
Research.
Plasmonic nanoparticles.
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
frequencydependent 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 [13]. 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.
 S.
Gerber et al., Phys.
Rev. B 75,
073404 (2007). (PDF) Phys. Rev.
 F. Reil et al.,
Nano Lett. 8, 4128 (2008).
(PDF) Nano Lett.
 D. Koller et al.,
Phys. Rev. Lett. 104,
143901 (2010).
(PDF)
Phys.
Rev.
 B. Schaffer et al., Phys. Rev. B 79, 041401(R) (2009).
(PDF) Phys.
Rev.
 U. Hohenester et al., Phys. Rev. Lett. 103, 106801 (2009). (PDF) Phys.
Rev.
 J. Becker et al.,
Plasmonics 5,
161 (2010).
(PDF)
 A. Jakab et al.,
ACS Nano 5, 6880 (2011). (PDF) ACS Nano
 T. Hanke et al., Nano Letters 12, 992 (2012).
(PDF) Nano Lett.
Quantum control of ultracold atoms
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 [24]. 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
nonequilibrium state of the system, analogous to a laser gain medium
after a pump pulse, and in the ensuing relaxation twinatom 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 BoseEinstein 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 GrossPitaevskii equation, the
multiconfigurational Hartree method for bosons, and on generic
fewmode 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 solidstate structure, which introduces additional decoherence
channels limiting the performance of the atoms. Most importantly,
JohnsonNyquist noise currents in the dielectric or metallic surface
arrangements produce magneticfield fluctuations at the positions of
the atoms. Upon undergoing spinflip 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.
 U.
Hohenester et al., Phys.
Rev. A 75,
023602 (2007). (PDF) Phys. Rev.
 J. Grond et al., Phys. Rev. A 79, 021603(R) (2009).
(PDF) Phys.
Rev.
 J. Grond et al.,
Phys. Rev. A 80, 053625 (2009).
(PDF) Phys.
Rev.
 J. Grond et al.,
New J. Phys. 12,
065036 (2010).
(PDF)
Selected as "Best of 2010"
 R. Bücker et al, Nature Physics 7, 608 (2011).
(PDF) Nature
 U. Hohenester, to appear in Comp. Phys. Commun. (2013).
(PDF)
 B. S. Skagerstam et al., Phys.
Rev. Lett. 97, 070401 (2006).
(PDF) Phys. Rev.
 U.
Hohenester et al., Phys. Rev. A 76, 033618 (2007). (PDF) Phys. Rev.
Quantum optics with semiconductur quantum dots
Higherdimensional semiconductors, such as quantum wells or wires, are
usually considered as poor quantum devices because of the strong
coupling to various solidstate excitations (e.g. phonons). For quantum
dots things are much better due to the atomiclike 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 [13]
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
polaronmediated decoherence might be responsible for entanglement loss
in quantumdot based entangledphoton sources [4]. More recent work has
been concerned with cavityQED experiments, where we have shown that
phonon couplings lead to an efficient scattering from quantum dot
excitons to cavity photons [5,6].
 U. Hohenester et al., Phys.
Rev. Lett. 92, 196801 (2004).
(PDF), Phys. Rev.
 U. Hohenester, Phys.
Rev. B 74, 161307(R) (2006).
(PDF) Phys. Rev.
 U. Hohenester, Journal
of Physics B 40,
S315 (2007). (PDF)
 U.
Hohenester et al., Phys.
Rev. Lett. 99,
047402 (2007). (PDF) Phys. Rev
 U. Hohenester et al.,
Phys. Rev. B 80,
201311(R) (2009).
(PDF) Phys.
Rev.
 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 BoseEinstein 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, KarlFranzens Universität Graz,
Austria 
