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