Terahertz-Antwort von zweidimensionalen Ladungsträgersystemen in GaAs-basierten Heterostrukturen

This thesis deals with the THz response of two-dimensional charge carrier systems in different semiconductor heterostructures under varying conditions. The utilized spectrometer is suitable for time-resolved optical pump - THz probe experiments, as well as for optical pump-probe experiments in the near infrared for identical conditions. It allows the investigation of the transverse dielectric function of both, a (GaIn)As/GaAs quantum well and a two-dimensional electron gas in a GaAs-based heterostructure. First, the THz response of an electron-hole plasma is examined for different carrier densities. The plasma is generated by interband transitions in a (GaIn)As/GaAs quantum well. The measured transverse dielectric function reveals that the plasma behaves in accordance with the classical Drude oscillator model. It also conforms to the microscopic theory of the THz response of corresponding many-body systems. Evidence of a plasma resonance in the negative imaginary part of the inverse dielectric function is found. The squared peak frequency of the resonance is proportional to the carrier density of the plasma. This behavior corresponds to the plasma frequency of a three-dimensional plasma. Overall, it can be shown that the transverse THz response of a two-dimensional electron-hole plasma behaves like the response of a three-dimensional plasma. Therefore, the transversal THz response of an electron-hole plasma seems to be independent of the dimension of the charge carrier system. Secondly, the behavior of the quantum well for a 1s-exciton dominated carrier system is investigated. A good agreement between experiment and microscopic theory is obtained for the dielectric function. The negative imaginary part of the inverse dielectric function shows a resonance at the intraexcitonic 1s-2p transition frequency, even in weakly excited excitonic systems. Increasing the carrier density leads to a plasma-like behavior of the system. However, in these densities a significant exciton fraction remains in the carrier system. The exciton population is not yet completely ionized. This is supported by matching optical pump-probe experiments. The carrier system begins to show plasma-like behavior, before the Mott density is reached. Finally, a pure two-dimensional electron gas (2DEG) in a GaAs heterostructure is investigated. A behavior similar to a three-dimensional plasma is observed both in the dielectric function, as well as in the imaginary part of the inverse dielectric function. However, in contrast to the electron-hole plasma of the quantum well, this cannot be described by a Drude oscillator model in the case of the 2DEG. Nevertheless, there is a good agreement with the microscopic theory. It seems that many-body effects appear to play a greater role, significantly modifying the ponderomotive contribution of the THz response.