Resonant optical control of magnetism on ultrashort timescales

Excitation of optical transitions in solids using ultrashort pulses of light allows to selectively perturb microscopic degrees of freedom in order to change and control material properties on very short timescales. In this thesis we study how ultrafast resonant excitation of optical transitions can induce coherent structural dynamics in wide-bandgap insulators and control magnetic interactions, manipulate magnetic order and induce (propagating) spin dynamics in insulating antiferromagnets. In time-resolved all-optical pump-probe experiments,we use ultrashort pulses of light to target specific lattice vibrations, orbital resonances and electronic transitions in various insulating materials and optically probe the structural and magnetic dynamics on the picosecond timescale.
Chapter 1 provides an introduction to the field that studies ultrafast optical control of solids, with a focus on resonant optical control of magnetic properties and the generation of propagating excitations. Chapter 2 discusses the basic concepts of magnetic interactions, magnetic order and spin waves and chapter 3 contains the main experimental methods and experimental setups used in this work.
In chapter 4 we study the coherent structural dynamics initiated by ultrafast resonant excitation of an infrared-active lattice vibration in the wide-bandgap insulator LaAlO₃. We observe the excitation of a coherent THz phonon mode, corresponding to rotations of the oxygen octahedra around a high-symmetry axis, and identify the underlying nonlinear phonon-phonon coupling through density functional theory calculations. The resonant lattice excitation is also shown to generate both longitudinal and transverse strain wavepackets, the result of optically induced anisotropic strain.
In chapter 5 we demonstrate that light-driven infrared-active phonons can be used to control fundamental magnetic interactions and coherently manipulate magnetic states on picosecond timescales. Resonant optical excitation of lattice vibrations in the antiferromagnet DyFeO₃ results in nonthermal, ultrafast and long-living changes in the exchange interaction between the Dy orbitals and the Fe spins. We identify phononinduced coherent lattice distortions as the underlying mechanism and show that we can use this change in magnetic interaction to induce picosecond coherent switching from a collinear antiferromagnetic ground state to a weakly ferromagnetic phase.
Having explored the structural and magnetic dynamics following excitation of lattice vibrations, we explore the effect of optical excitation of orbital resonances in the van der Waals antiferromagnet NiPS₃ in chapter 6. We demonstrate that ultrashort pulses of light, with the photon energy tuned in resonance with orbital transitions within the magnetic nickel d-orbital manifold, can excite a subterahertz magnon mode with twodimensional behaviour. We show that this selective excitation results from a photoinduced transient magnetic anisotropy axis, which emerges in response to excitation of the ground-state electrons to orbital states with a lower orbital symmetry.Finally, we show in chapter 7 that ultrashort pulses of light can generate a wavepacket of coherent propagating spin waves in insulating antiferromagnets. The nanometer confinement of ultrafast optical excitation in resonance with electronic charge-transfer transitions in the antiferromagnet DyFeO₃ creates a strongly non-uniform spatial spin excitation profile close to the material surface. This results in the emission of a broadband wavepacket of coherent subterahertz spin waves into the material. We optically probe individual spectral components of this spin-wavepacket with wavelengths down to 125nm in a time-resolved fashion using the magneto-optical Kerr effect.
Chapter 8 provides the main conclusions of the work presented in this thesis. Wereflect on unanswered questions and give possible directions for future research.