Increasing the interaction strength between quasi-particles in solid-state materials can cause strong correlations, collective phenomena and the transition to macroscopic quantum phases, such as a Bose-Einstein condensate. Heterostructures made of semiconducting 2D materials, such as MoSe₂ and WSe₂, are ideal systems to realize interacting exciton ensembles, because they provide large exciton binding energies, long photoluminescence lifetimes, as well as a permanent exciton dipole. The latter allows the manipulation of the exciton ensembles, e.g. via electric fields. Recently, we have reported on several signatures regarding photoluminescence intensity, linewidth, as well as spatial and temporal coherences in accordance with the predicted degeneracy of an exciton ensemble at low temperature. The ongoing studies want to explore further predicted phases, such as an excitonic superfluidity and possible condensation phenomena in momentum space.

Collaborators: Ursula Wurstbauer (University of Münster), Andreas Knorr (TU Berlin).

We generate optically active defects in atomically thin 2D materials on a spatial scale of below 10 nm, such that scalable opto/electronics based on individual atomic states seems at reach. We use a helium-ion microscope (HIM) to generate individual defects, e.g. in semiconducting MoS₂. Our single-defect technology is principally applicable to the wide range of 2D materials with more than a thousand different materials and is compatible with standard cleanroom manufacturing steps. This combination makes it possible to realize first devices based on single defects, such as gate-switchable single photon emitters and photodetectors or even solar cells based on individual atomic defects. At the same time, fundamental processes, such as the coherent single electron tunneling dynamics and many-body interactions of localized states in a Fermion boson mixture are experimentally accessible.

Collaborators: Jonathan Finley (TUM), Kai Müller (TUM), Nicolas Leitherer-Stenger (DTU), Sivan Refaely-Abramson (Weizmann), Alex Weber-Bargioni (Berkeley)

We are heading the Center for Nanotechnologies and Nanomaterials (ZNN), which is a shared nanofabrication facility of the Walter Schottky Institute of TUM. Students, researchers, and scholars from the greater scientific Munich area have access to state-of-the-art nanolithography and nanoanalytic instruments for building nanoscale electronic, optoelectronic, and photonic circuits. The methodologies include electron beam-, focused-ion-beam-, and helium-ion-beam lithography.

The vision of this research topic is to generate electric current pulses with a duration of only a few femtoseconds for an on-chip signal conversion at the interface between electronics and optics, the so-called THz-gap. For signal generation we use amongst others, photo-emission processes, such as multiphoton absorption and strong field tunnel processes in plasmonic nanocontacts; but also femtosecond processes within solid state materials. The signal propagation occurs via electromagnetic THz modes in coplanar strip conductors. For the on-chip THz-detection, we are working on an electrical detection on the 100 fs scale and faster. Currently we reach 350 fs by using the relaxation dynamics in amorphous silicon photo-switches. Our group manufactures all circuits in our own laboratory, and we use phase-stable femtosecond lasers to drive the on-chip THz circuits coherently. Intriguingly, atomically thin 2D materials can be integrated into the THz circuits without much effort, such that the 2D materials can act as functional THz modulators. In the same way, the electron and heat dynamics in the 2D materials can be investigated on a femto- to picosecond timescale.

Collaborator: Reinhard Kienberger (TUM).

Topological materials are very promising for future opto/electronic circuits, since computing schemes can be envisaged where e.g. information-carrying surface states are protected by topology. Van der Waals materials and their heterostructure are an ideal platform to engineer and explore topological states. On the one hand, one can control and break the relevant symmetries of the Hamiltonian at will by interfacing different van der Waals materials with different symmetries. On the other hand, we can directly address the symmetry of the electron-Bloch states in the van der Waals crystal e.g. by external electric fields. In our research, we explore a wide range of non-trivial, i.e. topological quantum phenomena. The research is headed by Dr. Christoph Kastl.

Collaborator: Marko Burghard (MPI Stuttgart).