Laser spectroscopy of localized quantum dot statesudinteracting with electron reservoirs

摘要

Self-assembled InGaAs quantum dots are nano-objects embedded in the solid-state matrixudof GaAs. They act as natural potential traps for charge carriers and feature a numberudof quantized states due to the quantum confinement. When incorporated in a field effectudstructure the quantum dot states can be conveniently manipulated with an electric fieldudand probed by resonant laser spectroscopy. In this thesis self-assembled quantum dots wereudinvestigated with an emphasis on the study of interactions between localized quantum dotudstates and charge or spin reservoirs in the environment. Experimentally the quantum dotsudwere addressed in distinct regimes where the quantum dot spectrum was sensitive to individual charge fluctuations or mesoscopic reservoirs.udThe fundamental transition of a neutral quantum dot was found to exhibit a number ofuddiscontinuities in the usually linear dispersion of the exciton energy in external electrostatic fields. The discontinuities were identified to arise from charge fluctuations in theudsurrounding crystalline matrix in which impurity atoms can capture or release electrons.udAt characteristic conditions charging and discharging events lead to discrete changes ofudthe electrostatic environment which in turn gives rise to an energy shift of the opticaludresonance condition. An electrostatic model was developed for a quantitative analysis ofudcharging events and their signatures. On the basis of the model a comprehensive study ofudnearby quantum dots allowed to map out the relative spatial positions of quantum dots andudimpurities. In contrast to previous reports our results provide evidence for bulk impuritiesudas the main source of charge fluctuations.udBy means of resonant laser spectroscopy in the energy dispersion of the neutral exciton audkink with a continuous energy shift has been observed which only occurs close to the regimeudwhere an electron is tunneling between the quantum dot and a 2D electron reservoir. Theudtunneling induces a weak coupling between the localized electron state of the quantum dotudand the continuum of states in the reservoir. The tunnel coupling between the interactingudstates leads to hybridization into a new superposition state. In consequence the energyudof the transition is renormalized which explains the kink in the energy dispersion. Theudhybridization model based on an Anderson-Fano approach quantitatively agrees with theudexperimental data and allows to extract the coupling strength between the reservoir andudthe localized state. In addition to the neutral exciton hybridization effects were also ob-served on the charged exciton.udTo study optical signatures of many-body effects sub-K laser spectroscopy was establishedudand the setup performance was characterized with optical studies of a quantum dot in theudPauli-blockade regime. The electron bath temperature was determined using experimentaludand calculated electron spin populations as a function of magnetic field and temperature.udThe experiment provided quantitative access to all parameters except the electron bathudtemperature. With the optical Bloch equations the electron spin populations were modeledudtaking into account all relevant external parameters. An analysis of the evolution of theudspin 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 beingudslightly offset to the nominal base temperature of 250 mK. This proves the successfuludimplementation of the sub-K laser spectroscopy setup.