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  • Conference paper
    Colombi A, Roux P, Miniaci M, Craster R, Guenneau S, Gueguen Pet al., 2017,

    The role of large scale computing behind the development of seismic (and elastic) metamaterials.

    , 2017 11th International Congress on Engineered Material Platforms for Novel Wave Phenomena (METAMATERIALS), Publisher: IEEE, Pages: 406-408
  • Journal article
    Colombi A, Ageeva V, Smith RJ, Clare A, Patel R, Clark M, Colquitt D, Roux P, Guenneau S, Craster RVet al., 2017,

    Enhanced sensing and conversion of ultrasonic Rayleigh waves by elastic metasurfaces

    , Scientific Reports, Vol: 7, ISSN: 2045-2322

    Recent years have heralded the introduction of metasurfaces that advantageously combine the vision of sub- wavelength wave manipulation, with the design, fabrication and size advantages associated with surface excitation. An important topic within metasurfaces is the tailored rainbow trapping and selective spatial frequency separation of electromagnetic and acoustic waves using graded metasurfaces. This frequency dependent trapping and spatial frequency segregation has implications for energy concentrators and associated energy harvesting, sensing and wave filtering techniques. Different demonstrations of acoustic and electromagnetic rainbow devices have been performed, however not for deep elastic substrates that support both shear and compressional waves, together with surface Rayleigh waves; these allow not only for Rayleigh wave rainbow effects to exist but also for mode conversion from surface into shear waves. Here we demonstrate experimentally not only elastic Rayleigh wave rainbow trapping, by taking advantage of a stop-band for surface waves, but also selective mode conversion of surface Rayleigh waves to shear waves. These experiments performed at ultrasonic frequencies, in the range of 400-600 kHz, are complemented by time domain numerical simulations. The metasurfaces we design are not limited to guided ultrasonic waves and are a general phenomenon in elastic waves that can be translated across scales.

  • Journal article
    O'Neill J, Selsil O, Haslinger SG, Movchan NV, Craster RVet al., 2017,


    , SIAM Journal on Applied Mathematics, Vol: 77, Pages: 1115-1135, ISSN: 0036-1399

    This paper considers active cloaking of a square array of evenly spaced pins in a Kirchhoff plate in the presence of flexural waves. Active sources, modeled as ideal point sources, are represented by the nonsingular Green's function for the two-dimensional biharmonic operator and have an arbitrary complex amplitude. These sources are distributed exterior to the cluster, and their complex amplitudes are found by solving an algebraic system of equations. This procedure ensures that selected multipole orders of the scattered field are successfully annulled. For frequencies in the zero-frequency stop band, we find that a small number of active sources located on a grid is sufficient for cloaking. For higher frequencies, we achieve efficient cloaking with the active sources positioned on a circle surrounding the cluster. We demonstrate the cloaking efficiency with several numerical illustrations, considering key frequencies from band diagrams and dispersion surfaces for a Kirchhoff plate pinned in a doubly periodic fashion.

  • Journal article
    Lefebvre G, Antonakakis T, Achaoui Y, Craster RV, Guenneau S, Sebbah Pet al., 2017,

    Unveiling extreme anisotropy in elastic structured media

    , Physical Review Letters, Vol: 118, ISSN: 0031-9007

    Periodic structures can be engineered to exhibit unique properties observed at symmetry points, such as zero group velocity, Dirac cones, and saddle points; identifying these and the nature of the associated modes from a direct reading of the dispersion surfaces is not straightforward, especially in three dimensions or at high frequencies when several dispersion surfaces fold back in the Brillouin zone. A recently proposed asymptotic high-frequency homogenization theory is applied to a challenging time-domain experiment with elastic waves in a pinned metallic plate. The prediction of a narrow high-frequency spectral region where the effective medium tensor dramatically switches from positive definite to indefinite is confirmed experimentally; a small frequency shift of the pulse carrier results in two distinct types of highly anisotropic modes. The underlying effective equation mirrors this behavior with a change in form from elliptic to hyperbolic exemplifying the high degree of wave control available and the importance of a simple and effective predictive model.

  • Journal article
    Achaoui Y, Antonakakis T, Brule S, Craster RV, Enoch S, Guenneau Set al., 2017,

    Clamped seismic metamaterials: Ultra-low broad frequency stop-bands

    , New Journal of Physics, Vol: 9, ISSN: 1367-2630

    The regularity of earthquakes, their destructive power, and the nuisance of ground vibration in urbanenvironments, all motivate designs of defence structures to lessen the impact of seismic and groundvibration waves on buildings. Low frequency waves, in the range 1–10 Hz for earthquakes and up to afew tens of Hz for vibrations generated by human activities, cause a large amount of damage, orinconvenience; depending on the geological conditions they can travel considerable distances andmay match the resonant fundamental frequency of buildings. The ultimate aim of any seismicmetamaterial, or any other seismic shield, is to protect over this entire range of frequencies; the longwavelengths involved, and low frequency, have meant this has been unachievable to date. Notably thisis scalable and the effects also hold for smaller devices in ultrasonics. There are three approaches toobtaining shielding effects: bragg scattering, locally resonant sub-wavelength inclusions and zerofrequencystop-band media. The former two have been explored, but the latter has not and isexamined here. Elastic flexural waves, applicable in the mechanical vibrations of thin elastic plates, canbe designed to have a broad zero-frequency stop-band using a periodic array of very small clampedcircles. Inspired by this experimental and theoretical observation, all be it in a situation far removedfrom seismic waves, we demonstrate that it is possible to achieve elastic surface (Rayleigh)wavereflectors at very large wavelengths in structured soils modelled as a fully elastic layer periodicallyclamped to bedrock. We identify zero frequency stop-bands that only exist in the limit of columns ofconcrete clamped at their base to the bedrock. In a realistic configuration of a sedimentary basin 15 mdeep we observe a zero frequency stop-band covering a broad frequency range of 0–30 Hz.

  • Journal article
    Haslinger SG, Movchan NV, Movchan AB, Jones IS, Craster RVet al., 2017,

    Controlling flexural waves in semi-infinite platonic crystals with resonator-type scatterers

    , Quarterly Journal of Mechanics and Applied Mathematics, Vol: 70, Pages: 216-247, ISSN: 1464-3855

    We address the scattering and transmission of a plane flexural wave through a semi-infinite array of point scatterers/resonators, which take a variety of physically interesting forms. The mathematical model accounts for several classes of point defects, including mass-spring resonators attached to the top surface of the flexural plate and their limiting case of concentrated point masses. We also analyse the special case of resonators attached to opposite faces of the plate. The problem is reduced to a functional equation of the Wiener–Hopf type, whose kernel varies with the type of scatterer considered. A novel approach, which stems from the direct connection between the kernel function of the semi-infinite system and the quasi-periodic Green's functions for corresponding infinite systems, is used to identify special frequency regimes. We thereby demonstrate dynamically anisotropic wave effects in semi-infinite platonic crystals, with particular attention paid to designing systems that exhibit dynamic neutrality (perfect transmission) and localisation close to the structured interface.

  • Journal article
    Castro-Lopez M, Gaio M, Sellers S, Gkantzounis G, Florescu M, Sapienza Ret al., 2017,

    Reciprocal space engineering with hyperuniform gold disordered surfaces

    , APL Photonics, Vol: 2, ISSN: 2378-0967

    Hyperuniform geometries feature correlated disordered topologies which followfrom a tailored k-space design. Here, we study gold plasmonic hyperuniformdisordered surfaces and, by momentum spectroscopy, we report evidence of kspaceengineering on both light scattering and light emission. Even if the structureslack a well-defined periodicity, emission and scattering are directional inring-shaped patterns. The opening of these rotational-symmetric patterns scaleswith the hyperuniform correlation length parameter as predicted via the spectralfunction method.

  • Conference paper
    O'Neill J, Selsil Ö, Haslinger SG, Movchan NV, Craster RVet al., 2017,

    Active cloaking for flexural waves in a pinned kirchhoff plate

  • Journal article
    Nguyen N, Maier SA, Hong M, Oulton RFet al., 2016,

    Recovering parity-time symmetry in highly dispersive coupled optical waveguides

    , New Journal of Physics, Vol: 18, ISSN: 1367-2630

    Coupled photonic systems satisfying parity-time symmetry (PTS) provideexibility to engineer the ow of light including non-reciprocal propagation, perfectlaser-absorbers, and ultra-fast switching. Achieving the required index pro le foran optical system with ideal PTS, i.e. n(x) =n(-x)*, has proven to be difficult due to the challenge of controlling gain, loss and material dispersion simultaneously. Consequently, most research has focused on dilute or low gain optical systems where material dispersion is minimal. In this paper, we study a model system of coupled inorganic semiconductor waveguides with potentially high gain (>1,500 cm-1) and dispersion. Our analysis makes use of coupled mode theory's parameters to quantify smooth transitions between PTS phases under imperfect conditions. We find that the detrimental influence of gain-induced dispersion is counteracted and the key features of parity-time symmetric optical systems are recovered by working with non-identical waveguides and bias pumping of the optical waveguides. Our coupled mode theory results show excellent agreement with numerical solutions, proving the robustness of coupled mode theory in describing various degrees of imperfection in systems with PTS.

  • Journal article
    Colquitt DJ, Colombi A, Craster RV, Roux P, Guenneau SRLet al., 2016,

    Seismic metasurfaces: Sub-wavelength resonators and Rayleigh wave interaction

    , Journal of the Mechanics and Physics of Solids, Vol: 99, Pages: 379-393, ISSN: 0022-5096

    We consider the canonical problem of an array of rods, which act as resonators, placed on an elastic substrate; the substrate being either a thin elastic plate or an elastic half-space. In both cases the flexural plate, or Rayleigh surface, waves in the substrate interact with the resonators to create interesting effects such as effective band-gaps for surface waves or filters that transform surface waves into bulk waves; these effects have parallels in the field of optics where such sub-wavelength resonators create metamaterials, and metasurfaces, in the bulk and at the surface respectively. Here we carefully analyse this canonical problem by extracting the dispersion relations analytically thereby examining the influence of both the flexural and compressional resonances on the propagating wave. For an array of resonators atop an elastic half-space we augment the analysis with numerical simulations. Amongst other effects, we demonstrate the striking effect of a dispersion curve that transitions from Rayleigh wave-like to shear wave-like behaviour and the resultant change in displacement from surface to bulk waves.

  • Journal article
    Maling B, Colquitt D, Craster RV, 2016,

    Dynamic homogenisation of Maxwell’s equations with applications to photonic crystals and localised waveforms on gratings

    , Wave Motion, Vol: 69, Pages: 35-49, ISSN: 0165-2125

    A two-scale asymptotic theory is developed to generate continuum equations that model the macroscopic be-haviour of electromagnetic waves in periodic photonic structures when the wavelength is not necessarily longrelative to the periodic cell dimensions; potentially highly-oscillatory short-scale detail is encapsulated throughintegrated quantities. The resulting equations include tensors that represent effective refractive indices near bandedge frequencies along all principal axes directions, and these govern scalar functions providing long-scale mod-ulation of short-scale Bloch eigenstates, which can be used to predict the propagation of waves at frequenciesoutside of the long wavelength regime; these results are outside of the remit of typical homogenisation schemes.The theory we develop is applied to two topical examples, the first being the case of aligned dielectric cylin-ders, which has great importance in modelling photonic crystal fibres. Results of the asymptotic theory are veri-fied against numerical simulations by comparing photonic band diagrams and evanescent decay rates for guidedmodes. The second example is the propagation of electromagnetic waves localised within a planar array of di-electric spheres; at certain frequencies strongly directional propagation is observed, commonly described as dy-namic anisotropy. Computationally this is a challenging three-dimensional calculation, which we perform, andthen demonstrate that the asymptotic theory captures the effect, giving highly accurate qualitative and quantitativecomparisons as well as providing interpretation for the underlying change from elliptic to hyperbolic behaviour.

  • Journal article
    Mellor AV, Hylton N, Maier S, Ekins-Daukes Net al., 2016,

    Interstitial light-trapping design for multi-junction solar cells

    , Solar Energy Materials and Solar Cells, Vol: 159, Pages: 212-218, ISSN: 0927-0248

    We present a light-trapping design capable of significantly enhancing the photon absorption inany subcell of a multi-junction solar cell. The design works by coupling incident light intowaveguide modes in one of the subcells via a diffraction grating, and preventing these modesfrom leaking into lower subcells via a low-index layer and a distributed Bragg reflector, whichtogether form an omnidirectional mirror. This allows the thickness of the target subcell to bereduced without compromising photon absorption, which improves carrier collection, andtherefore photocurrent. The paper focuses on using the composite structure to improve theradiation hardness of a InGaP/Ga(In)As/Ge space solar cell. In this context, it is shown viasimulation that the Ga(In)As middle-cell thickness can be reduced from 3500 to 700 nm,whilst maintaining strong photon absorption, and that this leads to a significantly improvedend-of-life photocurrent in the Ga(In)As middle cell. However, the design can in general beapplied to a wide range of multi-junction solar cell types. We discuss the principles ofoperation of the design, as well as possible methods of its fabrication and integration intomulti-junction solar cells.

  • Journal article
    Pendry JB, Sasihithlu K, Craster RV, 2016,

    Phonon-assisted heat transfer between vacuum-separated surfaces

    , Physical Review B - Condensed Matter and Materials Physics, Vol: 94, ISSN: 1098-0121

    With increasing interest in nanotechnology, the question arises of how heat is exchanged between materials separated by only a few nanometers of vacuum. Here, we present calculations of the contribution of phonons to heat transfer mediated by van der Waals forces and compare the results to other mechanisms such as coupling through near field fluctuations. Our results show a more dramatic decay with separation than previous work.

  • Journal article
    Albella Echave P, Shibanuma T, Maier S, Shibanuma T, Albella P, Maier SAet al., 2016,

    Unidirectional light scattering with high efficiency at optical frequencies based on low-loss dielectric nanoantennas

    , Nanoscale, Vol: 8, Pages: 14184-14192, ISSN: 2040-3372

    Dielectric nanoparticles offer low optical losses and access to both electric and magnetic Mie resonances. This enables unidirectional scattering along the incident axis of light, owing to the interference between these two resonances. Here we theoretically and experimentally demonstrate that an asymmetric dimer of dielectric nanoparticles can provide unidirectional forward scattering with high efficiency. Theoretical analyses reveal that the dimer configuration can satisfy the first Kerker condition at the resonant peaks of electric and magnetic dipolar modes, therefore showing highly efficient directional forward scattering. The unidirectional forward scattering with high efficiency is confirmed in our experiments using a silicon nanodisk dimer on a transparent substrate. This study will boost the realization of practical applications using low-loss and efficient subwavelength all-dielectric nanoantennas.

  • Journal article
    Fitzgerald JM, Narang P, Craster RV, Maier SA, Giannini Vet al., 2016,

    Quantum Plasmonics

    , Proceedings of the IEEE, Vol: 104, Pages: 2307-2322, ISSN: 0018-9219

    Quantum plasmonics is an exciting subbranch of nanoplasmonics where the laws of quantum theory are used to describe light–matter interactions on the nanoscale. Plasmonic materials allow extreme subdiffraction confinement of (quantum or classical) light to regions so small that the quantization of both light and matter may be necessary for an accurate description. State-of-the-art experiments now allow us to probe these regimes and push existing theories to the limits which opens up the possibilities of exploring the nature of many-body collective oscillations as well as developing new plasmonic devices, which use the particle quality of light and the wave quality of matter, and have a wealth of potential applications in sensing, lasing, and quantum computing. This merging of fundamental condensed matter theory with application-rich electromagnetism (and a splash of quantum optics thrown in) gives rise to a fascinating area of modern physics that is still very much in its infancy. In this review, we discuss and compare the key models and experiments used to explore how the quantum nature of electrons impacts plasmonics in the context of quantum size corrections of localized plasmons and quantum tunneling between nanoparticle dimers. We also look at some of the remarkable experiments that are revealing the quantum nature of surface plasmon polaritons.

  • Journal article
    Gennaro SD, Rahmani M, Giannini V, Aouani H, Sidiropoulos TP, Navarro-Cía M, Maier SA, Oulton RFet al., 2016,

    The Interplay of Symmetry and Scattering Phase in Second Harmonic Generation from Gold Nanoantennas

    , Nano Letters, Vol: 16, Pages: 5278-5285, ISSN: 1530-6992

    Nonlinear phenomena are central to modern photonics but, being inherently weak, typically require gradual accumulation over several millimeters. For example, second harmonic generation (SHG) is typically achieved in thick transparent nonlinear crystals by phase-matching energy exchange between light at initial, ω, and final, 2ω, frequencies. Recently, metamaterials imbued with artificial nonlinearity from their constituent nanoantennas have generated excitement by opening the possibility of wavelength-scale nonlinear optics. However, the selection rules of SHG typically prevent dipole emission from simple nanoantennas, which has led to much discussion concerning the best geometries, for example, those breaking centro-symmetry or incorporating resonances at multiple harmonics. In this work, we explore the use of both nanoantenna symmetry and multiple harmonics to control the strength, polarization and radiation pattern of SHG from a variety of antenna configurations incorporating simple resonant elements tuned to light at both ω and 2ω. We use a microscopic description of the scattering strength and phases of these constituent particles, determined by their relative positions, to accurately predict the SHG radiation observed in our experiments. We find that the 2ω particles radiate dipolar SHG by near-field coupling to the ω particle, which radiates SHG as a quadrupole. Consequently, strong linearly polarized dipolar SHG is only possible for noncentro-symmetric antennas that also minimize interference between their dipolar and quadrupolar responses. Metamaterials with such intra-antenna phase and polarization control could enable compact nonlinear photonic nanotechnologies.

  • Journal article
    Maling B, Craster RV, 2016,

    Whispering Bloch modes

    , Proceedings of the Royal Society A: Mathematical, Physical & Engineering Sciences, Vol: 472, ISSN: 1364-5021

    We investigate eigenvalue problems for the planar Helmholtz equation in open systems with a high order of rotational symmetry. The resulting solutions have similarities with the whispering gallery modes exploited in photonic micro-resonators and elsewhere, but unlike these do not necessarily require a surrounding material boundary, with confinement instead resulting from the geometry of a series of inclusions arranged in a ring. The corresponding fields exhibit angular quasi-periodicity reminiscent of Bloch waves, and hence we refer to them as whispering Bloch modes (WBMs). We show that if the geometry of the system is slightly perturbed such that the rotational symmetry is broken, modes with asymmetric field patterns can be observed, resulting in field enhancement and other potentially desirable effects. We investigate the WBMs of two specific geometries first using expansion methods and then by applying a two-scale asymptotic scheme.

  • Journal article
    Harutyunyan D, Milton GW, Craster RV, 2016,

    High-frequency homogenization for travelling waves in periodic media

    , Proceedings of the Royal Society A: Mathematical, Physical & Engineering Sciences, Vol: 472, ISSN: 1364-5021

    We consider high-frequency homogenization in periodic media for travelling waves of several different equations: the wave equation for scalar-valued waves such as acoustics; the wave equation for vector-valued waves such as electromagnetism and elasticity; and a system that encompasses the Schrödinger equation. This homogenization applies when the wavelength is of the order of the size of the medium periodicity cell. The travelling wave is assumed to be the sum of two waves: a modulated Bloch carrier wave having crystal wavevector [Formula: see text] and frequency ω 1 plus a modulated Bloch carrier wave having crystal wavevector [Formula: see text] and frequency ω 2. We derive effective equations for the modulating functions, and then prove that there is no coupling in the effective equations between the two different waves both in the scalar and the system cases. To be precise, we prove that there is no coupling unless ω 1=ω 2 and [Formula: see text] where Λ=(λ1λ2…λ d ) is the periodicity cell of the medium and for any two vectors [Formula: see text] the product a⊙b is defined to be the vector (a 1 b 1,a 2 b 2,…,a d b d ). This last condition forces the carrier waves to be equivalent Bloch waves meaning that the coupling constants in the system of effective equations vanish. We use two-scale analysis and some new weak-convergence type lemmas. The analysis is not at the same level of rigour as that of Allaire and co-workers who use two-scale convergence theory to treat the problem, but has the advantage of simplicity which will allow it to be easily extended to the case where there is degeneracy of the Bloch eigenvalue.

  • Journal article
    Schnitzer O, Giannini V, Maier SA, Craster RV, Schnitzer O, Giannini V, Maier SA, Craster RVet al., 2016,

    Surface-plasmon resonances of arbitrarily shaped nanometallic structures in the small-screening-length limit

    , Proceedings of the Royal Society A: Mathematical, Physical & Engineering Sciences, Vol: 472, ISSN: 1364-5021

    According to the hydrodynamic Drude model, surface plasmon resonances of metallic nanostructures blueshift owing to the non-local response of the metal’s electron gas. The screening length characterizing the non-local effect is often small relative to the overall dimensions of the metallic structure, which enables us to derive a coarse-grained non-local description using matched asymptotic expansions; a perturbation theory for the blueshifts of arbitrary-shaped nanometallic structures is then developed. The effect of non-locality is not always a perturbation and we present a detailed analysis of the ‘bonding’ modes of a dimer of nearly touching nanowires where the leading-order eigenfrequencies and eigenmode distributions are shown to be a renormalization of those predicted assuming a local metal permittivity.

  • Journal article
    Grinblat G, Li Y, Nielsen MP, Oulton R, Maier SAet al., 2016,

    Enhanced third harmonic generation in single Germanium nanodisks excited at the anapole mode

    , Nano Letters, Vol: 16, Pages: 4635-4640, ISSN: 1530-6992

    We present an all-dielectric germanium nanosystem exhibiting a strong third ordernonlinear response and efficient third harmonic generation in the optical regime. A thin germaniumnanodisk shows a pronounced valley in its scattering cross section close to the dark anapole mode,while the electric field energy inside the disk is maximized due to high confinement within thedielectric. We investigate the dependence of the third harmonic signal on disk size and pumpwavelength to reveal the nature of the anapole mode. Each germanium nanodisk generates a higheffective third order susceptibility of (3) = 4.3 10−9 , corresponding to an associated thirdharmonic conversion efficiency of 0.0001% at a wavelength of 1650 nm, which is four orders ofmagnitude greater than the case of an unstructured germanium reference film. Furthermore, thenonlinear conversion via the anapole mode outperforms that via the radiative dipolar resonancesby about one order of magnitude, which is consistent with our numerical simulations. Thesefindings open new possibilities for the optimization of upconversion processes on the nanoscalethrough the appropriate engineering of suitable dielectric materials.

  • Journal article
    Grinblat G, Li Y, Nielsen MP, Oulton RF, Maier SAet al., 2016,

    Enhanced Third Harmonic Generation in Single Germanium Nanodisks Excited at the Anapole Mode

    , Nano Letters, Vol: 16, Pages: 4635-4640, ISSN: 1530-6992

    We present an all-dielectric germanium nanosystem exhibiting a strong third order nonlinear response and efficient third harmonic generation in the optical regime. A thin germanium nanodisk shows a pronounced valley in its scattering cross section at the dark anapole mode, while the electric field energy inside the disk is maximized due to high confinement within the dielectric. We investigate the dependence of the third harmonic signal on disk size and pump wavelength to reveal the nature of the anapole mode. Each germanium nanodisk generates a high effective third order susceptibility of χ(3) = 4.3 × 10–9 esu, corresponding to an associated third harmonic conversion efficiency of 0.0001% at an excitation wavelength of 1650 nm, which is 4 orders of magnitude greater than the case of an unstructured germanium reference film. Furthermore, the nonlinear conversion via the anapole mode outperforms that via the radiative dipolar resonances by about 1 order of magnitude, which is consistent with our numerical simulations. These findings open new possibilities for the optimization of upconversion processes on the nanoscale through the appropriate engineering of suitable dielectric materials.

  • Journal article
    Vanel AL, Craster RV, Colquitt DJ, Makwana Met al., 2016,

    Asymptotics of dynamic lattice Green’s functions

    , Wave Motion, Vol: 67, Pages: 15-31, ISSN: 0165-2125

    In the study of periodic problems it is natural and commonplace to use Fourier transforms to obtain explicit lattice Green’s functions in the form of multidimensional integrals. Considerable physical information is encapsulated within the Green’s function and our aim is to extract the behaviour near critical frequencies by creating connections with multiple-scale homogenisation methods recently applied to partial differential equations. We show that the integrals naturally contain two-scales, a short-scale on the scale of the lattice and a long-scale envelope. For pedagogic purposes we first consider the well-known two dimensional square lattice, followed by the three dimensional cubic lattice. The features we uncover, and the asymptotics, are generic for many lattice structures. Finally we consider a topical three dimensional example from structural mechanics showing dynamic anisotropy, that is, at specific frequencies all the energy is directed along specific characteristic directions.

  • Journal article
    Colombi A, Colquitt D, Roux P, Guenneau S, Craster RVet al., 2016,

    A seismic metamaterial: the resonant metawedge

    , Scientific Reports, Vol: 6, ISSN: 2045-2322

    Critical concepts from three different fields, elasticity, plasmonics and metamaterials, are brought together to design a metasurface at the geophysical scale, the resonant metawedge, to control seismic Rayleigh waves. Made of spatially graded vertical subwavelength resonators on an elastic substrate, the metawedge can either mode convert incident surface Rayleigh waves into bulk elastic shear waves or reflect the Rayleigh waves creating a "seismic rainbow" effect analogous to the optical rainbow for electromagnetic metasurfaces. Time-domain spectral element simulations demonstrate the broadband efficacy of the metawedge in mode conversion while an analytical model is developed to accurately describe and predict the seismic rainbow effect; allowing the metawedge to be designed without the need for extensive parametric studies and simulations. The efficiency of the resonant metawedge shows that large-scale mechanical metamaterials are feasible, will have application, and that the time is ripe for considering many optical devices in the seismic and geophysical context.

  • Journal article
    Huidobro PA, Kraft M, Maier SA, Pendry JB, Maier SA, Arroyo Huidobro P, Kraft M, Pendry JBet al., 2016,

    Graphene as a Tunable Anisotropic or IsotropicPlasmonic Metasurface

    , ACS Nano, Vol: 10, Pages: 5499-5506, ISSN: 1936-086X

    We demonstrate a tunable plasmonic metasurface by considering a graphene sheetsubject to a periodically patterned doping level. The unique optical properties ofgraphene result in electrically tunable plasmons that allow for extreme confinementof electromagnetic energy in the technologically significant regime of THz frequencies.Here we add an extra degree of freedom by using graphene as a metasurface, proposingto dope it with an electrical gate patterned in the micron or sub-micron scale. Byextracting the effective conductivity of the sheet we characterize metasurfaces periodicallymodulated along one or two directions. In the first case, and making use of theanalytical insight provided by transformation optics, we show an efficient control ofTHz radiation for one polarization. In the second case, we demonstrate a metasurfacewith an isotropic response that is independent of wave polarization and orientation.

  • Journal article
    Craster RV, Lowe M, Shi F, Xi Xet al., 2016,

    Diffuse scattered field of elastic waves from randomly rough surfaces using an analytical Kirchhoff theory

    , Journal of the Mechanics and Physics of Solids, Vol: 92, Pages: 260-277, ISSN: 0022-5096

    We develop an elastodynamic theory to predict the diffuse scattered field ofelastic waves by randomly rough surfaces, for the first time, with the aid of theKirchhoff approximation (KA). Analytical expressions are derived incorporatingsurface statistics, to represent the expectation of the angular distribution of thediffuse intensity for different modes. The analytical solutions are successfullyverified with numerical Monte Carlo simulations, and also validated by comparisonwith experiments. We then apply the theory to quantitatively investigatethe effects of the roughness and the shear-to-compressional wave speed ratio onthe mode conversion and the scattering intensity, from low to high roughnesswithin the valid region of KA. Both the direct and the mode converted intensitiesare significantly affected by the roughness, which leads to distinct scatteringpatterns for different wave modes. The mode conversion effect is very strongaround the specular angle and it is found to increase as the surface appearsto be more rough. In addition, the 3D roughness induced coupling betweenthe out-of-plane shear horizontal (SH) mode and the in-plane modes is studied.The intensity of the SH mode is shown to be very sensitive to the out-of-planecorrelation length, being influenced more by this than by the RMS value of theroughness. However, it is found that the depolarization pattern for the diffusefield is independent of the actual value of the roughness.

  • Journal article
    Gennaro SD, Roschuk TR, Maier SA, Oulton RFet al., 2016,

    Measuring chromatic aberrations in imaging systems using plasmonic nanoparticles

    , Optics Letters, Vol: 41, Pages: 1688-1691, ISSN: 1539-4794

    We demonstrate a method to measure chromatic aberrations of microscope objectives with metallic nanoparticles using white light. Extinction spectra are recorded while scanning a single nanoparticle through a lens’s focal plane. We show a direct correlation between the focal wavelength and the longitudinal chromatic focal shift through our analysis of the variations between the scanned extinction spectra at each scan position and the peak extinction over the entire scan. The method has been tested on achromat and apochromat objectives using aluminum disks varying in size from 260–520 nm. Our method is straightforward, robust, low cost, and broadband with a sensitivity suitable for assessing longitudinal chromatic aberrations in high-numerical-aperture apochromatic corrected lenses.

  • Journal article
    Arroyo Huidobro P, Kraft M, Ren K, Maier SA, Pendry Jet al., 2016,

    Graphene, plasmons and transformation optics

    , Journal of Optics, Vol: 18, ISSN: 2040-8978

    Here we study subwavelength gratings for coupling into graphene plasmons by means of an an-alytical model based on transformation optics that is not limited to very shallow gratings. Weconsider gratings that consist of a periodic modulation of the charge density in the graphene sheet,and gratings formed by this conductivity modulation together with a dielectric grating placed inclose vicinity of the graphene. Explicit expressions for the dispersion relation of the plasmon po-laritons supported by the system, and reectance and transmittance under plane wave illuminationare given. We discuss the conditions for maximising the coupling between incident radiation andplasmons in the graphene, finding the optimal modulation strength for a conductivity grating.

  • Journal article
    Roeder R, Sidiropoulos TPH, Buschlinger R, Riediger M, Peschel U, Oulton RF, Ronning Cet al., 2016,

    Mode Switching and Filtering in Nanowire Lasers

    , Nano Letters, Vol: 16, Pages: 2878-2884, ISSN: 1530-6992

    Coherent light sources confining the light below the vacuum wavelength barrier will drive future concepts of nanosensing, nanospectroscopy, and photonic circuits. Here, we directly image the angular emission of such a light source based on single semiconductor nanowire lasers. It is confirmed that the lasing switches from the fundamental mode in a thin ZnO nanowire to an admixture of several transverse modes in thicker nanowires approximately at the multimode cutoff. The mode competition with higher order modes substantially slows down the laser dynamics. We show that efficient photonic mode filtering in tapered nanowires selects the desired fundamental mode for lasing with improved performance including power, efficiency, and directionality important for an optimal coupling between adjacent nanophotonic waveguides.

  • Journal article
    Duncan AB, Pavliotis GA, Lelievre T, 2016,

    Variance Reduction using Nonreversible Langevin Samplers

    , Journal of Statistical Physics, Vol: 163, Pages: 457-491, ISSN: 1572-9613

    A standard approach to computing expectations with respect to a given target measure is to introduce an overdamped Langevin equation which is reversible with respect to the target distribution, and to approximate the expectation by a time-averaging estimator. As has been noted in recent papers, introducing an appropriately chosen nonreversiblecomponent to the dynamics is beneficial, both in terms of reducing the asymptotic variance and of speeding up convergence to the target distribution. In this paper we present a detailed study of the dependence of the asymptotic variance on the deviation from reversibility. Our theoretical findings are supported by numerical simulations.

  • Journal article
    Bak AO, Yoxall EO, Sarriugarte P, Giannini V, Maier SA, Hillenbrand R, Pendry JB, Phillips CCet al., 2016,

    Harnessing a Quantum Design Approach for Making Low-Loss Superlenses

    , NANO LETTERS, Vol: 16, Pages: 1609-1613, ISSN: 1530-6984

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