Publications Project 06 -
Simulation of Plasmonic Nanoparticles

Project Leader: Ulrich Hohenester

Mapping vibrational surface and bulk modes in a single nanocube

M. J. Lagos, A. Trügler, U. Hohenester & P. E. Batson

  Imaging of vibrational excitations in and near nanostructures is essential for developing low-loss infrared nanophotonics, controlling heat transport in thermal nanodevices, inventing new thermoelectric materials and understanding nanoscale energy transport. Spatially resolved electron energy loss spectroscopy has previously been used to image plasmonic behaviour in nanostructures in an electron microscope, but hitherto it has not been possible to map vibrational modes directly in a single nanostructure, limiting our understanding of phonon coupling with photons and plasmons. Here we present spatial mapping of optical and acoustic, bulk and surface vibrational modes in magnesium oxide nanocubes using an atom-wide electron beam. We find that the energy and the symmetry of the surface polariton phonon modes depend on the size of the nanocubes, and that they are localized to the surfaces of the nanocube. We also observe a limiting of bulk phonon scattering in the presence of surface phonon modes. Most phonon spectroscopies are selectively sensitive to either surface or bulk excitations; therefore, by demonstrating the excitation of both bulk and surface vibrational modes using a single probe, our work represents advances in the detection and visualization of spatially confined surface and bulk phonons in nanostructures.

Published: 2017-03-22
Nature 543, 533 (2017)
DOI: 10.1038/nature21699

Mapping the local particle plasmon sensitivity with a scanning probe

M. Krug, G. Schaffernak, M. Belitsch, M. Gašparic, V. Leitgeb, A. Trügler, U. Hohenester, J. R. Krenn, and A. Hohenau

  We probe the local sensitivity of an optically excited plasmonic nanoparticle by changing the local dielectric environment through a scanning glass fiber tip. Recording the particle plasmon scattering spectrum for each tip position allows us to observe spectral resonance shifts concurrent with changes in scattering intensity and plasmon damping. For the tip-induced spectral shifts we find the strongest sensitivity at the particle edges, in accordance with the spatial plasmonic field profile. In contrast, the strongest sensitivity occurs at the center of the particle if the scattering intensity is probed at the short wavelength slope of the plasmon resonance instead of the resonance position. This bears important implications for plasmonic sensing, in particular when done at a single light wavelength.

Published: 2016-08-30
Nanoscale, 2016
DOI: 10.1039/C6NR05800K
(--> also listed in Project 05)

Edge mode coupling within a plasmonic nanoparticle

F-P. Schmidt, H. Ditlbacher, A. Hohenau, U. Hohenester, F. Hofer, and J.R. Krenn

  The coupling of plasmonic nanoparticles can strongly modify their optical properties. Here, we show that the coupling of the edges within a single rectangular particle leads to mode splitting and the formation of bonding and antibonding edge modes. We are able to unambiguously designate the modes due to the high spatial resolution of electron microscopy-based electron energy loss spectroscopy and the comparison with numerical simulations. Our results provide simple guidelines for the interpretation and the design of plasmonic mode spectra.

Published: 2016-07-18
Nano Letters, article ASAP
DOI: 10.1021/acs.nanolett.6b02097
(--> also listed in Project 05)

Exciton-exciton annihilation and stimulated biexciton stimulated emission in graphene nanoribbons

Soavi, G., S. Dal Conte, C. Manzoni, D. Viola, A. Narita, X. Feng, U. Hohenester, E. Molinari, D. Prezzi, K. Müllen, and G. Cerullo

  Graphene nanoribbons display extraordinary optical properties due to one-dimensional quantum-confinement, such as width-dependent bandgap and strong electron–hole interactions, responsible for the formation of excitons with extremely high binding energies. Here we use femtosecond transient absorption spectroscopy to explore the ultrafast optical properties of ultranarrow, structurally well-defined graphene nanoribbons as a function of the excitation fluence, and the impact of enhanced Coulomb interaction on their excited states dynamics. We show that in the high-excitation regime biexcitons are formed by nonlinear exciton–exciton annihilation, and that they radiatively recombine via stimulated emission. We obtain a biexciton binding energy of ≈250 meV, in very good agreement with theoretical results from quantum Monte Carlo simulations. These observations pave the way for the application of graphene nanoribbons in photonics and optoelectronics.

Published: 2016-03-17
Nature Communications 7, 11010 (2016)
DOI: 10.1038/ncomms11010

Gap plasmonics of silver nanocube dimers

D. Knebl, A. Hörl, A. Trügler, J. Kern, J. Krenn, P. Puschnig, U. Hohenester

  We theoretically investigate gap plasmons for two silver nanocubes coupled through a molecular tunnel junction. In the absence of tunneling, the redshift of the bonding mode saturates with decreasing gap distance. Tunneling at small gap distances leads to a damping and slight blueshift of the bonding mode, but no low-energy charge transfer plasmon mode appears in the spectra. This finding is in stark contrast to recent work of Tan et al. [Science 343, 1496 (2014)].

Published: 2016-02-09
Physical Review B 93, 081405(R) (2016)
DOI: 10.1103/PhysRevB.93.081405
(--> also listed in Project 05)


Three dimensional sensitivity characterization of plasmonic nanorods for refractometric biosensors

V. Leitgeb, A. Trügler, S. Köstler, M.K. Krug, U. Hohenester, A. Hohenau, A. Leitner, J.R. Krenn

  An experimental three dimensional characterization of the local refractive index sensitivity of plasmonic gold nanorods is performed by controlled apposition of lithographic nanostructures. We show up to seven times higher sensitivity values to local changes in the refractive index at the particle tip than center. In addition, successive deposition of defined nm-thin dielectric layers on nanorods covered with stripe masks allows us to study the sensitivity decrease normal to the particle surface separately for different particle sites. Clear trends to a stronger sensitivity decay at sites of higher local sensitivity are demonstrated experimentally and theoretically. Our sensitivity characterization provides an important tool to find the most suitable particle type and particle site for specific bio-sensing applications.

Published: 2015-12-30
Nanoscale 8, 2974 (2016)
DOI: 10.1039/c5nr06401e
(--> also listed in Project 05)


Plasmon modes of a silver thin film taper probed with STEM-EELS

F-P. Schmidt, H. Ditlbacher, A. Trügler, U. Hohenester, A. Hohenau, F. Hofer, J. Krenn

  By focusing propagating surface plasmons, electromagnetic energy can be delivered to nanoscale volumes. In this context, we employ electron energy loss spectroscopy in a scanning transmission electron microscope to characterize the full plasmonic mode spectrum of a silver thin film tapered to a sharp tip. We show that the plasmon modes can be ordered in film and edge modes and corroborate our assignment through supplementary numerical simulations. In particular, we find that the focused plasmon field at the taper tip is fueled by edge modes.

Published: 2015-11-26
OSA, Optics Letters, 5670, Vol. 40, No.23 (2015)
DOI: 10.1364/OL.40.005670
(--> also listed in Project 05)


Correlated 3D Nanoscale Mapping and Simulation of Coupled Plasmonic Nanoparticles

G. Haberfehlner, A. Trügler, F.P. Schmidt, A. Hörl, F. Hofer, U. Hohenester, G. Kothleitner

  Electron tomography in combination with electron energy-loss spectroscopy (EELS) experiments and simulations was used to unravel the interplay between structure and plasmonic properties of a silver nanocuboid dimer. The precise 3D geometry of the particles fabricated by means of electron beam lithography was reconstructed through electron tomography, and the full three-dimensional information was used as an input for simulations of energy-loss spectra and plasmon resonance maps. Excellent agreement between experiment and theory was found throughout, bringing the comparison between EELS imaging and simulations to a quantitative and correlative level. In addition, interface mode patterns, normally masked by the projection nature of a transmission microscopy investigation, could be unambiguously identified through tomographic reconstruction. This work overcomes the need for geometrical assumptions or symmetry restrictions of the sample in simulations and paves the way for detailed investigations of realistic and complex plasmonic nanostructures.

Published: 2015-10-23
Nano Letters 2015, 15, 7726-7730
DOI: 10.1021/acs.nanolett.5b03780


Full Three-Dimensonal Reconstruction of the Dyadic Green Tensor from Electron Energy Loss Spectroscopy of Plasmonic Nanoparticles

A. Hörl, A. Trügler, U. Hohenester

  Electron energy loss spectroscopy (EELS) has emerged as a powerful tool for the investigation of plasmonic nanoparticles, but the interpretation of EELS results in terms of optical quantities, such as the photonic local density of states, remains challenging. Recent work has demonstrated that, under restrictive assumptions, including the applicability of the quasistatic approximation and a plasmonic response governed by a single mode, one can rephrase EELS as a tomography scheme for the reconstruction of plasmonic eigenmodes. In this paper we lift these restrictions by formulating EELS as an inverse problem and show that the complete dyadic Green tensor can be reconstructed for plasmonic particles of arbitrary shape. The key steps underlying our approach are a generic singular value decomposition of the dyadic Green tensor and a compressed sensing optimization for the determination of the expansion coefficients. We demonstrate the applicability of our scheme for prototypical nanorod, bowtie, and cube geometries.

Published: 2015-09-04
ACS Photonics, 2 (10), pp 1429–1435 (2015)
DOI: 10.1021/acsphotonics.5b00256


Imaging nanowire plasmon modes with two-photon polymerization

C. Gruber, A. Hirzer, V. Schmidt, A. Trügler, U. Hohenester, H. Ditlbacher, A. Hohenau, J. Krenn

  Metal nanowires sustain propagating surface plasmons that are strongly confined to the wire surface. Plasmon reflection at the wire end faces and interference lead to standing plasmon modes. We demonstrate that these modes can be imaged via two-photon (plasmon) polymerization of a thin film resist covering the wires and subsequent electron microscopy. Thereby, the plasmon wavelength and the phase shift of the nanowire mode picked up upon reflection can be directly retrieved. In general terms, polymerization imaging is a promising tool for the imaging of propagating plasmon modes from the nano- to micro-scale.

Published: 2015-02-23
Applied Physics Letters 106, 081101 (2015)
DOI: 10.1063/1.4913470
(--> also listed in Project 05)


Effect of multipole excitations in electron energy-loss spectroscopy of surface plasmon modes in silver nanowires

X. Zhou, A. Hörl, A. Trügler, U. Hohenester, T.B. Norris, A.A. Herzing

  We have characterized the surface plasmon resonance (SPR) in silver nanowires using spatially resolved electron energy loss spectroscopy (EELS) in the scanning transmission electron microscope. Non-symmetric EELS spectra due to high-k SPR propagation along the nanowire and spectral shifts due to higher-order mode excitation are observed when the beam is positioned near the tip of the nanowire. When the beam is far from the tip region and on the side of nanowire, no spectral shifts are observed as the beam is scanned in the radial direction of the nanowire. The experimental spectra are compared with three different theoretical approaches: direct numerical calculation of the energy loss, analytical models for energy loss, and numerical simulations using an optical model. All three models reproduce the spectral shifts as the electron beam approaches the cap of the nanowire. The analytical model reveals the origin of the shifts in high-order plasmon mode excitation.

Published: 2014-12-08
Journal of Applied Physics 116, 223101 (2014)
DOI: 10.1063/1.4903535


Probing plasmonic breathing modes optically

M.K. Krug, M. Reisecker, A. Hohenau, H. Ditlbacher, A. Trügler, U. Hohenester, J. Krenn

  The confinement of surface plasmon modes in flat nanoparticles gives rise to plasmonic breathing modes. With a vanishing net dipole moment, breathing modes do not radiate, i.e., they are optically dark. Having thus escaped optical detection, breathing modes were only recently revealed in silver nanodisks with electron energy loss spectroscopy in an electron microscope. We show that for disk diameters >200 nm, retardation induced by oblique optical illumination relaxes the optically dark character. This makes breathing modes and thus the full plasmonic mode spectrum accessible to optical spectroscopy. The experimental spectroscopy data are in excellent agreement with numerical simulations.

Published: 2014-10-16
Applied Physics Letters, 105, 171103 (2014)
DOI: 10.1063/1.4900615
(--> also listed in Project 05)


Morphing a plasmonic nanodisk into a nanotriangle

F.-P. Schmidt, H. Ditlbacher, F. Hofer, J. Krenn, U. Hohenester

  We morph a silver nanodisk into a nanotriangle by producing a series of nanoparticles with electron beam lithography. Using electron energy loss spectroscopy (EELS), we map out the plasmonic eigenmodes and trace the evolution of edge and film modes during morphing. Our results suggest that disk modes, characterized by angular order, can serve as a suitable basis for other nanoparticle geometries and are subject to resonance energy shifts and splittings, as well as to hybridization upon morphing. Similar to the linear combination of atomic orbitals (LCAO) in quantum chemistry, we introduce a linear combination of plasmonic eigenmodes to describe plasmon modes in different geometries, hereby extending the successful hybridization model of plasmonics.

Published: 2014-07-07
ACS Publications - Nano Letters
DOI: 10.1021/nl502027r
(--> also listed in Project 05)

Plasmon Mapping in Au@Ag Nanocube Assemblies

B. Goris, G. Guzzinati, C. Fernández-López, J. Pérez-Juste, L. M. Liz-Marzán, A. Trügler, U. Hohenester, J. Verbeeck, S. Bals, G. Van Tendeloo

  Surface plasmon modes in metallic nanostructures largely determine their optoelectronic properties. Such plasmon modes can be manipulated by changing the morphology of the nanoparticles or by bringing plasmonic nanoparticle building blocks close to each other within organized assemblies. We report the EELS mapping of such plasmon modes in pure Ag nanocubes, Au@Ag core−shell nanocubes, and arrays of Au@Ag nanocubes. We show that these arrays enable the creation of interesting plasmonic structures starting from elementary building blocks. Special attention will be dedicated to the plasmon modes in a triangular array formed by three nanocubes. Because of hybridization, a combination of such nanotriangles is shown to provide an antenna effect, resulting in strong electrical field enhancement at the narrow gap between the nanotriangles.

Published: 2014-06-27
J. Chem. Phys. C 118, 15356 (2014)
DOI: 10.1021/jp502584t


Universal dispersion of surface plasmons in flat nanostructures

F-P. Schmidt, H. Ditlbacher, U. Hohenester, A. Hohenau, F. Hofer, J. Krenn

  Dimensionality has a significant impact on the optical properties of solid-state nanostructures. For example, dimensionality-dependent carrier confinement in semiconductors leads to the formation of quantum wells, quantum wires and quantum dots. While semiconductor properties are governed by excitonic effects, the optical response of metal nanostructures is dominated by surface plasmons. Here we find that, in contrast to excitonic systems, the mode dispersions in plasmonic structures of different dimensionality are related by simple scaling rules. Employing electron energy loss spectroscopy, we show that the modes of silver nanodisks can be scaled to the surface and edge modes of extended silver thin films. We thereby introduce a general and intuitive ordering scheme for plasmonic excitations with edge and surface modes as the elementary building blocks.

Published: 2014-04-10
Nature Communications, Article Number: 3604
DOI: 10.1038/ncomms4604
(--> also listed in Project 05)


Near-field and SERS enhancement from rough plasmonic nanoparticles

A. Trügler, J-C. Tinguely, G. Jakopic, U. Hohenester, J. Krenn, A. Hohenau

  The lithographic fabrication of metal nanoparticles usually involves the thermal vacuum deposition of metals, which leads to polycrystallinity and surface roughness. In recent years, strong efforts have been made to clarify the role of such roughness in surface-enhanced Raman scattering (SERS). In this paper, we provide a systematic experimental and theoretical study of single lithographically fabricated nanoparticles to unravel the role of surface roughness and morphology on the optical far- and near-field properties. We find that the experimentally observed reduction of the SERS signal upon thermal annealing of particle arrays is caused by a complex interplay of changes in the dielectric response of gold, the resonance wavelength, and the reduced nanoscopic roughness.

Published: 2014-04-08
APS Physics, Physical Review B 89, 165409
DOI: 10.1103/PhysRevB.89.165409
(--> also listed in Project 05)

Simulating electron energy loss spectroscopy with the MNPBEM toolbox

U. Hohenester

  Within the MNPBEM toolbox, we show how to simulate electron energy loss spectroscopy (EELS) of plasmonic nanoparticles using a boundary element method approach. The methodology underlying our approach closely follows the concepts developed by Garca de Abajo and coworkers [for a review see Rev. Mod. Phys. 82, 209 (2010)]. We introduce two classes eelsret and eelsstat that allow in combination with our recently developed MNPBEM toolbox for a simple, robust, and ecient computation of EEL spectra and maps. The classes are accompanied by a number of demo programs for EELS simulation of metallic nanospheres, nanodisks, and nanotriangles, and for electron trajectories passing by or penetrating through the metallic nanoparticles. We also discuss how to compute electric elds induced by the electron beam and cathodoluminescence.

Published: 2014-03
Comp. Phys. Commun. 185, 1177 (2014)
DOI: 10.1016/j.cpc.2013.12.010

Optical near-field excitation at commercial scanning probe microscopy tips: a theoretical and experimental investigation

C. Huber, A. Trügler, U. Hohenester, Y. Prior, W. Kautek

  A systematic study of the influence of the excitation angle, the light polarization and the coating thickness of commercial SPM tips on the field enhancement in an apertureless scanning near-field optical microscope is presented. A new method to optimize the alignment of the electric field vector along the major tip axis by measuring the resonance frequency was developed. The simulations were performed with a MNPBEM toolbox based on the Boundary Element Method (BEM). The influence of the coating thickness was investigated for the first time. Coatings below 40 nm showed a drastic influence both on the resonance wavelength and the enhancement. A shift to higher angles of incidence for the maximum enhancement could be observed for greater tip radii.

Published: 2014-02-14
Phys. Chem. Chem. Phys. 16, 2289 (2014)
DOI: 10.1039/c3cp51730f

Spectral Modifications and Polarization Dependent Coupling in Tailored Assemblies of Quantum Dots and Plasmonic Nanowires

C. Gruber, A. Trügler, A. Hohenau, U. Hohenester, J. Krenn

  The coupling of optical emitters with a nanostructured environment is at the heart of nano- and quantum optics. We control this coupling by the lithographic positioning of a few (1−3) quantum dots (QDs) along plasmonic silver nanowires with nanoscale resolution. The fluorescence emission from the QD-nanowire systems is probed spectroscopically, by microscopic imaging and decay time measurements. We find that the plasmonic modes can strongly modulate the fluorescence emission. For a given QD position, the local plasmon field dictates the coupling efficiency, and thus the relative weight of free space radiation and emission into plasmon modes. Simulations performed with a generic few-level model give very good agreement with experiment. Our data imply that the 2D degenerate emission dipole orientation of the QD can be forced to predominantly emit to one polarization component dictated by the nanowire modes.

Published: 2013-08-22
NanoLetters 2013, 13, 4257−4262
DOI: 10.1021/nl4019947
(--> also listed in Project 05)

Tomography of Particle Plasmon Fields from Electron Energy Loss Spectroscopy

A. Hörl, A. Trügler, U. Hohenester

  We theoretically investigate electron energy loss spectroscopy (EELS) of metallic nanoparticles in the optical frequency domain. Using a quasistatic approximation scheme together with a plasmon eigenmode
expansion, we show that EELS can be rephrased in terms of a tomography problem. For selected single and coupled nanoparticles we extract the three-dimensional plasmon fields from a collection of rotated
EELS maps. Our results pave the way for a fully three-dimensional plasmon-field tomography and establish EELS as a quantitative measurement device for plasmonics.

Published: 2013-08-16
Physical Review Letters 111, 076801 (2013)
DOI: 10.1103/PhysRevLett.111.076801






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