Plenarvorträge - DPG-Tagungen

Halbleiterphysik Donnerstag
HL 36 Quantenpunkte und -drähte: Optische Eigenschaften I
Zeit: Donnerstag 10:15–13:30 Raum: H17
HL 36.1 Do 10:15 H17
Structural Relaxation and the Optical Properties of Ge and
Si Nanocrystals – ab initio Calculations — •Weißker Hans-
Christian 1 , Giancarlo Cappellini 2 , Jürgen Furthmüller 1 ,
and Friedhelm Bechstedt 1 — 1 IFTO, Friedrich-Schiller-Universität
Jena, Max-Wien-Platz 1, 07743 Jena — 2 Universitá di Cagliari, Cittadella
Universitaria, Strada Prov.le Monserrato-Sestu, Km 0.700, Italy
Though much effort has been put into the description of the properties
of nanocrystals, ab initio calculations of such systems are still a demanding
task. The ionic degrees of freedom, i.e., the structural relaxation, have
only recently become amenable to ab initio methods for larger clusters
because of the large computational requirements. The use of ultrasoft
pseudopotentials and of the projector-augmented-wave method helps to
alleviate this problem. Size-dependent electron-hole pair excitation energies
calculated by means of a DSCF method are presented.
The ionic relaxation is a twofold problem. First, there is the technical
problem of finding reasonable model structures for the crystallites. This
involves the question of the influence of the ground-state structural relaxation
on the desired quantities. For the optical properties, we demonstrate
that a proper account of the relaxation effects is indispensable
for the quantitative, and in some cases even the qualitative, description.
The second problem is the structural relaxation after electronic excitation,
which results in a contribution of a single nanocrystal to the Stokes
shift measurable for an experimental sample. Model calculations of the
Stokes shift are presented and discussed.
HL 36.2 Do 10:30 H17
Dephasing in Quantum Dots: Quadratic Coupling to Acoustic
Phonons — •Egor Muljarov 1,2 , Frank Grosse 1 , and Roland
Zimmermann 1 — 1 Institut für Physik der Humboldt-Universität zu
Berlin, Newtonstr. 15, 12489 Berlin, Germany — 2 General Physics Institute
RAS, Vavilova 38, Moscow 119991, Russia
A microscopic theory of optical transitions in quantum dots with
carrier-phonon interaction is developed. Virtual transitions into higher
confined states with acoustic phonon assistance add a quadratic phonon
coupling to the standard linear one, thus extending the independent Boson
model. Summing infinitely many diagrams in the cumulant, an exact
solution for the interband polarization is found. Its full time dependence
and the absorption lineshape of the quantum dot are calculated.
A temperature-dependent broadening of the zero-phonon line is obtained
and compared with available experimental data.
HL 36.3 Do 10:45 H17
Investigation of the coupling-dependence on the dephasing in
coupled quantum dots — •Matthias Schwab 1 , P. Borri 1 , W.
Langbein 1 , U. Woggon 1 , M. Bayer 1 , S. Fafard 2 , Z. Wasilewski 2 ,
and P. Hawrylak 2 — 1 Experimentelle Physik II, Otto-Hahn Strasse 4,
44221 Dortmund, Germany — 2 Institute for Microstructural Science, National
Research Concil, Ottawa, K1A 0R6, Canada
Due to their atom-like density of states semiconductor quantum dots
(QD) are often referred to as artificial atoms. QDs receive a lot of attention
as they are possible candidates in the field of quantum information
and cryptography. By shrinking the distance between two QDs, one can
bring the single-particle wave function to an overlap creating so called
semiconductor quantum dot molecules.
A prerequisite for any application of their quantum mechanical properties
is a detailed knowledge of their electronic levels and their coupling
to the environment.
In this talk we report on measurements of the dephasing times of the
exciton in InAs/GaAs quantum dot molecules having different interdot
barrier thicknesses. A heterodyne four-wave-mixing (FWM) technique is
used to measure the dephasing times in a temperature range from 5 K to
60 K. The results are compared to a quantum dot single layer. Systematic
dependencies of the homogeneous linewidth on barrier thickness and on
the temperature are reported.
HL 36.4 Do 11:00 H17
Structure Dependent Few-particle Effects in InAs Quantum
Dots — •Andrei Schliwa, Sven Rodt, Florian Guffarth,
Robert Heitz, Robert Seguin, and Dieter Bimberg — Institut
für Festkörperphysik, Technische Universität Berlin
Interaction between strongly localized charge carriers in zerodimensional
systems like quantum dots (QDs) depend sensitively on
their geometrical properties. Few-particle properties, like the biexciton
binding energies, may vary not only in size but also in sign. Based on
the configuration interaction method those properties are calculated
for realistic InAs QDs as function of their structural properties. The
wavefunctions are obtained from eight-band k·p calculations including
strain and piezoelectric effects.
The problem of correlating calculated and measured spectra can be circumvented
exploiting the monolayer splitting in photoluminescence (PL)
-spectra in appropriately grown samples. Detailed analysis of those spectra
shows that (i) the QDs are very flat, (ii) the QDs have well-defined
interfaces and (iii) each peak represents a subensemble of QDs sharing
the same height. With decreasing transition energy the height of the related
QD-ensemble increases subsequently by one monolayer. Single dot
spectra revealing details of few-particle effects are obtained by cathodoluminescence
measurements with shadow masks. Transition from positive
to negative biexciton binding energy with increasing exciton energy due
to decreasing dot size is observed and understood in the framework of
the above theory.
HL 36.5 Do 11:15 H17
Coulomb effects in the optical spectra of semiconductor quantum
dots — •Norman Baer, Paul Gartner, and Frank Jahnke
— Institute for Theoretical Physics, University Bremen
Emission spectra of individual self-assembeld quantum dots show a rich
structure due to Coulomb correlated multi-exciton states. We present a
theoretical investigation of this system based on a full diagonalisation
scheme for the localized states. In addition to a discussion of the renormalized
electronic energies this procedure allows a direct calculation of
the optical spectra in terms of the Coulomb correlated states.
The resulting multi-exciton spectra are discussed and a simplified
Hamiltonian is introduced in order to explain the main features of the
emission spectra. This simplified Hamiltonian, which is based on the relative
importance of the interaction matrix elements, provides an intuitive
picture for the structure of the multi-exciton emission spectra.
In addition to the spectra resulting from neutral excitons the emission
spectra of excitons in the presence of spectator charges will be discussed.
HL 36.6 Do 11:30 H17
Many-body theory of carrier capture and relaxation in semiconductor
quantum-dot lasers — •Torben Roland Nielsen, Paul
Gartner, and Frank Jahnke — Institute for Theoretical Physics,
University of Bremen, 28334 Bremen, Germany
In quantum-dot laser devices containing a quasi two-dimensional wetting
layer, a pump process initially populates the letting-layer states. The
scattering of carriers from these spatially-extended quasi two-dimensional
states into the quantum-dot states as well as the relaxation of carriers
between the quantum-dot levels are studied theoretically.
Based on the wave functions for the coupled quantum-dot/wettinglayer
system interaction matrix elements are calculated for carrier-carrier
Coulomb interaction and carrier-phonon interaction. Scattering rates
for various capture and relaxation processes are evaluated under quasiequilibrium
conditions.
For elevated carrier densities in the wetting layer, Coulomb scattering
provides processes with capture (relaxation) times typically faster
than 10ps (1ps). When energy conservation allows for interaction with
LO-phonons, comparable rates are obtained.
HL 36.7 Do 11:45 H17
Optical properties of semiconductor microcavity pillars — •J.
Wiersig 1 , N. Baer 1 , P. Gartner 1 , F. Jahnke 1 , M. Benyoucef
2 , S.M. Ulrich 2 und P. Michler 2 — 1 Universität Bremen —
2 Universität Stuttgart
A recent experiment demonstrated the generation of triggered photon
pairs due to cascaded biexciton-exciton emission from a single In-
GaAs/GaAs quantum dot in a cylindrical micropillar [1]. Such experiments
are based on the altered spontaneous emission of carriers in the
microcavity as well as on the selection of optical transitions by the resonator.
Within a detailed theory-experiment comparison we study op-