Suchergebnisse

The exact solution of Maxwell's equations in the presence of arbitrarily shaped dielectrics is expressed in terms of surface-integral equations evaluated at the interfaces. The electromagnetic field induced by the passage of an external electron is then calculated in terms of self-consistently obtained boundary charges and currents. This procedure is shown to be suitable for the simulation of electron energy loss spectra when the materials under consideration are described by local frequency-dependent response functions. The particular cases of translationally invariant interfaces and axially symmetric interfaces are discussed in detail. The versatility of this method is emphasized by examples of energy loss spectra for electrons passing near metallic and dielectric wedges, coupled cylinders, spheres, and tori, and other complex geometries, where retardation aspects and Cherenkov losses can sometimes be significant. ; One of the authors (F.J.G.A.) wants to acknowledge help and support from the Spanish Ministerio de Ciencia y Tecnologia and the Basque Government. ; Peer reviewed

8 pags, 8 figs ; The possibility has recently been reported of using spatially resolved electron energy loss spectroscopy and cathodoluminescence in scanning (transmission) electron microscopes to probe optical excitations'plasmons, photons, excitons'at a scale that could not have been considered only a few years ago. This allows these excitations to be studied at the relevant scale for the characterization of novel materials with potential applications in nanophotonics and nanoplasmonics. This review aims at describing the state-of-the art experimental and theoretical techniques of this emerging field and its major uses and applications. © 2012 Materials Research Society. ; The authors acknowledge financial support from the European Union under the Framework 6 program under a contract for an Integrated Infrastructure Initiative. Reference 026019 ESTEEM, and French ANR (1010 BLAN 1016 "HYNNA") support.

4 pags., 1 fig. ; The rich structure of surface plasmon modes localized in noble metal nanoparticles is explored by optical spectroscopy, spatially resolved electron energy-loss spectroscopy, and by electron beam-induced radiation emission. Spectral features and spatiallyresolved maps of surface plasmon modes are calculated by using the boundary element method. Our results show the unmatched capability of electron beams for spectrally and spatially probing plasmon modes in metal nanoparticles. ; We acknowledge support from the European Union (NMP4-SL-2008-213669-ENSEMBLE, ICT-2009-4-248909-LIMA, and FP7-ICT-2009-4-248855-N4E) and the Spanish MICINN (MAT2010-14885 and Consolider NanoLight.es). V.M. acknowledges the Spanish CSIC -JAE grant. ; Peer reviewed

The ability to control light over very small distances is a problem of fundamental importance for a vast range of applications in communications, nanophotonics, and quantum information technologies. For this purpose, several methods have been proposed and demonstrated to confine and guide light, for example in dielectric and surface plasmon polariton (SPP) waveguides. Here, we study the interaction between different kinds of planar waveguides, which produces dramatic changes in the dispersion relation of the waveguide pair and even leads to mode suppression at small separations. This interaction also produces a transfer of power between the waveguides, which depends on the gap and propagation distances, thus providing a mechanism for optical signal transfer. We analytically study the properties of this interaction and the power transfer in different structures of interest including plasmonic and particle-array waveguides, for which we propose an experimental realization of these ideas. © 2012 Optical Society of America. ; We acknowledge support from the European Union (NMP4-2006-016881-SPANS, FP7- ICT-2009-4-248855-Nanophotonics4Energy, FP7-ICT-2009-4-248909-LIMA and NMP4-SL- 2008-213669-ENSEMBLE) and the Spanish MICINN (MAT2010-14885 and Consolider NanoLight.es). X.M.B. acknowledges a JAE scholarship from CSIC. ; Peer Reviewed

12 pags., 3 figs., 1 app. -- Open Access funded by Creative Commons Atribution Licence 3.0 ; We predict unprecedentedly large values of the energy-transfer rate between an optical emitter and a layer of periodically doped graphene. The transfer exhibits divergences at photon frequencies corresponding to the Van Hove singularities of the plasmonic band structure of the graphene. In particular, we find flat bands associated with regions of vanishing doping charge, which appear in graphene when it is patterned through gates of spatially alternating signs, giving rise to intense transfer rate singularities. Graphene is thus shown to provide a unique platform for fast control of optical energy transfer via fast electrostatic inhomogeneous doping. © IOP Publishing and Deutsche Physikalische Gesellschaft. ; This work was partially supported by the European Union (FP7-ICT-2009-4-248855-N4E), the Spanish MEC (MAT2010-14885 and Consolider NanoLight.es) and Ibercivis. AM acknowledges financial support from the Spanish FPU.

The excitations produced by fast electrons impinging perpendicularly on both metallic and semiconductor cylindrical nanowires are investigated within the framework of dielectric theory. The dependence of the electron energy-loss spectra (EELS) on the nanowire radius is studied in detail, and so is the spatial extension of the induced-charge fluctuations associated to the modes that are excited during the loss process. The limits of applicability of dielectric theory to nanowires are discussed. In particular, comparison between the present theory and EELS measurements performed with silicon nanofibers support the use of dielectric theory at the scale of a few nanometers in diameter, and it is shown that this positive result is justified in terms of the longitudinal pattern of the induced surface plasmons. Finally, the effect of nanowire termination on the electron energy-loss probability for electrons passing near the edge is calculated using the boundary charge method, showing that the range of this effect can extend up to tens of nanometers for low-energy m=0 modes. ; Support from the Basque government and Basque Country University is acknowledged. One of the authors E.O. also acknowledges the Spanish Ministerio de Educacion y Cultura for financial support under Contract No. PB98-0780-C02. ; Peer reviewed

9 páginas, 3 figuras.-- et al. ; The inhibition in wave propagation at band gap energies plays a central role in many areas of technology such as electronics (electron gaps), nanophotonics (light gaps) and phononics (acoustic gaps), among others. Here we demonstrate that metal surfaces featuring free-electron-like bands may become semiconducting by periodic nanostructuration. We combine scanning tunneling spectroscopy and angle-resolved photoemisssion to accurately determine the energy-dependent local density of states and band structure of the Ag/Cu(111) noble metal interface patterned with an array of triangular dislocations, demonstrating the existence of a 25 meV band gap that extends over the entire surface Brillouin zone. We prove that this gap is a general consequence of symmetry reduction in close-packed metallic overlayers; in particular, we show that the gap opening is due to the symmetry lowering of the wave vector group at the K point from C3v to C3. ; This work was supported in part by the Spanish MICINN (MAT2007-66050, MAT2007-63083 and Consolider NanoLight.es), the EU (NMP4-SL-2008-213669-ENSEMBLE), the Basque Government (IT-257-07) and the Centre National de la Recherche Scientifique (CNRS). ; Peer reviewed

We identify weak and strong coupling regimes between a near-field probing tip and a plasmonic sample by imaging plasmon-resonant gold nanodisks with scattering-type scanning near-field optical microscopy (s-SNOM). By means of rigorous electrodynamical calculations based on a model system, we find that in the weak coupling regime, s-SNOM can be applied for direct mapping of plasmonic nanoantenna modes, while in the strong coupling regime, the near-field probe allows for high-precision opto-mechanical control of the antenna response. © 2009 The American Physical Society. ; We acknowledge P. Hanarp and D. S. Sutherland for provision of the disk samples, and financial support from the project ETORTEK-2008 of the Department of Industry of the Basque Government and project FIS2007-66711-C01-01 of the Spanish Ministry of Education and Science. ; Peer Reviewed

Au(111) vicinal surfaces are characteristic examples of two-phase segregation or faceting. Between ∼4° and 9.5° miscut, the surface exhibits hilland-valley structures formed by bunches of relatively wide (d w - 36-41 Å) and narrow (dn - 14 Å) terraces. The evolution of surface electronic states in such a faceted system is followed using a curved crystal. Beyond 4° the surface state splits into distinct dw and dn bands. Our analysis suggests the crucial role of surface states in defining the characteristic dw and dn sizes during Au faceting. © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft. ; Fruitful discussions with E Tosatti are acknowledged. The work is supported through projects of the Basque Government (GI1058) and the Spanish Ministerio de Educacion y Ciencia (MAT2007-63083, MAT2007-66050). The SRC is funded by the National Science Foundation (Award no. DMR-0084402). ; Peer Reviewed

We describe both theoretically and experimentally the existence and excitation of confined modes in planar arrays of gold nanodisks. Ordered 2D lattices of monodispersive nanoparticles are manufactured, embedded in a silica matrix, and exposed to evanescent prism-coupling illumination, leading to dark features in the reflectivity, which signal the presence of confined modes guided along the arrays. We find remarkable agreement between theory and experiment in the frequency-momentum dispersion of the resonances. Direct excitation of these modes reveals long propagation distances and deep extinction features. This combined experimental and theoretical characterization of guidedmodes shows a good understanding of the optical response of metallic particles arrays, which can be beneficial in future designs of optical-signal and distant-sensing applications. © 2013 Optical Society of America. ; This work has been supported in part by the European Union (NMP4-2006-016881-SPANS, NMP4-SL-2008-213669-ENSEMBLE,FP7-ICT-2009-4-248909-LIMA, and FP7-ICT-2009-4-248855-N4E), the Spanish MEC (MAT2010-14885 and Consolider Nano-Light.es), and the research program of the Foundation for Fundamental Research on Matter (FOM), which is financially supported by the Netherlands Organization for Fundamental Research (NWO) and is part of an industrial partnership program between Philips and FOM. It is also supported in part by NanoNextNL, a micro and nanotechnology consortium of the Government of the Netherlands and 130 partners. X.M.B. acknowledges a Spanish CSIC-JAE grant. ; Peer Reviewed

We report on spectral imaging within individual silver split-ring resonators (SRRs) operating in the near infrared-visible range. We classified the optical eigenmodes from the measurement of their energies and nanometer scale spatial distributions. They are plasmonic standing waves that show great similarities with that of nanoantennas. We, however, evidenced marked differences in the near-field electric field lines' spatial distribution and the energies' dispersion. We also showed that the subwavelength defect's influence on the SRRs' eigenmodes spatial distribution is small. © 2010 The American Physical Society. ; The authors acknowledge financial support from the European Union under the Framework 6 program under a contract for an Integrated Infrastructure Initiative. Reference 026019 ESTEEM. ; Peer Reviewed

5 páginas, 4 figuras.-- PACS numbers: 73.20.At, 73.22.-f, 79.60.Jv ; The two-dimensional, free-electron-like band structure of noble metal surfaces can be radically transformed by appropriate nanostructuration. A case example is the triangular dislocation network that characterizes the epitaxial Ag/Cu(111) system, which exhibits a highly featured band topology with a full band gap above EF and a hole-pocket-like Fermi surface. Here we show that controlled doping of the Ag/Cu(111) interface with Au allows one to observe a complete Lifshitz transition at 300 K; i.e., the hole pockets fill up, the band gap entirely shifts across EF, and the Fermi surface becomes electron-pocket-like. ; This work was supported by the Spanish MICINN (MAT2010-21156-C03-01 and -C03-03) and the Basque Government (IT-257-07). ; Peer reviewed

STM images show that vicinal Au(788) surfaces are made up of a uniform array of (111)-oriented terraces of similar width (∼3.8nm). This uniformity makes it possible to study the electronic structure of the resulting step superlattice by angle-resolved photoemission. We show that for this terrace array the surface state appears to be broken up into one-dimensional quantum-well levels, indicating total electron confinement within the terraces. The angular resolution allows the probability density of the terrace quantum well state to be mapped in reciprocal space, complementing nicely the wave function measured in real space by STM. ; A. Mu., F. J. G. de A., and J. E. O. are supported by the Universidad del País Vasco (1/UPV/EHU/00057.240-EA8078/2000) and the Max Planck Research Award Program. V. R. and S. R. are supported by the CNRS-ULTIMATECH program, the CRIF, and the Université de Paris 7. A. Ma. is supported by a Marie Curie Fellowship of the European Union, under Contract No. HPMF-CT-2000-00565. V. P.-D. is supported by the Comunidad Autónoma de Madrid (Project No. 07N/0042/98) and the DGICYT (Spain) (Grant No. PB-97-1199). ; Peer reviewed

We report on spatially resolved electron energy-loss spectroscopy studies of optical modes in individual star-shaped gold nanoparticles. We studied different morphologies, ranging from a spheroid to well-developed nanostars. For each shape, essentially two groups of modes are appearing: the first one is localized around the core of the nanostars and has an energy slightly less than the quasi-static dipolar mode of a gold sphere (about 2.2 eV); in the second group, the modes are localized at the end of the nanostar tips, with varying energies depending on the geometry of each tip and with energy down to 1.2 eV. The localization of the tip modes is interpreted with the help of boundary element methods simulations. © 2011 EDP Sciences. ; The authors acknowledge financial support from the European Union (E4) under the Framework 6 Program under a contract for an Integrated Infrastructure Initiative (reference 026019 ESTEEM), EU-FP6 (NMP4-2006-016881SPANS) and Marie Curie Program (MEST CT 2004 514307). ; Peer Reviewed

We present theoretical simulations of electron refraction at the lateral atomic interface between a 'homogeneous' Cu(111) surface and the >nanostructured> one-monolayer (ML) Ag/Cu(111) dislocation lattice. Calculations are performed for electron binding energies barely below the 1 ML Ag/Cu(111) M-point gap (binding energy E = 53 meV, below the Fermi level) and slightly above its Γ-point energy (E = 160 meV), both characterized by isotropic/circular constant energy surfaces. Using plane-wave-expansion and boundary-element methods, we show that electron refraction occurs at the interface, the Snell law is obeyed, and a total internal reflection occurs beyond the critical angle. Additionally, a weak negative refraction is observed for E = 53 meV electron energy at beam incidence higher than the critical angle. Such an interesting observation stems from the interface phase-matching and momentum conservation with the umklapp bands at the second Brillouin zone of the dislocation lattice. The present analysis is not restricted to our Cu-Ag/Cu model system but can be readily extended to technologically relevant interfaces with spin-polarized, highly featured, and anisotropic constant energy contours, such as those characteristic for Rashba systems and topological insulators. ; This work has been supported in part by the Spanish MINECO (Grant Nos. MAT2013–46593-C6–4-P, MAT2016–78293-C6–6-R, MAT2014-59096-P, and SEV2015-0522), the Basque Government (Grant No. IT621–13), the Catalan CERCA Program, Fundacio Privada Cellex, and AGAUR (Grant No. 2014 SGR 1400). ; Peer Reviewed