Coherent Terahertz Control and Ultrafast Spectroscopy of Layered Antiferromagnets

This thesis presents new insights into the strong interactions among electronic, lattice, spin, and orbital degrees of freedom in layered magnetic materials, as well as their emergent properties. Using a suite of spectroscopic techniques, both in equilibrium and out-of-equilibrium settings, several important findings have been made. In a family of transition metal thiophosphates, a novel bound state resulting from electronic transitions between d-orbitals and Raman-active phonons was observed in NiPS3, using femtosecond transient absorption spectroscopy. Furthermore, this phonon symmetry was employed to identify a new magnetostrictive effect in FePS3 through coherent phonon spectroscopy. These and other observations point to strong interactions between spin and lattice degrees of freedom in this system. This coupling has been harnessed to actively control the magnetic structure. Specifically, intense, tailored terahertz pulses were used to displace the lattice along particular phonon directions, inducing a new magnetic order characterized by net magnetization. This effect is notably more efficient and exhibits an increasingly longer lifetime near the phase transition point, highlighting the key role played by critical fluctuations. Finally, second harmonic generation, linear dichroism, and Raman spectroscopy were employed to discover a new type-II multiferroic phase that persists down to the atomic monolayer limit in NiI2.