Table of Contents:
The recent advances in THz technology led to the development of very
intense coherent sources. This technology opens a path to analyze and
control coherent excitonic systems. In atomic systems these techniques
were used for a variety of applications. These range from the control
of photoluminescence over the preparation of quantum states to the
entanglement of atoms or even QBits. The main focus of this thesis is
the analysis of these processes in excitonic ensembles.
For the generation of strong pulsed THz sources there are several different
established methods. Starting from an amplified fs-laser pulse
with high peak intensity, optical rectification is the method of choice.
This process is very efficient in LiNbO3 crystals. Due to the big difference
of the diffraction index comparing the NIR and the THz regime,
the conversion is done in Cherenkov geometry. Besides the efficient
conversion, LiNbO3 unfortunately also has a non negliable absorption
index for THz radiation. To optimizing the useable THz field strength,
a novel generation geometry is developed. The idea is to decouple generation
and out coupling of the THz radiation. Whereas LiNbO3 is
used for the optical rectification process, a Si prism is utilized for
the out coupling of the generated radiation. Having an absorption coefficient
of about a factor of 1000 less than LiNbO3, silicon is the
ideal material for the propagation of the created THz radiation. This
geometry overcomes the major drawback of the classical Cherenkov
approach. The generated THz pulses can reach field strength of more
than 50 kV/cm and cover a frequency range of 0.2 THz to 3 THz. In
addition the homogeneous emission profile allows realizing small focal
spots. The strong THz source was used to analyze the excitonic polarization
dynamic of a Ga(In)As semiconductor quantum well. The results
show for the first time the signatures of a Rabi-oscillation between
the |1s>- and |2p> state which lead to an Autler-Townes splitting of
the excitonic resonance. Besides this, the measurement shows a rich
dynamic, which is shown to be caused by broad range of exciton excitations
even up to ionization. The findings are further analyzed by
means of a many-particle microscopic theory. This reveals that the
dynamic is fundamentally driven by the many-body properties of the
system. Simple 2 states system descriptions as often used in the literature
neglect these properties and thus fail to describe the full dynamic
of the system.
For an even more controlled manipulation of the coherent system a
strong THz source with a very narrow spectral width is used. This
allows to precisely addressing the transitions between the excitonic
states. Most notably is the excitonic |1s> to |2s> transition. When applying
dipole selection rules, this transition should not be induced by
the THz radiation. The measured transient optical properties of the
excited system clearly show the presence of this excitation under the
presence of THz radiation. This observation is a result of the manybody
nature of the excitonic states. The coulomb-scattering induced
mixing of the |2p>- and |2s> states breaks the initial symmetry of the
This allows to directly preparing an excitonic state for which a recombination
of the exciton is forbidden. Thus suppressing it’s photoluminescence.
When driving the system to Rabi oscillations the succeeding
repopulation of the initial |1s> state also a recurreance of the photoluminescence
is observed. This is the first observation of this „shelving“
in the THz regime.
The analysis of the polarization dependence of the luminescences reveals
the secondary emission of the excitonic polarization.
The presence of Rabi oscillations between the |1s>-state and the |2s>-
state are a direct proof of mixing of the |2p>-state and the |2s>-state.
If there would only be a Coulomb scattering between two separated states, a Rabi-oscillation would not be possible.
In summary it was shown that the coherent control of en excitonic
system is possible. The inherent many-body interaction in this system
however leads to some unique behaviors. On the one hand, the
population of excitonic states with high quantum numbers was observed.
This is caused by multi-photon processes and the mixing of
the different quantum states. As this are the first steps to a full coherent
control of an excitonic system, the potential of this system is still