Optical and spin properties of individual GaAs droplet dots in AlGaAs barriers are studied in photoluminescence
experiments at 4K. First we report strong mixing of heavy hole-light hole states. Using the neutral and charged
exciton emission as a monitor we observe the direct consequence of quantum dot symmetry reduction in this strain free system. By fitting the polar diagram of the emission with simple analytical expressions obtained from k•p theory we are able to extract the mixing that arises from the heavy-light hole coupling due to the geometrical asymmetry of the quantum dot. Second we report optical orientation experiments. Circularly polarized optical excitation yields strong circular polarization of the resulting photoluminescence. Optical injection of spin polarized electrons into a GaAs dot gives rise to dynamical nuclear polarization that considerably changes the exciton Zeeman splitting (Overhauser shift). We show that the created nuclear polarization is bistable and present a direct measurement of the build-up time of the nuclear polarization in a single GaAs dot in the order of one second.
Developments in the self-assembly by droplet epitaxy in our research group have enabled us to fabricate various GaAs quantum nanostructures of high optical quality such as quantum dots, single quantum rings, and concentric double quantum rings. We clarified their electronic states and relaxation processes by micro photoluminescence experiments. We achieved lasing of lattice-matched GaAs quantum dots and their excitonic Rabi oscillation by resonant excitation. We succeeded in the control of their photon emission rate by photonic crystal micro cavities.
We report on observation of spontaneous emissions from a single quantum dot after resonant excitation. To capture
weak emissions from a single quantum dot, reflection of an excitation laser was reduced by applying obliquely incident
geometry and crossed polarization configuration. In addition, collected emissions were temporally resolved to be
separated from residual reflection. These allowed us to realize a simple manipulation and read out of an exciton qubit.
We observed an exciton Rabi oscillation in excitation amplitude dependence of the emission intensity. We determined an
exciton dipole moment from the Rabi oscillation and from the emission decay time, independently. Comparison between
the two values shows reasonable agreement.
Photoluminescence from a single semiconductor quantum dot after perfect resonant excitation has been temporally resolved with picosecond time resolution. Making use of non-confocal geometry for the micro photoluminescence setup, we can greatly remove the elastic scattering component, and successfully capture the resonantly-excited spontaneous emission from a single quantum dot. The emission intensity as a function of coherent photoinjection shows peculiar oscillatory dependence, i.e., Rabi oscillations of ground-state excitons. The transition strength of the groundstate exciton is precisely determined through the analysis. The value indicates a good agreement with that expected by the radiative decay rate of the quantum dots.