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 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.