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.


Ultrafast hot-carrier relaxation in silicon monitored by phase-resolved transient absorption spectroscopy

M. Wörle, A.W. Holleitner, R. Kienberger, H. Iglev

Physical Review B 104, L041201 (2021).

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The relaxation dynamics of hot carriers in silicon (100) is studied via a holistic approach based on phase-resolved transient absorption spectroscopy with few-cycle optical pulses. After excitation by a sub-5-fs light pulse, strong electron-electron coupling leads to an ultrafast single electron momentum relaxation time of 10 fs. The thermalization of the hot carriers is visible in the temporal evolution of the effective mass and the collision time as extracted from the Drude model. The optical effective mass decreases from 0.3m(e) to about 0.125m(e) with a time constants of 58 fs, while the collision time increases from 3 fs for the shortest timescales with a saturation at approximately 18 fs with a time constant of 150 fs. The observation shows that both Drude parameters exhibit different dependences on the carrier temperature. The presented information on the electron mass dynamics as well as the momentum-, and electron-phonon scattering times with unprecedented time resolution is important for all hot-carrier optoelectronic devices.

DOI: 10.1103/PhysRevB.104.L041201

The role of chalcogen vacancies for atomic defect emission in MoS2

E. Mitterreiter, B. Schuler, A. Micevic, D. Hernangómez-Pérez, K. Barthelmi, K.A. Cochrane, J. Kiemle, F. Sigger, J. Klein, E. Wong, E.S. Barnard, K. Watanabe, T. Taniguchi, M. Lorke, F. Jahnke, J.J. Finley, A.M. Schwartzberg, D.Y. Qiu, S. Refaely-Abramson, A.W. Holleitner, A. Weber-Bargioni, C. Kastl

Nature Communications 12, 3822 (2021).

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For two-dimensional (2D) layered semiconductors, control over atomic defects and understanding of their electronic and optical functionality represent major challenges towards developing a mature semiconductor technology using such materials. Here, we correlate generation, optical spectroscopy, atomic resolution imaging, and ab initio theory of chalcogen vacancies in monolayer MoS2. Chalcogen vacancies are selectively generated by in-vacuo annealing, but also focused ion beam exposure. The defect generation rate, atomic imaging and the optical signatures support this claim. We discriminate the narrow linewidth photoluminescence signatures of vacancies, resulting predominantly from localized defect orbitals, from broad luminescence features in the same spectral range, resulting from adsorbates. Vacancies can be patterned with a precision below 10nm by ion beams, show single photon emission, and open the possibility for advanced defect engineering of 2D semiconductors at the ultimate scale. The relation between the microscopic structure and the optical properties of atomic defects in 2D semiconductors is still debated. Here, the authors correlate different fabrication processes, optical spectroscopy and electron microscopy to identify the optical signatures of chalcogen vacancies in monolayer MoS2.

DOI: 10.1038/s41467-021-24102-y

Controlling exciton many-body states by the electric-field effect in monolayer MoS2

J. Klein, A. Hötger, M. Florian, A. Steinhoff, A. Delhomme, T. Taniguchi, K. Watanabe, F. Jahnke, A.W. Holleitner, M. Potemski, C. Faugeras, J.J. Finley, A.V. Stier

Physical Review Research 3, L022009 (2021).

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We report magneto-optical spectroscopy of gated monolayer MoS2 in high magnetic fields up to 28T and obtain new insights on the many-body interaction of neutral and charged excitons with the resident charges of distinct spin and valley texture. For neutral excitons at low electron doping, we observe a nonlinear valley Zeeman shift due to dipolar spin-interactions that depends sensitively on the local carrier concentration. As the Fermi energy increases to dominate over the other relevant energy scales in the system, the magneto-optical response depends on the occupation of the fully spin-polarized Landau levels (LL) in both K/K′ valleys. This manifests itself in a many-body state. Our experiments demonstrate that the exciton in monolayer semiconductors is only a single particle boson close to charge neutrality. We find that away from charge neutrality it smoothly transitions into polaronic states with a distinct spin-valley flavor that is defined by the LL quantized spin and valley texture.

DOI: 10.1103/PhysRevResearch.3.L022009

Engineering the Luminescence and Generation of Individual Defect Emitters in Atomically Thin MoS2

J. Klein, L. Sigl, S. Gyger, K. Barthelmi, M. Florian, S. Rey T. Taniguchi K. Watanabe, F. Jahnke, C. Kastl, V. Zwiller, K.D. Jons, K. Mueller, U. Wurstbauer, J.J. Finley, A.W. Holleitner

ACS Photonics 8 (2), 669-677 (2021).

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We demonstrate the on-demand creation and positioning of photon emitters in atomically thin MoS2 with very narrow ensemble broadening and negligible background luminescence. Focused helium-ion beam irradiation creates 100s to 1000s of such mono-typical emitters at specific positions in the MoS2 monolayers. Individually measured photon emitters show anti-bunching behavior with a g(2)(0) similar to 0.23 and 0.27. From a statistical analysis, we extract the creation yield of the He-ion induced photon emitters in MoS2 as a function of the exposed area, as well as the total yield of single emitters as a function of the number of He ions when single spots are irradiated by He ions. We reach probabilities as high as 18% for the generation of individual and spectrally clean photon emitters per irradiated single site. Our results firmly establish 2D materials as a platform for photon emitters with unprecedented control of position as well as photophysical properties owing to the all-interfacial nature.

DOI: 10.1021/acsphotonics.0c01907

Gate-Switchable Arrays of Quantum Light Emitters in Contacted Monolayer MoS2 van der Waals Heterodevices

A. Hoetger, J. Klein, K. Barthelmi, L. Sigl, F. Sigger, W. Manner, S. Gyger, M. Florian, M. Lorke, F. Jahnke, T. Taniguchi, K. Watanabe, K.D. Jons, U. Wurstbauer, C. Kastl, K. Mueller, J.J. Finley, A.W. Holleitner

Nano Letters 21 (2), 1040-1046 (2021).

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We demonstrate electrostatic switching of individual, site-selectively generated matrices of single photon emitters (SPEs) in MoS2 van der Waals heterodevices. We contact monolayers of MoS2 in field-effect devices with graphene gates and hexagonal boron nitride as the dielectric and graphite as bottom gates. After the assembly of such gate-tunable heterodevices, we demonstrate how arrays of defects, that serve as quantum emitters, can be site-selectively generated in the monolayer MoS2 by focused helium ion irradiation. The SPEs are sensitive to the charge carrier concentration in the MoS2 and switch on and off similar to the neutral exciton in MoS2 for moderate electron doping. The demonstrated scheme is a first step for producing scalable, gate-addressable, and gate-switchable arrays of quantum light emitters in MoS2 heterostacks.

DOI: 10.1021/acs.nanolett.0c04222

Signatures of a degenerate many-body state of interlayer excitons in a van der Waals heterostack

L. Sigl, F. Sigger, F. Kronowetter, J. Kiemle, J. Klein, K. Watanabe, T. Taniguchi, J.J. Finley, U. Wurstbauer, A.W. Holleitner

Physical Review Research 2, 042044(R) (2020).

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Atomistic van der Waals heterostacks are ideal systems for high-temperature exciton condensation because of large exciton binding energies and long lifetimes. Charge transport and electron energy-loss spectroscopy showed first evidence of excitonic many-body states in such two-dimensional materials. Pure optical studies, the most obvious way to access the phase diagram of photogenerated excitons, have been elusive. We observe several criticalities in photogenerated exciton ensembles hosted in MoSe2-WSe2 heterostacks with respect to photoluminescence intensity, linewidth, and temporal coherence pointing towards the transition to a coherent many-body quantum state, consistent with the predicted critical degeneracy temperature. For this state, the estimated occupation is approximately 100% and the phenomena survive above 10 K.Y

DOI: 10.1103/PhysRevResearch.2.042044

Light-field and spin-orbit-driven currents in van der Waals materials

J. Kiemle, P. Zimmermann, A.W. Holleitner, C. Kastl

Nanophotonics 9 (9), 2693-2708 (2020).

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This review aims to provide an overview over recent developments of light-driven currents with a focus on their application to layered van der Waals materials. In topological and spin-orbit dominated van der Waals materials helicity-driven and light-field-driven currents are relevant for nanophotonic applications from ultrafast detectors to onchip current generators. The photon helicity allows addressing chiral and non-trivial surface states in topological systems, but also the valley degree of freedom in two-dimensional van der Waals materials. The underlying spinorbit interactions break the spatiotemporal electrodynamic symmetries, such that directed currents can emerge after an ultrafast laser excitation. Equally, the light-field of few-cycle optical pulses can coherently drive the transport of charge carriers with sub-cycle precision by generating strong and directed electric fields on the atomic scale. Ultrafast light-driven currents may open up novel perspectives at the interface between photonics and ultrafast electronics.

DOI: 10.1515/nanoph-2020-0226

Atomistic defects as single-photon emitters in atomically thin MoS2

K. Barthelmi, J. Klein, A. Hoetger, L. Sigl, F. Sigger, E. Mitterreiter, S. Rey, S. Gyger, M. Lorke, M. Florian, F. Jahnke, T: Taniguchi, K. Watanabe, V. Zwiller, K.D. Jons, U. Wurstbauer, C. Kastl, A. Weber-Bargioni, J.J. Finley, K. Mueller, A.W. Holleitner

Applied Physics Letters 117 (7), 070501 (2020).

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Precisely positioned and scalable single-photon emitters (SPEs) are highly desirable for applications in quantum technology. This Perspective discusses single-photon-emitting atomistic defects in monolayers of MoS2 that can be generated by focused He-ion irradiation with few nanometers positioning accuracy. We present the optical properties of the emitters and the possibilities to implement them into photonic and optoelectronic devices. We showcase the advantages of the presented emitters with respect to atomistic positioning, scalability, long (microsecond) lifetime, and a homogeneous emission energy within ensembles of the emitters. Moreover, we demonstrate that the emitters are stable in energy on a timescale exceeding several weeks and that temperature cycling narrows the ensembles' emission energy distribution.

DOI: 10.1063/5.0018557

Atomistic Positioning of Defects in Helium Ion Treated Single-Layer MoS2

E. Mitterreiter, B. Schuler, K.A. Cochrane, U. Wurstbauer, A: Weber-Bargioni, C. Kastl, A.W. Holleitner

Nano Letters 20 (6), 4437-4444 (2020).

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Structuring materials with atomic precision is the ultimate goal of nanotechnology and is becoming increasingly relevant as an enabling technology for quantum electronics/spintronics and quantum photonics. Here, we create atomic defects in monolayer MoS2 by helium ion (He-ion) beam lithography with a spatial fidelity approaching the single-atom limit in all three dimensions. Using low-temperature scanning tunneling microscopy (STM), we confirm the formation of individual point defects in MoS2 upon He-ion bombardment and show that defects are generated within 9 nm of the incident helium ions. Atom-specific sputtering yields are determined by analyzing the type and occurrence of defects observed in high-resolution STM images and compared with with Monte Carlo simulations. Both theory and experiment indicate that the He-ion bombardment predominantly generates sulfur vacancies.

DOI: 10.1021/acs.nanolett.0c01222

On-site tuning of the carrier lifetime in silicon for on-chip THz circuits using a focused beam of helium ions

P. Zimmermann, A.W. Holleitner

Applied Physics Letters 116 (7), 073501 (2020).

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In this study, we demonstrate that a focused helium ion beam allows the local adjustment and optimization of the carrier lifetime in silicon-based photoswitches integrated in ultrafast on-chip terahertz-circuits. Starting with a carrier lifetime of 5.3 ps for as-grown silicon on sapphire, we monotonously reduce the carrier lifetime in integrated switches to a minimum of similar to 0.55 ps for a helium ion fluence of 20x10(15) ions/cm(2). By introducing an analytical model for the carrier lifetimes in the photoswitches, we particularly demonstrate that the carrier lifetime can be adjusted locally even within single photoswitches. In turn, the demonstrated on-site tuning allows optimizing ultrafast high-frequency circuits, into which radiation-sensitive nanoscale materials, such as two-dimensional materials, are embedded. Published under license by AIP Publishing.

DOI: 10.1063/1.5143421

Impact of substrate induced band tail states on the electronic and optical properties of MoS2

J. Klein, A. Kerelsky, M. Lorke, M. Florian, F. Sigger, J. Kiemle, M. C. Reuter, T. Taniguchi, K. Watanabe, J. Finley, A. N. Pasupathy, A. Holleitner, F. M. Ross, U. Wurstbauer

Applied Physics Letters 115 (26), 261603 (2019).

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Substrate, environment, and lattice imperfections have a strong impact on the local electronic structure and the optical properties of atomically thin transition metal dichalcogenides. We find by a comparative study of MoS2 on SiO2 and hexagonal boron nitride (hBN) using scanning tunneling spectroscopy (STS) measurements that the apparent bandgap of MoS2 on SiO2 is significantly reduced compared to MoS2 on hBN. The bandgap energies as well as the exciton binding energies determined from all-optical measurements are very similar for MoS2 on SiO2 and hBN. This discrepancy is found to be caused by a substantial amount of band tail states near the conduction band edge of MoS2 supported by SiO2. The presence of those states impacts the local density of states in STS measurements and can be linked to a broad red-shifted photoluminescence peak and a higher charge carrier density that are all strongly diminished or even absent using high quality hBN substrates. By taking into account the substrate effects, we obtain a quasiparticle gap that is in excellent agreement with optical absorbance spectra and we deduce an exciton binding energy of about 0.53 eV on SiO2 and 0.44 eV on hBN.

DOI: 10.1063/1.5131270

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).

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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

Quantized Conductance in Topological Insulators Revealed by the Shockley-Ramo Theorem

P. Seifert, M. Kundinger, G. Shi, X.Y. He, K.H. Wu, Y.Q. Li, A. Holleitner, C. Kastl

Physical Review Letters 122 (14), 146804 (2019).

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Crystals with symmetry-protected topological order, such as topological insulators, promise coherent spin and charge transport phenomena even in the presence of disorder at room temperature. We demonstrate how to image and read out the local conductance of helical surface modes in the prototypical topological insulators Bi2Se3 and BiSbTe3. We apply the so-called Shockley-Ramo theorem to design an optoelectronic probe circuit for the gapless surface states, and we find a well-defined conductance quantization at le(2)/h within the experimental error without any external magnetic field. The unprecedented response is a clear signature of local spin-polarized transport, and it can be switched on and off via an electrostatic field effect. The macroscopic, global readout scheme is based on an electrostatic coupling from the local excitation spot to the readout electrodes, and it does not require coherent transport between electrodes, in contrast to the conventional Landauer-Biittiker description. It provides a generalizable platform for studying further nontrivial gapless systems such as Weyl semimetals and quantum spin-Hall insulators.

DOI: 10.1103/PhysRevLett.122.146804

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).

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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

Toward femtosecond electronics up to 10 THz

N. Fernandez, P. Zimmermann, P. Zechmann, M. Worle, R. Kienberger, A.W. Holleitner

Ultrafast Phenomena and Nanophotonics XXIII 10916, 109160R (2019).

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We numerically compute the effective diffraction index and attenuation of coplanar stripline circuits with microscale lateral dimensions on various substrates including sapphire, GaN, silica glass, and diamond grown by chemical vapor deposition. We show how to include dielectric, radiative and ohmic losses to describe the pulse propagation in the striplines to allow femtosecond on-chip electronics with frequency components up to 10 THz.

DOI: 10.1117/12.2511668

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