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Research

Laboratories of the group

We have five laboratories in our group. One lab for the preparation of 2D crystals and their heterostructures via mechanical exfoliation and deterministic gel-film transfer (2D Crystal Fab), and four labs for optical spectroscopy.


The 2D Crystal Fab

Experiments:

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

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

Pictures: 

Microscope setup for 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.



The Ultrafast Spectroscopy Lab I

Experiments:

  • 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
     

Pictures:

Lab with suspended device rack, Streak camera, microscopy setup and the He3 cryostat.

Lab with dimmed lighting for spectroscopic measurements.

Ti:Sa-laser, frequency doubler, Streak camera und delay line.



The Ultrafast Spectroscopy Lab II

Experiments:

  • Time- and spatially resolved Kerr rotation with mode-locked Ti:Sapphire laser
    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

Pictures:

Lab with microscope and second-harmonic generation setups.

Setup with laser safety housing.



The Raman Lab I

Experiments:
  • 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


Pictures:

Triple Raman spectrometer

Micro-Raman setup



The Raman Lab II

Experiment:

  • 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 

Pictures:

Scanning-Micro-Raman setup in 2.0.22



Research projects

Our projects are funded by the DFG in the individual project SCHU1171/8, the DFG Priority Program SPP2244, and within the Collaborative Research Center SFB 1277


Exciton and valley dynamics in 2D crystals and heterostructures

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.

Left: Photoluminescence spectrum of a MoSe2-WSe2 heterostructure. Right: Schematic picture of the charge separation in a type-II heterostructure and formation of the interlayer exciton (IEX).

Ongoing PhD projects:

M. Sc. Johannes Holler (supervised by Prof. Dr. Tobias Korn, University of Rostock)

M. Sc. Simon Raiber

M. Sc. Andreas Beer



Ultrafast magneto spectroscopy of 2D crystals

In this project, the aim is to explore the coherence properties of excitons and of excitonic complexes - like charged excitons, neutral and charged biexcitons - in atomically thin transition-metal dichalcogenide monolayers in external magnatic fields by time-integrated four-wave-mixing.

The experiments will be performed in three-beam technique by applying different polarization configurations in an optical split-coil magnet cryostat. The focus of the investigations will be on MoSe2 monolayers, where different excitonic complexes can be specifically targeted by choosing different polarization configurations.

 

 

 

Spectrally-resolved four-wave mixing experiment on GaAs-AlGaAs superlattice, showing coherent Bloch oscillations.

Ongoing PhD project:

M. Sc. Simon Raiber



Resonant Raman spectroscopy of 2D crystal heterostructures

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. Sebastian Meier

M. Sc. Philipp Parzefall



Inelastic light scattering on GaAs-AlGaAs nanostructures

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. 

Ongoing PhD project:

M. Sc. Sebastian Meier



  1. Universität
  2. Physik

Optics of Semiconductor Quantum Structures

Research


Contact

Prof. Dr. Christian Schüller 

Tel. +49 (0)941 943 2078

christian.schueller@ur.de