Plenarvorträge - DPG-Tagungen

Halbleiterphysik Montag
HL 3.12 Mo 13:00 H17
Structural properties of self-assembled InAs quantum dots —
•Christian Heyn 1 , Arne Bolz 1 , Theophilos Maltezopoulos 1 ,
Robert L. Johnson 2 , and Wolfgang Hansen 1 — 1 Institut für
Angewandte Physik, Universität Hamburg, Jungiusstr. 11, 20355 Hamburg,
Germany — 2 Institut für Experimentalphysik, Universität Hamburg,
Luruper Chaussee 149, 22761 Hamburg, Germany
We fabricate self-assembled InAs quantum dots (QDs) with solid-
HL 4 Ultrakurzzeitphänomene
source molecular beam epitaxy (MBE) on GaAs and AlAs substrates.
The structural properties of the QDs are studied using in situ electron
diffraction (RHEED), atomic force microscopy (AFM), and x-raydiffraction,
with emphasis on the growth parameter dependence of the
QD density, size, and composition as well as on the critical coverage up to
QD formation. In a second step, we develop a kinetic rate-equations based
model of the strain-induced InAs quantum dot formation and refine the
model assumptions by comparing calculation results with experimental
data.
Zeit: Montag 10:15–13:15 Raum: H13
HL 4.1 Mo 10:15 H13
Microscopic theory of carrier-wave Rabi flopping in semiconductors
— Q. T. Vu 1 , L. Bányai 1 , H. Haug 1 , O. D. Mücke 2 , •T.
Tritschler 2 , M. Wegener 2 , U. Morgner 3 , and F. X. Kärtner 4
— 1 Institut für Theoretische Physik, Universität Frankfurt, 60054 Frankfurt
am Main, Germany — 2 Institut für Angewandte Physik, Wolfgang-
Gaede-Straße 1, Universität Karlsruhe (TH), 76131 Karlsruhe, Germany
— 3 Max-Planck-Institut für Kernphysik, Saupfercheckweg 1, 69117 Heidelberg,
Germany — 4 Department of Electrical Engineering and Computer
Science and Research Laboratory of Electronics, MIT, Cambridge,
MA 02139, U.S.A.
Carrier-wave Rabi flopping refers to a Rabi oscillation, the frequency of
which becomes comparable to the carrier frequency of light. Recently, we
have performed corresponding experiments on 50-100nm thin GaAs films
excited with intense 5fs pulses. However, the semiconductor problem has
only been discussed in terms of two-level systems [1]. Consequently, scepticism
has been expressed that this simple although intuitive description
is able to properly catch the underlying physics. Here, we will show that
the experimental spectra are nicely reproduced by a microscopic theory
based on the semiconductor Bloch equations, treating the GaAs band
structure via a tight-binding approach and accounting for the wave-vector
and carrier-density dependent relaxation and dephasing processes in a realistic
manner. This shows that our intuitive interpretation in terms of
carrier-wave Rabi flopping is correct indeed.
[1] O.D. Mücke et al., Phys. Rev. Lett. 89, 127401 (2002)
HL 4.2 Mo 10:30 H13
Dephasing of excitonic and biexcitonic polarization: Intensity
and temperature dependence — •Lars Wischmeier, Hans
Georg Breunig, Tobias Voss, Ilja Rückmann, and Jürgen
Gutowski — Institut für Festkörperphysik, Universität Bremen, P.O.
Box 330440, D-28334 Bremen
The decay of coherent excitonic and biexcitonic polarization was investigated
in a high quality 10 nm ZnSe/ZnS0,07Se single quantum well
as a function of intensity, temperature (4 − 70 K), and polarization
of the 110 fs excitation pulses. The spectral position of the pulses was
tuned such that the resonances of the 1s heavy-hole exciton and the
biexciton were exclusively excited. The dephasing rates were obtained
by both four-wave-mixing (FWM) and pulse-transmission measurements.
The experimental results were fitted with the temperature dependence of
the homogeneous linewidth due to phonon scattering. From the temperature
dependence of the decay of the FWM signal of the excitonic and
biexcitonic polarizations the acoustic- and optical-phonon-scattering coefficients
were determined.
Additionally the contrast ratio obtained from coherent-control measurements
with two phase locked pulses was studied as a function of
temperature and intensity.
HL 4.3 Mo 10:45 H13
Kohärente und inkohärente Ladungsträgerdynamik in
Quantentopf-Infrarot-Photodetektoren — •Thomas Maier 1 ,
Harald Schneider 1 , Hui Chun Liu 2 , Martin Walther 1 und Peter
Koidl 1 — 1 Fraunhofer-Institut für angewandte Festkörperphysik,
Tullastr. 72, D-79108 Freiburg — 2 Institute for Microstructural Sciences,
National Research Council, Ottawa K1A 0R6, Canada
Wir untersuchen Quantentopf-Infrarot-Photodetektoren (QWIPs), die
so konzipiert sind, dass drei energetisch äquidistante Niveaus entstehen.
Während die ersten beiden Zustände gebunden sind, stellt das dritte
Niveau eine Kontinuumsresonanz dar. Da Elektronen, die zum Photostrom
beitragen, über eine Zwei-Photon-Absorption in das Kontinuum
gelangen, ergibt sich eine quadratische Abhängigkeit des Photostroms
von der einfallenden Leistungsdichte. Führt man nun interferometrische
Autokorrelationsmessungen zweiter Ordnung an ultrakurzen Pulsen mit
einem solchen QWIP als Detektor durch, ist dessen Ansprechverhalten
durch die Lebensdauer und die Dephasierungszeit des Zwischenzustands
bestimmt. Eine numerische Analyse basierend auf den optischen Bloch-
Gleichungen ergibt Lebensdauern im Bereich von 500 fs (750 fs) und Dephasierungszeiten
von ca. 100 fs (250 fs) in topf-(modulations-)dotierten
Strukturen.
HL 4.4 Mo 11:00 H13
THz emission from GaSb samples with modified surfaces —
•Stephan Winnerl, Thomas Dekorsy und Manfred Helm — Institut
für Ionenstrahlphysik und Materialforschung, Forschungszentrum
Rossendorf, PF 510119, 01314 Dresden
The optimization of emitters for THz radiation pulses has gained a
lot of interest in recent years. They can be based on accelerating charge
carriers, which are photoexcited by fs-laser pulses in the surface field
of a semiconductor. We studied the THz emission from GaSb samples
that were annealed at different temperatures in the range from 300 ◦ C
to 700 ◦ C. Without annealing, no THz emission was observed under fs
excitation at 800 nm. This is consistent with the fact, that for GaSb no
surface states in the band gap have been found [1]. The THz emission
was strongest for the annealing temperature of 450 ◦ C with amplitudes
comparable to that of an InGaAs emitter. We attribute the THz emission
to a surface field caused by decomposition of the surface of the annealed
samples. The decomposition is confirmed by measuring the surface
stoichiometry using Auger electron spectroscopy. We suggest that the
decrease of THz intensity for the higher annealing temperatures is due
to a lowering of the carrier mobility. Our experiment demonstrates the
possibility to modify materials for THz emission in a very simple way.
[1] P.W. Chey, I.A. Babalado, T. Sukegawa, and W.E. Spicer, Phys.
Rev. Lett. 35, 1602 (1975).
HL 4.5 Mo 11:15 H13
Coherent switching of polariton modes and their quantum beats
in four-wave mixing and real-time resolved pulse-transmission
experiments on ZnSe SQWs — •I. Kudyk, T. Voss, H. G. Breunig,
I. Rückmann, and J. Gutowski — Institute of Solid-State
Physics, University of Bremen, POB 330440, D-28334 Bremen, Germany
For layer thicknesses above 20 nm the optical properties of ZnSe nanostructures
are usually dominated by the existence of different polariton
modes and their interactions.
Optical coherent control of these polariton modes is investigated by
use of a pair of 100 fs long phase-locked pulses in both pulse-transmission
experiments with real-time resolution and four-wave-mixing (FWM) experiments.
It is shown that for low excitation densities the contributions
of the different resonances can be selectively enhanced or diminished.
When continuously changing the inter-pulse delay time a succession of
constructive and destructive interference is observed separately for each
resonance. The coherent control of polariton modes also allows for a directed
manipulation of the beat structures on the real-time transients as
well as in the FWM signals.
Furthermore, the dephasing time of the polariton modes is analyzed in
spectrally resolved, two-pulse degenerate FWM experiments and studied
as a function of the excitation density and temperature. By additional
measurements of the polarization states of the different spectral components
the relative phase of the polariton modes is investigated.