Ultrafast spectroscopy of two-dimensional crystals in external magnetic fields

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In this project, the coherence properties of excitons and excitonic complexes - like charged excitons, neutral and charged biexcitons - in atomically thin transition-metal dichalcogenide monolayers in an external magnetic field shall be explored by time-integrated four-wave-mixing spectroscopy. The experiments will be performed in three-beam technique by applying different polarization configurations in an optical split-coil magnet cryostat. The investigations will be focused on MoSe2 monolayers. In this material, coherent excitonic complexes will be excited via distinct choices of different polarization configurations. The aim is to investigate of the quantum mechanical coupling of the complexes and the four- and many-particle correlations in external magnetic fields. We will start by applying the magnetic field in the Faraday configuration. In this configuration we expect a drastic amplification of many-particle correlations and signatures of non-Markovian memory effects. On the other hand, the experiments will also be performed in Voigt configuration, where a mixing of different spin states can be achieved. We will study the impact of the spin mixing on the coherence properties of excitonic complexes. Towards the end of the project also experiments on MoSe2 monolayers and MoSe2-WSe2 heterostructures, which are encapsulated in hexagonal Boron nitride (hBN), are planned. Encapsulation in hBN reduces significantly the inhomogeneous broadening and the disorder potential for charged excitons in TMDC monolayers and can potentially also influence the coherence times. In MoSe2-WSe2 heterostructures, the electron density in the MoSe2 can be tuned via an additional laser illumination, due to the type-II band alignment of the heterostructure and the resulting charge separation of photo-excited electron-hole pairs. We will explore the influence of the background charge-carrier concentration on the coherence properties of excitonic complexes.