Publications Project 05 -
Plasmonic Sources and Interaction

Project Leader: Joachim Krenn


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 06)


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 06)


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 06)


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 06)

 

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 06)

 

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 06)

 

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 06)

 

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 06)

 

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 06)

 

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 06)

 

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 06)


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 06)


 

 

 

 

 

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