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Seilmeier, Florian (2013): Laser spectroscopy of localized quantum dot states interacting with electron reservoirs. Dissertation, LMU München: Fakultät für Physik



Self-assembled InGaAs quantum dots are nano-objects embedded in the solid-state matrix of GaAs. They act as natural potential traps for charge carriers and feature a number of quantized states due to the quantum confinement. When incorporated in a field effect structure the quantum dot states can be conveniently manipulated with an electric field and probed by resonant laser spectroscopy. In this thesis self-assembled quantum dots were investigated with an emphasis on the study of interactions between localized quantum dot states and charge or spin reservoirs in the environment. Experimentally the quantum dots were addressed in distinct regimes where the quantum dot spectrum was sensitive to individual charge fluctuations or mesoscopic reservoirs. The fundamental transition of a neutral quantum dot was found to exhibit a number of discontinuities in the usually linear dispersion of the exciton energy in external electrostatic fields. The discontinuities were identified to arise from charge fluctuations in the surrounding crystalline matrix in which impurity atoms can capture or release electrons. At characteristic conditions charging and discharging events lead to discrete changes of the electrostatic environment which in turn gives rise to an energy shift of the optical resonance condition. An electrostatic model was developed for a quantitative analysis of charging events and their signatures. On the basis of the model a comprehensive study of nearby quantum dots allowed to map out the relative spatial positions of quantum dots and impurities. In contrast to previous reports our results provide evidence for bulk impurities as the main source of charge fluctuations. By means of resonant laser spectroscopy in the energy dispersion of the neutral exciton a kink with a continuous energy shift has been observed which only occurs close to the regime where an electron is tunneling between the quantum dot and a 2D electron reservoir. The tunneling induces a weak coupling between the localized electron state of the quantum dot and the continuum of states in the reservoir. The tunnel coupling between the interacting states leads to hybridization into a new superposition state. In consequence the energy of the transition is renormalized which explains the kink in the energy dispersion. The hybridization model based on an Anderson-Fano approach quantitatively agrees with the experimental data and allows to extract the coupling strength between the reservoir and the localized state. In addition to the neutral exciton hybridization effects were also ob-served on the charged exciton. To study optical signatures of many-body effects sub-K laser spectroscopy was established and the setup performance was characterized with optical studies of a quantum dot in the Pauli-blockade regime. The electron bath temperature was determined using experimental and calculated electron spin populations as a function of magnetic field and temperature. The experiment provided quantitative access to all parameters except the electron bath temperature. With the optical Bloch equations the electron spin populations were modeled taking into account all relevant external parameters. An analysis of the evolution of the spin population in magnetic fields with the electron bath temperature as the only free fitting parameter was performed. An electron bath temperature of 380 mK was derived being slightly offset to the nominal base temperature of 250 mK. This proves the successful implementation of the sub-K laser spectroscopy setup.