Dokument

Summary:
In the major part of this Thesis, we discuss the linear THz response of semiconductor nanostructures based on a microscopic theory. Here, two different problems are investigated: intersubband transitions in optically excited quantum wells and the THz plasma response of two-dimensional systems. In the latter case, we analyze the response of correlated electron and electron-hole plasmas. Extracting the plasma frequency from the linear response, we find significant deviations from the commonly accepted two-dimensional plasma frequency. Besides analyzing the pure plasma response, we also consider an intermediate regime where the response of the electron-hole plasma consists of a mixture of plasma contributions and excitonic transitions. A quantitative experiment-theory comparison provides novel insights into the behavior of the system at the transition from one regime to the other. The discussion of the intersubband transitions mainly focuses on the coherent superposition of the responses from true THz transitions and the ponderomotively accelerated carriers. We present a simple method to directly identify ponderomotive effects in the linear THz response. Apart from that, the excitonic contributions to intersubband transitions are investigated. The last part of the present Thesis deals with a completely different regime. Here, the extreme nonlinear optical response of low-dimensional semiconductor structures is discussed. Formally, extreme nonlinear optics describes the regime of light-matter interaction where the exciting field is strong enough such that the Rabi frequency is comparable to or larger than the characteristic transition frequency of the investigated system. Here, the Rabi frequency is given by the product of the electrical field strength and the dipole-matrix element of the respective transition. Theoretical investigations have predicted a large number of novel nonlinear effects arising for such strong excitations. Some of them have been observed in experiments performed on semiconductors. Previous theoretical works often modeled the semiconductor as an ensemble of independent two-level systems. Such an approach does surely not account for many-body interactions among the carriers. Only very few publications exist that include Coulomb effects in the extreme nonlinear regime. Furthermore, these studies concentrated exclusively on the optically induced interband transitions. For the strong fields considered here, however, the ponderomotive intraband acceleration of the photo-excited carriers cannot be neglected a priori. In our discussion of the extreme nonlinear optical response of semiconductors, we will analyze both the influence of the Coulomb interaction and the effect of carrier accelerations. The Thesis is organized as follows. In Chap. 2, we give an overview of our microscopic theory that has been used to obtain the results presented in this work. Chapter 3 discusses intersubband transitions of optically excited quantum wells. Besides a purely theoretical analysis of excitonic effects, a detailed experiment-theory comparison is presented. Chapter 4 deals with the intraband dynamics in two-dimensional semiconductor systems. Here, our results are also compared to recent experiments. In Chap. 5, we explore the extreme nonlinear optical response of semiconductor nanostructures. Finally, we summarize our findings and give a short outlook in Chap. 6.

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