Andreas Trügler

MNPBEM - A Matlab toolbox for the simulation of plasmonic nanoparticles ✩

Abstract MNPBEM is a Matlab toolbox for the simulation of metallic nanoparticles (MNP), using a boundary element method (BEM) approach. The main purpose of the toolbox is to solve Maxwellʼs equations for a dielectric environment where bodies with homogeneous and isotropic dielectric functions are separated by abrupt interfaces. Although the approach is in principle suited for arbitrary body sizes and photon energies, it is tested (and probably works best) for metallic nanoparticles with sizes ranging from a few to a few hundreds of nanometers, and for frequencies in the optical and near-infrared regime. The toolbox has been implemented with Matlab classes. These classes can be easily combined, which has the advantage that one can adapt the simulation programs flexibly for various applications. Program summary Program title: MNPBEM Catalogue identifier: AEKJ_v1_0 Program summary URL: http://cpc.cs.qub.ac.uk/summaries/AEKJ_v1_0.html Program obtainable from: CPC Program Library, Queenʼs University, Belfast, N. Ireland Licensing provisions: GNU General Public License v2 No. of lines in distributed program, including test data, etc.: 15 700 No. of bytes in distributed program, including test data, etc.: 891 417 Distribution format: tar.gz Programming language: Matlab 7.11.0 (R2010b) Computer: Any which supports Matlab 7.11.0 (R2010b) Operating system: Any which supports Matlab 7.11.0 (R2010b) RAM: ⩾1 GByte Classification: 18 Nature of problem: Solve Maxwellʼs equations for dielectric particles with homogeneous dielectric functions separated by abrupt interfaces. Solution method: Boundary element method using electromagnetic potentials. Running time: Depending on surface discretization between seconds and hours.

Ultrafast strong-field photoemission from plasmonic nanoparticles.

We demonstrate strong-field photoemission from plasmonic nanoparticles by ultrashort pulses. Significant (x110) field enhancement attributed to surface plasmons enable 25-eV electron generation in nano-localized fields around nanoparticles. Correlation between plasmonic resonance and electron spectra is shown.

Tailoring Spatiotemporal Light Confinement in Single Plasmonic Nanoantennas

Plasmonic nanoantennas are efficient devices to concentrate light in spatial regions much smaller than the wavelength. Only recently, their ability to manipulate photons also on a femtosecond time scale has been harnessed. Nevertheless, designing the dynamical properties of optical antennas has been difficult since the relevant microscopic processes governing their ultrafast response have remained unclear. Here, we exploit frequency-resolved optical gating to directly investigate plasmon response times of different antenna geometries resonant in the near-infrared. Third-harmonic imaging is used in parallel to spatially monitor the plasmonic mode patterns. We find that the few-femtosecond dynamics of these nanodevices is dominated by radiative damping. A high efficiency for nonlinear frequency conversion is directly linked to long plasmon damping times. This single parameter explains the counterintuitive result that rod-type nanoantennas with minimum volume generate by far the strongest third-harmonic emission as compared to the more bulky geometries of bow-tie-, elliptical-, or disk-shaped specimens.

Highly sensitive plasmonic silver nanorods.

We compare the single-particle plasmonic sensitivity of silver and gold nanorods with similar resonance wavelengths by monitoring the plasmon resonance shift upon changing the environment from water to 12.5% sucrose solution. We find that silver nanoparticles have 1.2 to 2 times higher sensitivity than gold, in good agreement with simulations based on the boundary-elements-method (BEM). To exclude the effect of particle volume on sensitivity, we test gold rods with increasing particle width at a given resonance wavelength. Using the Drude-model of optical properties of metals together with the quasi-static approximation (QSA) for localized surface plasmons, we show that the dominant contribution to higher sensitivity of silver is the lower background polarizability of the d-band electrons and provide a simple formula for the sensitivity. We improve the reversibility of the silver nanorod sensors upon repeated cycles of environmental changes by blocking the high energy parts of the illumination light.

Influence of surface roughness on the optical properties of plasmonic nanoparticles

Summary . In summary, we have investigated the influenceof surface roughness on the optical properties of plasmonicnanoparticlesandhavefoundasurprisinglysmalleffect.Usinga simulation and perturbation theory approach, we have beenable to trace back our findings to a motional narrowing, wherethe plasmon averages over the random height fluctuations. Asno corresponding conclusions prevail for the near-field opticalproperties, our results are in accordance with the findingsof “hot spots” in fluorescence or surface enhanced Ramanscattering experiments. Acknowledgments . This work has been supported in part bythe Austrian Science Fund FWF under Project No. P21235–N20. * ulrich.hohenester@uni-graz.at 1 S. A. Maier, Plasmonics: Fundamentals and Applications (Springer, Berlin, 2007). 2 J. A. Schuller, E. S. Barnard, W. Cai, Y. C. Jun, J. S. White, andM. L. Brongersma, Nat. Mater. 9 , 193 (2010). 3 P.Anger,P.Bharadwaj,andL.Novotny,Phys.Rev.Lett. 96 ,113002(2006). 4 K. Kneipp, M. Moskovits, and H. Kneipp (eds.),

Tomography of Particle Plasmon Fields from Electron Energy Loss Spectroscopy

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.

Mapping vibrational surface and bulk modes in a single nanocube

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.

Interaction of Single Molecules With Metallic Nanoparticles

We theoretically investigate the interaction between a single molecule and a metallic nanoparticle (MNP). We develop a general quantum mechanical description for the calculation of the enhancement of radiative and nonradiative decay channels for a molecule situated in the near-field regime of the MNP. Using a boundary element method approach, we compute the scattering rates for several nanoparticle shapes. We also introduce an eigenmode expansion and quantization scheme for the surface plasmons, which allows us to analyze the scattering processes in simple physical terms. An intuitive explanation is given for the large quantum yield of quasi-1-D and quasi-2-D nanostructures. Finally, we briefly discuss resonant Forster energy transfer in presence of MNPs.

Spectral modifications and polarization dependent coupling in tailored assemblies of quantum dots and plasmonic nanowires.

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.

Plasmon Mapping in Au@Ag Nanocube Assemblies

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.