Nonlinear Terahertz Phononics: A Novel Route to Controlling Matter

Periodically arranged atoms are the fundamental building blocks of solids, and determine the mechanical, thermal, and electric properties of a material. Thus, it is no surprise that lattice vibrations (phonons) govern a number of exciting phenomena such as spin transport in thermal gradients, phase transitions and superconductivity. In this thesis, we take advantage of phonons as a novel and specific pathway to drive ultrafast processes in solids. By direct excitation with intense, ultrashort THz electric-field transients, high frequency phonons in insulating solids are accessed on their intrinsic time- and energy-scales, while avoiding parasitic electronic processes. These studies were enabled by the design and implementation of a high-field THz source, which allows for phase-sensitive pump-probe experiments over multiple time scales: from femto- to microseconds. This work provides new insights into the coupling between the lattice and magnetic ordering, which is of central relevance for rapid data processing and information storage in future technological applications. Furthermore, fundamental dynamic processes such as magnetization switching and transport of spin angular momentum require an understanding of the way spins interact with oscillations of the crystal lattice. In order to gain such fundamental insights we investigate pure spin- lattice coupling by resonant excitation of infrared-active phonon modes of the textbook ferrimagnetic insulator Yttrium Iron Garnet. Remarkably, two distinctive time scales for phonon-magnon equilibration are revealed. A surprisingly rapid change of magnetic order with a time constant of ~1 ps is found to be driven by phonon-induced fluctuations of the exchange coupling, which leads to a sublattice demagnetization under the constraint of conserved total spin angular momentum. The resulting metastable state persists for nanoseconds until the spin angular momentum is released to the lattice via weaker coupling mechanisms. The experimental observations can be reproduced by atomistic spin-dynamics simulations. These findings have important implications for contemporary research fields like the spin Seebeck effect, antiferromagnetic spintronics and ultrafast magnetization switching. In contrast, phonon modes with vanishing electric dipole moments were so far excluded from such direct THz excitation. In this thesis, a novel type of light-matter interaction is presented that enables coherent-phonon excitation via non-resonant two-photon absorption of intense THz fields. This second- order nonlinear process is the so far neglected up-conversion counterpart of stimulated Raman scattering. Here, it is demonstrated by the coherent control of the 40 THz Raman-active optical phonon in diamond via the sum frequency of two intense terahertz field components. Remarkably, the CEP of the driving pulse is directly imprinted on the lattice vibration. This study opens up a novel pathway to the phase-sensitive coherent control of phonons that were previously inaccessible by THz radiation. Furthermore, new prospects in vibrational and magnon spectroscopy, lattice trajectory control and laser machining emerge from this work. In conclusion, this thesis demonstrates that phonons are a key component for controlling ultrafast processes in solids.