Ulrich Hohenester

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.

Few-Particle Effects in Semiconductor Quantum Dots: Observation of Multicharged Excitons

We investigate experimentally and theoretically few-particle effects in the optical spectra of single quantum dots (QDs). Photodepletion of the QD together with the slow hopping transport of impurity-bound electrons back to the QD are employed to efficiently control the number of electrons present in the QD. By investigating structurally identical QDs, we show that the spectral evolutions observed can be attributed to intrinsic, multi-particle-related effects, as opposed to extrinsic QD-impurity environment-related interactions. From our theoretical calculations we identify the distinct transitions related to excitons and excitons charged with up to five additional electrons, as well as neutral and charged biexcitons.

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.

Exploiting exciton-exciton interactions in semiconductor quantum dots for quantum-information processing

We propose an all-optical implementation of quantum-information processing in semiconductor quantum dots, where electron-hole excitations (excitons) serve as the computational degrees of freedom (qubits). We show that the strong dot confinement leads to an overall enhancement of Coulomb correlations and to a strong renormalization of the excitonic states, which can be exploited for performing conditional and unconditional qubit operations.

Dark plasmonic breathing modes in silver nanodisks.

We map the complete plasmonic spectrum of silver nanodisks by electron energy loss spectroscopy and show that the mode which couples strongest to the electron beam has radial symmetry with no net dipole moment. Therefore, this mode does not couple to light and has escaped from observation in optical experiments. This radial breathing mode has the character of an extended two-dimensional surface plasmon with a wavenumber determined by the circular disk confinement. Its strong near fields can impact the hybridization in coupled plasmonic nanoparticles as well as couplings with nearby quantum emitters.

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.

Optical excitations of a self-assembled artificial ion

By use of magnetophotoluminescence spectroscopy, we demonstrate bias-controlled single-electron charging of a single quantum dot. Neutral, single, and double charged excitons are identified in the optical spectra. At high magnetic fields one Zeeman component of the single charged exciton is found to be quenched, which is attributed to the competing effects of tunneling and spin-flip processes. Our experimental data are in good agreement with theoretical model calculations for situations where the spatial extent of the hole wave functions is smaller as compared to the electron wave functions. Semiconductor quantum dots ~QD’s ! are often referred to as artificial atoms. Different levels of neutral occupancies in QD’s have been obtained in the last years by power dependent optical excitation. The associated few-particle states were intensively studied by multiexciton photoluminescence ~PL! spectroscopy and corresponding theoretical investigations. 1‐7 Occupancies with different numbers of electrons and holes result in charged exciton complexes. In analogy to QD’s with neutral occupancy—artificial atoms— charged exciton complexes may be considered as artificial ions. In the field of low-dimensional semiconductors charged excitons were first observed in quantum-well structures. 8 In QD’s, charged excitons were studied in inhomogeneously broadened ensembles by PL ~Ref. 9! as well as in interband transmission experiments, 10 and recently also in single, optically tunable QD’s, 11 as well as in electrically tunable quantum rings by PL. 12 Few-particle theory predicts binding energies for charged QD excitons on the order of some meV. 11,13 This allows for the controlled manipulation of energetically well-separated few-particle states under the action of an external gate electrode. Discrete and stable numbers of extra charges are thereby possible via the Coulomb blockade mechanism. In future experiments and possible applications, the resonant optical absorption and emission of such systems is expected to be tunable between discrete and characteristic energies. Moreover such few-particle systems are expected to exhibit an interesting variety of spin configurations, which can be controlled by an external magnetic field, gate-induced occupancy, and spin-selective optical excitation. In the present paper we present, for the first time to our knowledge, experimental and theoretical results on the gate-controlled charging of a single InxGa12xAs QD with an increasing number of electrons probed by magneto-PL. For controlled charging of individual QD’s a special electric-field tunable n-i structure has been grown by molecular-beam epitaxy. In0.5Ga0.5As QD’s are embedded in an i-GaAs region 40 nm above an n-doped GaAs layer (5 310 18 cm 23 ) which acts as a back contact. The growth of the QD’s is followed by 270-nm i-GaAs, a 40-nm-thick Al0.3Ga0.7As blocking layer, and a 10-nm i-GaAs cap layer. As a Schottky gate we use a 5-nm-thick semitransparent Ti layer. The samples were processed as photodiodes combined with electron-beam-structured shadow masks with apertures ranging from 200 to 800 nm. Schematic overviews of the sample and the band diagram are shown in Figs. 1~a! and 1~b!. The occupation of the QD with electrons can be controlled by an external bias voltage VB on the Schottky gate with respect to the back contact. For increasing VB the band flattens, and the electron levels of the QD are shifted below the Fermi energy of the n-GaAs back contact, which results in successive single-electron charging of the QD. In our experiments excitons are generated optically at low rate and form charged excitons together with the VB induced extra electrons in the QD. We used a HeNe laser ~632.8 nm! for

Simulating electron energy loss spectroscopy with the MNPBEM toolbox

Abstract 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 Garcia de Abajo and coworkers (Garcia de Abajo, 2010). We introduce two classes eelsret and eelsstat that allow in combination with our recently developed MNPBEM toolbox for a simple, robust, and efficient 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 fields induced by the electron beam and cathodoluminescence. Program summary Program title: MNPBEM toolbox Catalogue identifier: AEKJ_v2_0 Program summary URL: http://cpc.cs.qub.ac.uk/summaries/AEKJ_v2_0.html Program obtainable from: CPC Program Library, Queen’s University, Belfast, N. Ireland Licensing provisions: Standard CPC licence, http://cpc.cs.qub.ac.uk/licence/licence.html No. of lines in distributed program, including test data, etc.: 38886 No. of bytes in distributed program, including test data, etc.: 1222650 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 GB Classification: 18. Catalogue identifier of previous version: AEKJ_v1_0 Journal reference of previous version: Comput. Phys. Comm. 183 (2012) 370 External routines: MESH2D available at www.mathworks.com Does the new version supersede the previous version?: Yes Nature of problem: Simulation of electron energy loss spectroscopy (EELS) for plasmonic nanoparticles. Solution method: Boundary element method using electromagnetic potentials. Reasons for new version: The new version of the toolbox includes two additional classes for the simulation of electron energy loss spectroscopy (EELS) of plasmonic nanoparticles, and corrects a few minor bugs and inconsistencies. Summary of revisions: New classes “ eelsstat ” and “ eelsret ” for the simulation of electron energy loss spectroscopy (EELS) of plasmonic nanoparticles have been added. A few minor errors in the implementation of dipole excitation have been corrected. Running time: Depending on surface discretization between seconds and hours.

Twin-atom beams

Twin photons — pairs of highly correlated photons — are one of the building blocks for quantum optics, and are used in both fundamental tests of quantum physics and technological applications. Now an efficient source for correlated atom pairs is demonstrated, promising to enable a wide range of experiments in the field of quantum matter-wave optics.