The main interests of our research group are focussed on the use of optical spectroscopy techniques to investigate the electronic and optical properties of low dimensional semiconductor nanostructures in the III/V material system. In our studies we explore fundamental issues of light-matter interaction as well as the potential of semiconductor nanostructures to act as novel light sources.
In particular, we are interested in the optical and electronic properties of single self assembled InGaAs quantum dot nanostructures embedded in high-finesse micropillar resonators. This semiconductor system allows us to investigate basic phenomena in the domain of quantum optics called cavity quantum electrodynamics (cQED). cQED which has been a major topic in atomic physics in the past decades describes the behaviour of atom like emitters placed in optical cavities. In such an environment the spontaneous emission rate of an emitter can be strongly modified in comparison to free space. E.g. the spontaneous emission rate can either be enhanced or suppressed due to the Purcell-effect.
Due to the tremendous progress in nanostructure technology it has become feasible in recent years to perform cQED experiments also in semiconductor systems. Especially the unique properties of self assembled quantum dots in combination with microcavities providing a three dimensional photon confinement on a scale of the photon wavelength and long photon lifetimes have initialized cQED studies in semiconductor systems. Just recently, we were able to realize a long sought strongly coupled light-matter system in which the spontaneous emission becomes reversible and photons emitted by a quantum dot inside a high-finesse cavity becomes reabsorbed and reemitted, etc.. Such a coherent interaction is not only of fundamental interest for cQED experiments but also for novel quantum mechanical devices such as quantum gates.
High-finesse microcavities with quantum dots in the active layer are also promising candidates for novel light sources like single photon emitters or low threshold microlasers. In microlasers a significant fraction b of the spontaneously emitted photons is coupled into the lasing mode which lowers the threshold power significantly in comparison to usual laser diodes. We are investigating low threshold lasing in micropillar cavities with InGaAs QDs acting as active medium. Due to the high quality of these structures we have been able to observe lasing associated with b-values almost approaching unity which finally is expected to result in thresholdless lasing. Even single quantum dot lasers seem to be feasible in this kind of microstructures.