Thesis defense of Fabian Mertens

2D materials, like the antiferromagnetic van der Waals semiconductors FePS3 studied in this work, open up new possibilities for technological applications due to the unique interaction of their magnetization with electronic, optical, and mechanical properties. Furthermore, they provide the potential to study magnetism and magnetization dynamics in reduced dimensions. Up do date, the coherent control of the magnetization of these materials has barely been studied. Our research addresses this gap by using ultrashort light pulses. In this context, time-resolved studies can give an insight into the evolution of the light-induced dynamics, which essentially require a dedicated experimental setup. In this thesis, we present a comprehensive study on the development and application of a table-top laser setup designed for magneto-optical pump-probe experiments and adaptable for the investigation of microscopic samples. The system employs two optical parametric amplifiers, with a tunable photon-energy range of 0.5 eV - 3.5 eV for both the pump and the probe beam. Remarkable is the high pump amplitude modulation rate at 50 % of the laser repetition rate, realized via the integration of an electro-optical modulator, blocking every second pump pulse. Combined with a high-frequency digitizer, performing single pulse detection, our system can achieve a high sensitivity, down to 50 µdeg of the probe polarization rotation. The setup can apply magnetic fields of up to +-9 T, and voltages in the kV regime while providing a temperature control between 4 K-420 K. The functionality of the setup’s systems is demonstrated by performing static Kerr-rotation and ultrafast demagnetization measurements in a cobalt single crystal as a function of the most important experimental parameters. The major part of this thesis is dedicated to our studies on a coherent optical lattice mode of terahertz frequency triggered by femtosecond laser pulses in the antiferromagnetic van der Waals semiconductor FePS3. This specific 3.2 THz phonon mode shows a close relation to the antiferromagnetic order, as it vanishes above the Néel temperature and hybridizes with a magnon mode in the presence of a magnetic field. We investigate it as a function of sample temperature, probe polarization, excitation photon energy and externally applied magnetic fields. The resonant excitation of a crystal-field split electronic d-d transition efÏciently pumps the displacive excitation process of the mode, while the magnetic linear dichroism is identified as the magneto-optical effect, which reflects the phonon mode in the probe rotation. By applying magnetic fields of up to 9 T we can generate and observe the coherent hybridized phonon-magnon mode, thus exploiting the hybridization to excite coherent spin-dynamics. Furthermore, we investigate the coherent phonons in the bulk form of FePS3 and in an exfoliated flake with a thickness of 380 nm.

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