The Holleitner group studies correlation effects of light excitations in solids - so-called excitons - which are confined in low-dimensional quantum traps. The experiments allow to control and probe single excitons and their mutual interactions up to many-body interactions of dipolar exciton ensembles. The interacting droplets of excitons are confined in electrostatic traps in semiconductor heterostructures, which are built by state-of-the-art nanofabrication methods. Depending on quantum confinement, excitonic densities, and temperature, the interactions result in quantum phase transitions ranging from Wigner crystallization of such dipolar excitons over Bose-Einstein condensation in fully confined systems to a Mott transition into an electron-hole plasma at highest densities. A particular emphasis is put on the cross-over of many particle states to few and even individual excitons. The envisaged goals of the studies are nanofabricated, excitonic circuits based on coherent many-body correlations of photo-generated electrons and holes.
Non-equilibrium optoelectronic transport phenomena in nanostructured circuits comprise the relaxation and thermalization dynamics of optically excited charge and spin carriers. The Holleitner group established a real-time read-out of such transport dynamics with a picosecond time-resolution. The experimental approach exploits an ultrafast optical pump-probe scheme in combination with coplanar stripline circuits, and the optoelectronic response of the investigated circuits is sampled on-chip by a field probe. This allows to access quantum states, which are topologically protected, as well as the nonradiative transfer of spin information of optical emitters to excitation scavangers, and the coherent collective charge excitations in the nanoscale circuits in a real-time fashion.