presence of amorphous Ag nanoclusters with an average size of 1.1 nm. The luminescence spectra of the untreated glass can also be ascribed to these Ag nanoclusters. Upon heat-treatment, the clusters coalesce into Ag nanoparticles with an average size of 2.3 nm, and the glasses show an extra surface plasmon absorption band in the visible. These particles, however, cease to emit due to ascribing plasmonic properties of bulk silver.

Due to the relationship between charge dimensionality and singular field behavior, the stand-alone half rooftop is allowed to exist in the volumetric description of a topology carrying a three dimensional current. This leads to the possibility of using multi-level meshing to describe nanotopologies. The meshing scheme is validated by numerical calculations and experiments.

The formulation of Integral Equations for a problem, to be solved with the Method of Moments, has been a cornerstone in the field of computational electromagnetics for components and antennas in the microwave and millimeter wave frequency range for decades. In this invited paper, the applicability, advantages and disadvantages of this method are discussed in view of applying it at much higher frequencies, namely in the IR and optical frequency range. This invited presentation is based on an overview paper on this topic recently published.

In superconducting films under an applied dc current, we analyze experimentally and theoretically the influence of engineered pinning on the vortex velocity at which the flux-flow dissipation undergoes an abrupt transition from low to high resistance. We argue, based on a nonuniform distribution of vortex velocity in the sample, that in strongly disordered systems the mean critical vortex velocity for flux-flow instability (i) has a nonmonotonic dependence on magnetic field and (ii) decreases as the pinning strength is increased. These findings challenge the generally accepted microscopic model of Larkin and Ovchinnikov (1979 J. Low. Temp. Phys. 34 409) and all subsequent refinements of this model which ignore the presence of pinning centers. ispartof: New Journal of Physics vol:14 issue:053006 pages:1-11 status: published

Computational electromagnetics has become a real cornerstone in the design of microwave and millimeter wave components and systems. One of the major computational techniques used in these designs is to solve integral equations with the Method of Moments (MoM). Traditionally, MoM has been used extensively for antennas, transmission lines, filters, couplers, etc., mainly in the MHz and GHz range. However, MoM can also be applied at much higher frequencies. It will be shown that at these frequencies, this computational technique is just emerging. One of the reasons is that there are serious differences with the classical implementations, which prohibits the direct use of the well-known commercially available MoM solvers. For example, one of the differences is the fact that the concept of a surface current, confined to a very thin layer at the surface of a conductor, is not valid any more. In this presentation, the different computational techniques will be reviewed and compared, not only in the traditional microwave and millimeter wave range, but especially at THz, IR, and optical frequencies. In concreto, the MAGMAS MoM solver will be compared to a numerical FDTD solver well-known in the plasmonics research community: Lumerical. It will be shown that also at these very high frequencies, MoM has its proper place in the pallet of computational algorithms.

Solving Integral Equations with the Method of Moments has been a cornerstone in the field of computational electromagnetics for components and antennas in the microwave frequency range. In this invited paper, the applicability, advantages and disadvantages of this method are discussed in view of applying it at much higher frequencies, namely in the IR and optical frequency range. This invited presentation is based on an overview paper on this topic recently published.

[1] In the last 40 years the method of moments (MOM) has been a cornerstone in the field of computational electromagnetics. Traditionally, this method has been used to solve integral equations formulated for antennas and other components in the microwave frequency range and below. In this paper, the application of MOM in the field of plasmonics is briefly reviewed. First, existing literature is referenced. Then the differences with the classical implementations are pointed out. Finally, its applicability, advantages, and disadvantages are discussed. This is done by comparing it with a numerical finite difference time domain solver well-known in the plasmonics research community for a number of example structures. It is shown that also at these higher frequencies, namely in the IR and optical range, MOM is a very powerful technique.

In the last 40 years the Method of Moments has been a cornerstone in the field of computational electromagnetics for antennas and other components in the microwave frequency range. In this paper, the applicability, advantages and disadvantages of this method are discussed in view of applying it at much higher frequencies, namely in the field of plasmonics.

We present a new electromagnetic phenomenon—the asymmetric second-harmonic generation from planar chiral structures. The effect consists in distinguishing the handedness of a chiral material by rotating the sample in an experiment involving solely linearly polarized light. This phenomenon originates in the surface plasmon resonance of chiral gold nanostructures, where homodyne interference of anisotropic and chiral electric and/or magnetic multipoles appears to play an important role. The remarkable expansion of nanoscience and nanotechnology during the past decade has led to strong interest in the investigation of nanoscale optical fields and the development of tools for their study. Consequently, the optical properties of metallic nanostructures are of great current significance from both fundamental and practical points of view [1]. One of the most important effects in light scattering by these nanostructures is the enhancement, up to several orders of magnitude, of the field intensity, which occurs in a confined nanoscale region around the particles. This enhancement originates from a combination of the electrostatic lightning-rod effect, due to the geometric singularity of sharp curvatures, and localized surface plasmon resonances. The latter are a consequence of the metal nanoparticle’s ability to build up structural resonances of the collective oscillations of the metal electron plasma. The resonances depend on the particle’s geometry and dielectric properties, as well as on the refractive index of its surroundings. Second-harmonic generation (SHG) is known to be surface and interface sensitive on the atomic scale. Therefore, its application to the study of the nonlinear optical response of metal nanostructures, with their pronounced optical resonances and high surface-to-volume ratio, is particularly suitable for improving the insight into the relationship between optical properties and morphology at the nanoscale. Symmetry issues are closely related to the polarization dependence of the nonlinear optical responses since, within the electric dipole approximation, SHG is forbidden in materials with inversion symmetry. Planar nanostructures are particularly interesting because, using modern lithography techniques, dielectric and metallic forms of complicated geometry can be created, including planar chiral designs [2,3]. The types of structures have recently

Subradiant and superradiant plasmon modes in concentric ring/disk nanocavities are experimentally observed. The subradiance is obtained through an overall reduction of the total dipole moment of the hybridized mode due to antisymmetric coupling of the dipole moments of the parent plasmons. Multiple Fano resonances appear within the superradiant continuum when structural symmetry is broken via a nanometric displacement of the disk, due to coupling with higher order ring modes. Both subradiant modes and Fano resonances exhibit substantial reductions in line width compared to the parent plasmon resonances, opening up possibilities in optical and near IR sensing via plasmon line shape design.

We show experimentally the successful use of colloidal lithography for the fabrication of negative index metamaterials in the near-infrared wavelength range. In particular, we investigated a specific implementation of the widely studied double fishnet metamaterials, consisting of a gold-silica-gold layer stack perforated by a hexagonal array of round holes. Tuning of the hole diameter allows us to tailor these self-assembled metamaterials both as single- ({\epsilon} < 0) and double ({\epsilon} < 0 and {\mu} < 0) negative metamaterials.

Plasmonic nanostructures can serve as unit cells of new types of optical metamaterials with carefully engineered optical properties. For example, hybridized plasmonic systems consisting of several metallic subunits separated by nanoscale gaps can exhibit superradient and subradient modes due to dipolar coupling between the individual units. Additionally, in nanostructures with broken symmetry, Fano resonances can arise due to the interaction of narrow dark modes with broad bright modes. We have recently identified non-concentric ring/disk cavities as a system with a highly tunable Fano resonance and exceptionally large refractive index sensitivity and localized surface plasmon resonance figure of merit [1]. Here we present an experimental demonstration of Fano resonances in two plasmonic systems: firstly a dimer/monomer slab slab structure (“dolmen”), first proposed by Zhang et al [2], and secondly for a side-by-side arrangement of a Au ring and a Au disk (ring near disk cavity, RNDC), both fabricated using e-beam lithography. We analyze the dependence of the Fano lineshapes on the polarization of the incident light, and of the strength of the feature on the separation between individual nanoscale sub-units. We further show that for significantly broad dipolar modes, multiple Fano resonances can arise. The observed Fano-type dispersive features constitute a classical analogue [3] to the well-known phenomenon of electromagnetically induced transparency (EIT).

We observe the appearance of Fano resonances in the optical response of plasmonic nanocavities due to the coherent coupling between their superradiant and subradiant plasmon modes. Two reduced-symmetry nanostructures probed via confocal spectroscopy, a dolmen-style slab arrangement and a ring/disk dimer, clearly exhibit the strong polarization and geometry dependence expected for this behavior at the individual nanostructure level, confirmed by full-field electrodynamic analysis of each structure. In each case, multiple Fano resonances occur as structure size is increased.

We study the vortex structure in a Pb film deposited on top of a periodic array of ferromagnetic square microrings by combining two high resolution imaging techniques: Bitter decoration and scanning Hall probe microscopy (SHPM). The periodicity and strength of the magnetic pinning potential generated by the square microrings are controlled by the magnetic history of the template. When the square rings are in the magnetized dipolar state, known as the onion state, the strong stray field generated at the domain walls prevents the decoration of vortices. SHPM images show that the stray field generated by the dipoles is much stronger than the vortex field in agreement with the results of simulations. Real space vortex imaging has revealed that, in the onion state, the corners of the square rings act as effective pinning centers for vortices., Comment: To be published in Phys. Rev. B

Design principles and implementations of plasmonic nanocavities sustaining sub-radiant and Fano-type modes are described. These structures show high sensing figures of merit and could form a basis for nanoscale optical sensors and metamaterials.

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.

We study the flux flow state in superconducting materials characterized by rather strong intrinsic pinning, such as Nb, NbN, and nanostructured Al thin films, in which we drag the superconducting dissipative state into the normal state by current biasing. We modify the vortex pinning strength either by ion irradiation, by tuning the measuring temperature or by including artificial pinning centers. We measure critical flux flow voltages for all materials and the same effect is observed: switching to low flux flow dissipations at low fields for an intermediate pinning regime. This mechanism offers a way to additionally promote the stability of the superconducting state. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4718309]

A metallic nanocross geometry sustaining broad dipole and sharp higher order localized surface plasmon resonances is investigated. Spectral tunability is achieved by changing the cross arm length and the angle between the arms. The degree of rotational symmetry of the nanocross is varied by adding extra arms, changing the arm angle and shifting the arm intersection point. The particle's symmetry is shown to have a crucial influence on the plasmon coupling to incident radiation. Pronounced dipole, quadrupole, octupole and Fano resonances are observed in individual cross structures. Furthermore, the nanocross geometry proves to be a useful building block for coherently coupled plasmonic dimers and trimers where the reduced symmetry results in hybridized subradiant and superradiant modes and multiple Fano interferences. Finite difference time domain calculations of absorption and scattering cross-sections as well as charge density profiles are used to reveal the nature of the different plasmon modes. Experimental spectra for the discussed geometries support the calculations.

We study the spectral properties of a triangular plasmonic nanoslit-cavity with periodic triangular grooves to optimize the field enhancement inside the nanoslit. This work is mainly based on numerical calculations and also partly supported by experimental evidence. In the nanoslit-cavity, we can distinguish following three main contributions to the field enhancement: electrostatic interaction in the nanoslit, surface plasmon polariton standing waves in the cavity and excitation, and reflection of surface plasmon polaritons by the grating. The importance of phase matching between surface plasmons generated at the nanoslit and the gratings is also investigated in order to optimize the local field intensity in the nanoslit.

We present evidence of magnetically controlled guided vortex motion in a hybrid superconductor/ferromagnet nanosystem consisting of an Al film on top of a square array of permalloy square rings. When the rings are magnetized with an in-plane external field H, an array of point-like dipoles with moments antiparallel to H, is formed. The resulting magnetic template generates a strongly anisotropic pinning potential landscape for vortices in the superconducting layer. Transport measurements show that this anisotropy is able to confine the flux motion along the high symmetry axes of the square lattice of dipoles. This guided vortex motion can be either re-routed by 90 degrees by simply changing the dipole orientation or even strongly suppressed by inducing a flux-closure magnetic state with very low stray fields in the rings.

We catalog known optical moir\'e lattices and uncover exotic lattice configurations following a geometric analog of the ancient sieve of Eratosthenes algorithm for finding prime numbers. Rich dynamics of Bose-Einstein condensates loaded into these optical lattices is revealed from numerical simulations of time-of-flight interference patterns. What sets this method apart is the ability to tune the periodicity of the optical lattices without changing the wavelength of the laser, yet maintaining the local potential at the individual lattices sites. In addition, we discuss the ability to spatially translate the optical lattice through applying a structured phase only.

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.

Optical antennas transform light from freely propagating waves into highly localized excitations that interact strongly with matter. Unlike their radio frequency counterparts, optical antennas are nanoscopic and high frequency, making amplitude and phase measurements challenging and leaving some information hidden. Here we report a novel spectral interferometric microscopy technique to expose the amplitude and phase response of individual optical antennas across an octave of the visible to near-infrared spectrum. Although it is a far-field technique, we show that knowledge of the extinction phase allows quantitative estimation of nanoantenna absorption, which is a near-field quantity. To verify our method we characterize gold ring-disk dimers exhibiting Fano interference. Our results reveal that Fano interference only cancels a bright mode’s scattering, leaving residual extinction dominated by absorption. Spectral interference microscopy has the potential for real-time and single-shot phase and amplitude investigations of isolated quantum and classical antennas with applications across the physical and life sciences., Absorption by an optical nanoantenna determines its interaction strength with light, yet this quantity is hidden from conventional spectroscopy. Gennaro et al. now demonstrate a spectroscopic technique that reveals a nanoantenna’s absorption by recovering its amplitude and phase response.

Plasmonic nanostructures can serve as model systems for a variety of fascinating classical oscillator physics. For example, in nanostructures with broken symmetry, Fano resonances can arise due to the interaction of narrow dark modes with broad bright modes. We have recently identified non-concentric ring/disk cavities as a system with a highly tunable Fano resonance and exceptionally large refractive index sensitivity and localized surface plasmon resonance figure of merit [1]. Using numerical electromagnetic simulations, Zhang and co-workers have further shown that for dolmen-shaped slab structures with optimized geometries a full plasmonic-induced optical transparency can develop [2]. This constitutes a classical analogue [3] to the well-known phenomenon of electromagnetically induced transparency (EIT). Here we present an experimental demonstration of Fano resonances in two plasmonic systems: firstly the dimer/monomer dolmen slab structure proposed by Zhang et al [2], and secondly for a side-by-side arrangement of a Au ring and a Au disk (ring near disk cavity, RNDC), both fabricated using e-beam lithography. We analyze the dependence of the Fano lineshapes on the polarization of the incident light, and of the strength of the feature on the separation between individual nanoscale sub-units. We further show that for significantly broad dipolar modes, multiple Fano resonances can arise.

We explore the use of inverse design methods for the generation of periodic optical patterns in photonic integrated circuits. A carefully selected objective function based on the integer lattice method, which is an algebraic technique for optical lattice generation, is shown to be key for successful device design. Furthermore, we present a polychromatic pattern generating device that switches between optical lattices with different symmetry and periodicity depending on the operating wavelength. Important links are drawn between optical coherent lattices and optical potentials, pointing towards practical applications in the fields of quantum simulations and computing, optical trapping, and bio-sensing. ispartof: OPTICS EXPRESS vol:30 issue:7 pages:11384-11393 ispartof: location:United States status: published