Terahertz studies on semiconductor quantum heterostructures in the low and high field regime

In this thesis we investigate experimentally certain aspects of the interaction of terahertz (THz) radiation with intersubband transitions and excitonic transitions in semiconductor quantum wells.
The first part deals with a more fundamental view on an intersubband transition in a symmetric, undoped GaAs/AlGaAs multiple quantum well. After optical excitation of carriers, the considered electronic conduction intersubband transition is probed in the low-intensity linear regime using broadband THz pulses. These pulses are detected via field-resolved electro-optic sampling. While the sample’s terahertz absorption shows the expected single peak of the resonant intersubband transition, the differential transmission spectra, i.e. the photoexcitation-induced changes in transmission, display strong Fano signatures. On the basis of a microscopic theory, we show that they originate from a phase sensitive superposition of THz current and ponderomotive current. The latter one results from the wiggling motion of carriers induced by the accelerating THz field. Our findings demonstrate for the first time that the ponderomotive contribution has to be taken into account also at the lowest THz intensities. The following issues consider the interaction with THz pulses of higher intensity from the free-electron laser (FEL) of the Forschungszentrum Dresden-Rossendorf.
In one experiment we investigate efficient second order sideband generation in the GaAs/AlGaAs multiple quantum well mentioned above. To this end a near-infrared laser tuned to excitonic interband transitions is mixed inside the sample with the inplane polarized FEL beam to create the sum- and difference-frequencies between them. We compare the sideband efficiencies for the THz beam tuned to the interexcitonic heavy-hole light-hole transition and to the intraexcitonic heavy-hole 1s-2p transition. In the latter case we achieve a ten times higher n=+2 low-temperature efficiency around 0.1%. This value is comparable to previous studies in the literature, but our approach involves different transitions in a much simpler geometry. At room temperature the efficiency drops only by a factor of 7 for low THz powers.
The last part of this thesis addresses another fundamental quantum-mechanical phenomenon: the splitting of an absorption line in a strong THz field. In the same abovementioned quantum well sample the FEL wavelength is tuned near the intraexcitonic 1s-2p heavy-hole transition. The THz radiation induces a power-dependent splitting of the heavy-hole 1s exciton absorption line which manifests itself in the transmitted spectrum of a broadband near-infrared probe beam. The FEL-wavelength-dependent strength of this so-called Autler-Townes splitting is discussed on the basis of a simple two-level model.