Thesis defense of Marc Terschanski

In the first part of this thesis, a newly constructed set-up is described that allows simultaneous optical and magneto-optical pump-probe experiments to be performed as a function of temperature, magnetic field or an applied electric field. In the second part of this work, optical methods are used to investigate whether a coupling exists between the spin system and the electronic system in the magnetic semiconductor α-MnTe. For this purpose, a 200 nm thick layer of α-MnTe is measured with different experimental methods. The band gap of α-MnTe experiences an additional blue shift in the antiferromagnetic phase, which scales proportionally with the sublattice magnetisation. This static coupling between the band gap and the spin system is successfully reproduced in our sample. Based on this, the dynamics of the band gap is investigated using the pump-probe method. The transient reflectivity shows that the band gap is coherently modulated by optical active  E2g-phonons. In the final step, the band gap is coherently excited and the response of the spin system, the rotation of the polarisation, is detected. Again the rotation of polarisation is coherently modulated by the  E2g-phonon and shows an additional incoherent contribution in the antiferromagnetic phase, which is ascribed to the magnetic order. The decay time of this contribution corresponds to the time scale of the sublattice demagnetisation. Thus, a phonon-assisted coupling between the electronic system and the spin system on the femtosecond timescale in α-MnTe has been demonstrated in this work.