Dynamische Kontrolle exzitonischer Systeme
Durch die fortschreitende Entwicklung starker, gepulster THz-Quellen in den letzten Jahren besteht die Möglichkeit, gezielt die Dynamik exzitonischer Systeme zu manipulieren. Von besonderem Interesse ist in diesem Fall die große Analogie zu bekannten atomaren Systemen. Die Manipulation atomarer...
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- 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 states. 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 unclear.