Alexander Holleitner

Hybrid Nanosystems and Nanoscale Optoelectronics

Technical University of Munich

Walter Schottky Institute

Am Coulombwall 4

85748 Garching

Tel. +49 89 289 11575


Group Webpage


Research focus: quantum nano-systems, quantum many-body physics, optoelectronics

Quantum traps of single excitons

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.

Real-time read-out of quantum states in optoelectronic circuits

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.


Site-selectively generated photon emitters in monolayer MoS2 via local helium ion irradiation

J. Klein, M. Lorke, M. Florian, F. Sigger, J. Wierzbowski, J. Cerne, K. Müller, T. Taniguchi, K. Watanabe, U. Wurstbauer, M. Kaniber, M. Knap, R. Schmidt, J. Finley, A. Holleitner.

Nature Communications 10, Article number: 2755 (2019).

Show Abstract

Quantum light sources in solid-state systems are of major interest as a basic ingredient for integrated quantum photonic technologies. The ability to tailor quantum emitters via site-selective defect engineering is essential for realizing scalable architectures. However, a major difficulty is that defects need to be controllably positioned within the material. Here, we overcome this challenge by controllably irradiating monolayer MoS2 using a sub-nm focused helium ion beam to deterministically create defects. Subsequent encapsulation of the ion exposed MoS2 flake with high-quality hBN reveals spectrally narrow emission lines that produce photons in the visible spectral range. Based on ab-initio calculations we interpret these emission lines as stemming from the recombination of highly localized electron–hole complexes at defect states generated by the local helium ion exposure. Our approach to deterministically write optically active defect states in a single transition metal dichalcogenide layer provides a platform for realizing exotic many-body systems, including coupled single-photon sources and interacting exciton lattices that may allow the exploration of Hubbard physics.

DOI: 0.1038/s41467-019-10632-z

Tuning the Frohlich exciton-phonon scattering in monolayer MoS2

B. Miller, J. Lindlau, M. Bommert, A. Neumann, H. Yamaguchi, A. Holleitner, A. Hoegele, U. Wurstbauer

Nature Communications 10, 807 (2019).

Show Abstract

Charge carriers in semiconducting transition metal dichalcogenides possess a valley degree of freedom that allows for optoelectronic applications based on the momentum of excitons. At elevated temperatures, scattering by phonons limits valley polarization, making a detailed knowledge about strength and nature of the interaction of excitons with phonons essential. In this work, we directly access exciton-phonon coupling in charge tunable single layer MoS2 devices by polarization resolved Raman spectroscopy. We observe a strong defect mediated coupling between the long-range oscillating electric field induced by the longitudinal optical phonon in the dipolar medium and the exciton. This so-called Frohlich exciton phonon interaction is suppressed by doping. The suppression correlates with a distinct increase of the degree of valley polarization up to 20% even at elevated temperatures of 220 K. Our result demonstrates a promising strategy to increase the degree of valley polarization towards room temperature valleytronic applications.

DOI: 10.1038/s41467-019-08764-3

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