Semiconductor quantum posts (QPs) - nanowire-like InGaAs heterostructures in a GaAs matrix - resemble many
properties of regular self-assembled quantum dots (QDs), to which they are closely related. Due to their increased size as
compared to QDs, QPs have proven to be suitable for very low threshold interband lasers. However, their well
controllable height makes them attractive for precise tuning of the interband energy spacing that in QDs can only be
achieved via post-growth annealing. Specifically, the 1s - 2p transition energy is expected to drop below LO-phonon
energies at post heights of more than 30 nm, making them attractive as frequency-agile structures at terahertz
frequencies. In the work presented here we explore the capture dynamics of QP structures after photoexcitation into the
GaAs matrix. While the combined electron-hole dynamics are studied using time-resolved photoluminescence
spectroscopy, optical pump - THz probe experiments were performed in order to solely study the electron dynamics. The
results of the THz experiment show that after ultrafast excitation, electrons relax within a few picoseconds into the
quantum posts, which act as efficient traps. The saturation of the quantum post states, probed by photoluminescence, was
reached at approximately ten times the quantum post density in the samples. Also studied was the presence of possible
electronic resonances after direct photoexcitation into QPs where a broad absorption around 1.5 THz was observed.
S. Winnerl, D. Stehr, M. Wagner, H. Schneider, M. Helm, W. Seidel, P. Michel, E. Zibik, B. Carpenter, N. Porter, M. Skolnick, L. Wilson, T. Grange, R. Ferreira, G. Bastard, M. Orlita, P. Plochocka, P. Kossacki, M. Potemski, M. Sprinkle, C. Berger, W. de Heer
The free-electron laser FELBE at the Helmholtz-Zentrum Dresden Rossendorf enables experiments with spectral,
temporal, and, by means of near-field microscopy, also high spatial resolution. FELBE delivers picosecond IR and THz
pulses in a wavelength range from 4 μm to 280 μm. Here we review the potential of the laser and focus on two highlight
pump-probe experiments. In the first experiment, the relaxation dynamics in self assembled InGaAs quantum dots at
energies below the Reststrahlen band is studied. Long intradot relaxation times (1.5 ns) are found for level separations of
14 meV (3.4 THz), decreasing very strongly to ~ 2 ps at 30 meV (7 THz). The results are in very good agreement with
our microscopic theory of the carrier relaxation process, taking into account polaron decay via acoustic phonons. In the
second experiment, the relaxation dynamics in graphene is investigated at photon energies E = 20 - 250 meV. For
excitations below the energy of the optical phonon (G mode), the relaxation times are more than one order of magnitude
longer as compared to the relaxation times observed for near infrared excitation.
KEYWORDS: Electrons, Picosecond phenomena, Absorption, Superlattices, Free electron lasers, Temperature metrology, Scattering, Monte Carlo methods, Quantum wells, Infrared radiation
In this work we investigate the miniband relaxation dynamics of electrons in doped GaAs/AlGaAs superlattices by twocolor
infrared pump-probe experiments using a free electron laser synchronized to a table top broadband IR source. In
contrast to single color experiments, by this technique we are able to separate the different contributions from inter- and
intraminiband relaxation to the transient behavior after an ultrafast excitation. In particular, the intraminiband relaxation
is studied for different miniband widths, below and above the optical phonon energy of GaAs. For minibands wider than
this critical value we find fast relaxation, nearly constant for different excitation intensities whereas for narrow
minibands, a strong temperature and intensity dependence of the relaxation is found. The results are in good agreement
with previously published Monte Carlo simulations.
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