Research

• Preparation of single- and few-layer samples via mechanical exfoliation

• Preparation of artificial hetero- or multi-layer structures via a deterministic transfer process

(a) Microscope image of WSe₂ layer, encapsulated in hBN layers. (b) Exfoliated MoSe₂ sample. The numbers of layers are indicated.

Microscope image of WSe₂-MoSe₂ heterostructure, prepared by deterministic transfer of single layers.

• Time-resolved photoluminescence with streak-camera system
  2 ps maximum time-resolution, microscope setup for spatially-resolved experiments, temperatures down to 4 K
• Pump-probe spectroscopy, e.g., time-resolved Faraday or Kerr rotation, four-wave mixing, with mode-locked Ti-Sapphire femtosecond laser
  Pulse length 100 fs - 600 fs, wavelength range 700 nm - 830 nm, magnetic fields up to 11.5 T, temperatures down to 0.4 K
• cw photoluminescence spectroscopy
  Spectrometer with 0.5 m focal length and CCD camera, various laser sources, magnetic fields up to 11.5 T, temperatures down to 0.4 K

• Time- and spatially resolved two-color Kerr rotation with mode-locked Ti:Sapphire laser and white-light source
  Microscope setup for time- and spatially-resolved experiments, temperatures down to 4 K

• Second-harmonic generation with mode-locked Ti-Sapphire femtosecond laser
  Spectrometer with 0.25 m focal length and CCD camera

• Photoluminescence and white-light reflectance
  Spectrometer with 0.25 m focal length and CCD camera

• Resonant Magneto-Raman Spectroscopy
  Triple Raman spectrometer with LN2-cooled CCD detector, tunable cw Ti:Sapphire laser, magnetic fields up to 9 T, temperatures down to 2 K

• Resonant Micro-Raman Spectroscopy
  Microscope setup for spatially-resolved experiments, tunable cw Ti:Sapphire laser, temperatures down to 4 K

• Micro-Raman Spectroscopy and Mapping
  Microscope setup with automated mapping option, spectrometer with 0.5 m focal length and CCD camera, Bragg filter sets for stray-light reduction, various laser sources 
• White-light reflectance experiments
  white-light source, spectrometer with 0.5 m focal length and CCD camera

We study the optical and electronic properties of monolayer transition metal dichalcogenides and a variety of heterostructures consisting of different two-dimensional materials via cw- and time-resolved optical spectroscopy. The aim is to investigate spin- and valley-polarization dynamics in monolayers, as well as the properties of interlayer excitons in heterostructures. The employed spectroscopic methods are cw- and time-resolved photoluminescence, using a streak camera system, and, time-resolved pump-probe Faraday rotation. In the focus are time-resolved studies of the spin- and valley dynamics in high magnetic fields.

In this project, the aim is to explore the dynamics and coherence properties of excitons  and excitonic complexes in atomically thin transition-metal dichalcogenide mono- and multilayers in external magnetic fields by time-resolved Faraday rotation and time-integrated four-wave-mixing.

Ongoing PhD projects:

M. Sc. Simon Raiber

M. Sc. Anna Weindl

We apply the technique of low-frequency Raman scattering to a variety of hetero- and homobilayer transition-metal dichalcogenide structures. The aims are the investigation of the domain structure of atomic reconstructions in twisted heterobilayers, the low-frequency modes of correlated quantum phases in the magic-angle continuum in homobilayers, and the exploration of internal electronic excitations of excitonic complexes in heterobilayers. 

Left: Microscope picture of MoSe₂-WSe₂ heterostructure. Right: Second-harmonic generation experiment for the determination of the crystal orientations of the constituent layers.

Ongoing PhD projekt:

M. Sc. Philipp Parzefall

In collaboration with the group of Dominique Bougeard we are investigating electronic excitations in wurtzite-phase GaAs-AlGaAs core-shell nanowires. The nanowires have core diameters down to about 20 nm and show distinct two-dimensional quantization effects. Via inelastic light scattering we are exploring the electronic excitations of photo-excited electron-hole plasmas in those quantum structures. 

Left: Schematic picture of the Raman experiment on a single nanowire. Middle: Scanning electron microscope picture of a single nanowire. Right: Raman spectra of 3 nanowires with different diameters, as given in the figure. Intersubband excitations between size-quantized one-dimensional subbands can be observed.

Finished PhD project:

M. Sc. Sebastian Meier