Ulrich Hohenester
 

Research.

Plasmonic nanoparticles.

plasmonics
Metal nanoparticles can sustain local surface plasmon excitations, particle plasmons, which are
hybrid modes of a light field coupled to a coherent electron charge oscillation. The properties of these excitations depend strongly on particle geometry and interparticle coupling, and give rise to a variety of effects, such as frequency-dependent absorption and scattering or near field enhancement. Particle plasmons enable the concentration of light fields to nanoscale volumes and play a key role in surface enhanced spectroscopy. The properties of molecules can be strongly modified upon their electromagnetic interaction with particle plasmons.

In cooperation with the experimental nanooptics group in Graz, we have investigated the fluorescence properties of molecules interacting with lithographically fabricated metal nanoparticles [1-3]. Other work has been devoted to electron energy loss microscopy of plasmonic nanoparticles [4,5], sensor applications [6,7], and third harmonic generation using plasmonic nanoantennas [8].

In the past years we have developed
a Matlab toolbox MNPBEM for the simulation of metallic nanoparticles (MNP), using a boundary element method (BEM) approach, which allows to solve Maxwell's equations for a dielectric environment where bodies with homogeneous and isotropic dielectric functions are separated by abrupt interfaces. Details about the MNPBEM toolbox can be found here.
  1. S. Gerber et al., Phys. Rev. B 75, 073404 (2007). (PDF) Phys. Rev.
  2. F. Reil et al.,  Nano Lett. 8, 4128 (2008).  (PDF)   Nano Lett.
  3. D. Koller et al.,  Phys. Rev. Lett. 104, 143901 (2010).  (PDF)  Phys. Rev.
  4. B. Schaffer et al., Phys. Rev. B 79, 041401(R) (2009).   (PDF)  Phys. Rev.
  5. U. Hohenester et al., Phys. Rev. Lett. 103, 106801 (2009).  (PDF)  Phys. Rev.
  6. J. Becker et al.,  Plasmonics 5, 161 (2010).  (PDF)
  7. A. Jakab et al.,  ACS Nano 5, 6880 (2011).  (PDF)  ACS Nano
  8. T. Hanke et al., Nano Letters 12, 992 (2012).  (PDF)   Nano Lett.

Quantum control of ultracold atoms

atomchip Trapping and coherent manipulation of cold neutral atoms in microtraps near surfaces of atomic chips is a promising approach towards full control of matter waves on small scales. This field of atom optics is making rapid progress, driven both by the fundamental interest in quantum systems and by the prospect of new devices based on quantum manipulations of neutral atoms.

In collaboration with the atomchip group of Jörg Schmiedmayer, we have investigated optimal quantum control of ultracold atoms in magnetic microtraps
[1] and the possibility to create and exploit number squeezing for atom interferometery [2-4]. More recently, we have brought our optimal quantum control protocols to the lab and have devised a control sequence for shaking up a 1D condensate from the ground to the first excited state [5]. This excited state represents a highly non-equilibrium state of the system, analogous to a laser gain medium after a pump pulse, and in the ensuing relaxation twin-atom pairs are produced.

In the past we have developed a Matlab toolbox OCTBEC designed for optimal quantum control, within the framework of optimal control theory (OCT), of Bose-Einstein condensates (BEC) [6]. The systems we have in mind are ultracold atoms in confined geometries, where the dynamics takes place in one or two spatial dimensions, and the confinement potential can be controlled by some external parameters. The toolbox provides a variety of Matlab classes for simulations based on the Gross-Pitaevskii equation, the multi-configurational Hartree method for bosons, and on generic few-mode models, as well as optimization problems.
Details about the OCTBEC toolbox can be found here.

Older work has been concerned with the proximity of the ultracold atoms to the solid-state structure, which introduces additional decoherence channels limiting the performance of the atoms. Most importantly, Johnson-Nyquist noise currents in the dielectric or metallic surface arrangements produce magnetic-field fluctuations at the positions of the atoms. Upon undergoing spin-flip transitions, the atoms become more weakly trapped or are even lost from the microtrap. In [7,8] we have shown that such decoherence could be almost completely suppressed by using superconducting wires.

  1. U. Hohenester et al., Phys. Rev. A 75, 023602 (2007). (PDF) Phys. Rev.
  2. J. Grond et al., Phys. Rev. A 79, 021603(R) (2009).  (PDF)  Phys. Rev.
  3. J. Grond et al., Phys. Rev. A 80, 053625 (2009).  (PDF)   Phys. Rev.
  4. J. Grond et al.,  New J. Phys. 12, 065036 (2010). (PDF)   Selected as "Best of 2010"
  5. R. Bücker et al, Nature Physics 7, 608 (2011).  (PDF)  Nature
  6. U. Hohenester, to appear in Comp. Phys. Commun. (2013).  (PDF) 
  7. B. S. Skagerstam et al., Phys. Rev. Lett. 97, 070401 (2006). (PDF) Phys. Rev.
  8. U. Hohenester et al., Phys. Rev. A 76, 033618 (2007). (PDF)  Phys. Rev.

Quantum optics with semiconductur quantum dots

quantumdot Higher-dimensional semiconductors, such as quantum wells or wires, are usually considered as poor quantum devices because of the strong coupling to various solid-state excitations (e.g. phonons). For quantum dots things are much better due to the atomic-like carrier density of states, which results from the confinement in all spatial directions. In consequence, only a very limited number of scatterings is possible in these structures. Very long coherence times have indeed been observed in quantum dots. These studies have also revealed that a very specific coupling mechanism, namely the formation of a lattice distortion in the vicinity of the dot  usually called "polaron",  constitutes at low temperature one of the major decoherence channels.  In [1-3] we have shown that an optimization of control fields, such as external laser or voltage pulses, would allow to almost completely suppress such decoherence. We have also suggested that the polaron-mediated decoherence might be responsible for entanglement loss in quantum-dot based entangled-photon sources [4]. More recent work has been concerned with cavity-QED experiments, where we have shown that phonon couplings lead to an efficient scattering from quantum dot excitons to cavity photons [5,6].
  1. U. Hohenester et al., Phys. Rev. Lett. 92, 196801 (2004). (PDF), Phys. Rev.
  2. U. Hohenester, Phys. Rev. B 74, 161307(R) (2006). (PDF) Phys. Rev.
  3. U. Hohenester, Journal of Physics B 40, S315 (2007). (PDF)
  4. U. Hohenester et al., Phys. Rev. Lett. 99, 047402 (2007). (PDF) Phys. Rev
  5. U. Hohenester et al., Phys. Rev. B 80, 201311(R) (2009).  (PDF)  Phys. Rev.
  6. U. Hohenester, Phys. Rev. B 81, 155303 (2010).  (PDF)   Phys. Rev.

Matlab Toolboxes.

  • MNPBEM - A Matlab toolbox for the simulation of plasmonic nanoparticles.
  • OCTBEC - A Matlab toolbox for optimal quantum control of Bose-Einstein condensates.

Collaborations.

Joachim Krenn (Graz, Austria)
Ferdinand Hofer (Graz, Austria)
Wolfgang Kautek (Vienna, Austria)
Jörg Schmiedmayer (Vienna, Austria)
Carsten Sönnichsen (Mainz, Germany)
Alfred Leitenstorfer (Konstanz, Germany)
Rudolf Bratschitsch (Chemnitz, Germany)
Jonathan Finley (WSI Munich, Germany)
Elisa Molinari (Modena, Italy)
Atac Imamoglu (ETHZ Zürich, Switzerland)

Ulrich Hohenester
Institut für Physik, Karl-Franzens Universität Graz, Austria