Interaction of radiation and fast electrons with clusters of dielectrics: A multiple scattering approach

4 pages, 3 figures.-- PACS numbers: 73.20.Mf, 41.20.Jb, 61.16.Bg, 61.46.+wA fast, accurate, and general technique for solving Maxwell's equations in arbitrarily disposed clusters of dielectric objects is presented, based upon multiple scattering of electromagnetic multipole fields. Examples of application to the simulation of electron energy loss, radiation emission induced by fast electrons, and light scattering are offered. Large rates of Smith-Purcell radiation are predicted in the interaction of fast electrons with strings of Al and SiO2 spheres, suggesting its possible application in tunable soft UV light generation. Mutual electromagnetic interaction among objects in the different clusters under consideration is shown to be of primary importance.Help and support from the University of the Basque Country, the Spanish Ministerio de Educación y Cultura under Fulbright Grant No. FU-98-22726216, and the U.S. DOE under Contract No. DE-AC03-76SF00098 are gratefully acknowledged.Peer reviewe

Nonlinear Plasmonic Sensing with Nanographene

Plasmons provide excellent sensitivity to detect analyte molecules through their strong interaction with the dielectric environment. Plasmonic sensors based on noble metals are, however, limited by the spectral broadening of these excitations. Here we identify a new mechanism that reveals the presence of individual molecules through the radical changes that they produce in the plasmons of graphene nanoislands. An elementary charge or a weak permanent dipole carried by the molecule are shown to be sufficient to trigger observable modifications in the linear absorption spectra and the nonlinear response of the nanoislands. In particular, a strong second-harmonic signal, forbidden by symmetry in the unexposed graphene nanostructure, emerges due to a redistribution of conduction electrons produced by interaction with the molecule. These results pave the way toward ultrasensitive nonlinear detection of dipolar molecules and molecular radicals that is made possible by the extraordinary optoelectronic properties of graphene.Peer ReviewedPostprint (published version

Relativistic description of valence energy losses in the interaction of fast electrons with clusters of dielectrics: Multiple-scattering approach

A fully relativistic description of valence energy losses suffered by fast electrons passing near finite clusters of arbitrarily disposed dielectric objects is presented using an accurate technique suited to solve Maxwell’s equations. The method is based upon an expansion of the electromagnetic field in terms of multipoles around each of the objects of the cluster. Multiple elastic scattering of those multipole expansions is then performed until convergence is achieved. The energy loss, obtained from the induced electric field acting back on the electron, is computed in a time proportional to the square of the number of objects in the cluster, N2. Numerical examples are presented for various clusters formed by N=1–198 homogeneous spheres made of SiO2 and Al, and also for clusters of Si spheres coated with SiO2. Both relativistic effects and the interaction between the constituents of the cluster are shown to be of primary importance in the understanding of the position and magnitude of the features exhibited by the calculated electron-energy-loss spectra.Help and support from the University of the Basque Country and the Spanish Ministerio de Educacion y Cultura, under Fulbright Grant No. FU-98-22726216, is gratefully acknowledged. Part of this work was supported by the U.S. Department of Energy under contract No. DEAC03-76SF00098.Peer reviewe

Radiative heat transfer between neighboring particles

The near-field interaction between two neighboring particles is known to produce enhanced radiative heat transfer. We advance in the understanding of this phenomenon by including the full electromagnetic particle response, heat exchange with the environment, and important radiative corrections both in the distance dependence of the fields and in the particle absorption coefficients. We find that crossed terms of electric and magnetic interactions dominate the transfer rate between gold and SiC particles, whereas radiative corrections reduce it by several orders of magnitude even at small separations. Radiation away from the dimer can be strongly suppressed or enhanced at low and high temperatures, respectively. These effects must be taken into account for an accurate description of radiative heat transfer in nanostructured environments. © 2012 American Physical Society.This work has been supported by the Spanish Ministry of Science and Innovation (MAT2010-14885 and Consolider NanoLight.es) and the European Commission (FP7-ICT-2009-4-248909-LIMA and FP7-ICT-2009-4-248855-N4E). A.M. acknowledges financial support through FPU from ME.Peer Reviewe

Optically tunable surfaces with trapped particles in microcavities

We introduce optically tunable surfaces based upon metallic gold nanoparticles trapped in open, water-filled gold cavities. The optical properties of the surfaces change dramatically with the presence and location of the particles inside the cavities. The precise position of the particles is shown to be controllable through optical forces exerted by external illumination, thus leading to all-optical tunability, whereby the optical response of the surfaces is tuned through externally applied light. We discuss the performance of the cavity-particle complex in detail and provide theoretical support for its application as a novel concept of a large-scale optically tunable system. © 2008 The American Physical Society.This work was supported by the Spanish MEC (NAN2004- 08843-C05-05 and MAT2007-66050) and by the EU-FP6 (NMP4-2006-016881 ‘‘SPANS’’).Peer Reviewe

Electron diffraction by vacuum fluctuations

Vacuum fluctuations are known to produce electron diffraction leading to decoherence and self-interference. These effects have so far been studied as either an extension of the Aharonov–Bohm effect in front of a planar perfect conductor or through path integral analysis. Here, we present a simpler, general, and rigorous derivation based on a direct solution of the quantum electrodynamic aloof interaction between the electron and a material structure in the temporal gauge. Our approach allows us to study dissipative media, for which we show examples of electron wave function shaping due to the interaction with real-metal surfaces. We further present a proof of the relation between the phase associated with vacuum fluctuations and the Aharonov–Bohm effect produced by the image self-interaction that is valid for arbitrary geometries. Besides their fundamental interest, our results could be useful for on-demand patterning of electron beams with potential application in nondestructive nanoscale imaging and spectroscopy.Peer ReviewedPostprint (published version

Spontaneous light emission in complex nanostructures

The spontaneous emission of an excited atom surrounded by different materials is studied in the framework of a semiclassical approach, where the transition dipole moment acts as the source of the emission field. The emission in the presence of semiinfinite media, metallic nanorings, spheres, gratings, and other complex geometries is investigated. Strong emission enhancement effects are obtained in some of these geometries associated to the excitation of plasmons (e.g., in nanorings or spheres). Furthermore, the emission is shown to take place only along narrow angular distributions when the atom is located inside a low-index dielectric and near its planar surface, or when metallic nanogratings are employed at certain resonant wave lengths. In particular, axially symmetric gratings made of real silver metal are considered, and both emission rate enhancement and focused far-field emission are achieved simultaneously when the grating is decorated with further nanostructures.This work has been supported in part by the Basque Departamento de Educacion, Universidades e Investigacion, the University of the Basque Country UPV/EHU (Contract No. 00206.215-13639/2001) and the Spanish Ministerio de Ciencia y Tecnologia (Contract No. MAT2001-0946).Peer reviewe

Near-field focusing with optical phase antennas

We investigate the near-field focusing properties of three- dimensional phase antennas consisting of concentric rings designed to have source and image spots separated by several microns from the lens. Tight focal spots are obtained for silicon or gold rings patterned in a silica matrix. We analyze in detail the dependence of the performance of these lenses on geometrical parameters such as the number of rings, the ring thickness, and the focal distance. Subwavelength focal spots are found to form at distances of tens of wavelengths from the lens, thus suggesting applications to remote sensing and penlight microscopy and lithography. © 2009 Optical Society of America.This work has been supported by the Spanish MICINN (MAT2007-66050 and Consolider NanoLight.es) and by the EU (NMP4-2006-016881-SPANS and NMP4-SL-2008-213669- ENSEMBLE).Peer Reviewe

Nanoscale force manipulation in the vicinity of a metal nanostructure

The tight focus of Gaussian beams is commonly used to trap dielectric particles in optical tweezers. The corresponding field distribution generates a well-defined trapping potential that is only marginally controllable on a nanometre scale. Here we investigate the influence of a metal nanostructure that is located in the vicinity of the trapping focus on the trapping potential by calculating the corresponding field and force distributions. Even for an excitation wavelength that is tuned far from the plasmonic resonance of the nanostructure, the presence of the latter alters significantly the trap potential. For the given nanostructure, a ring of spheres that is illuminated in the axial direction, a smaller focus volume is observed in comparison to free focus. The superposition of this non-resonant Gaussian field with a planar wave illumination that is tuned to the plasmonic resonance gives a handle to modify the trapping potential. Polarization and intensity of the resonant illumination allows modifying the equilibrium position of the trapping potential, thus providing means to steer dielectric particles with nanometre precision. © 2007 IOP Publishing Ltd.TB thanks the DFG for an Emmy Noether Fellowship.Peer Reviewe