Ultrafast Probing of Nonlinear Dynamics in Quantum Materials: Beyond Linear Response Probes

Abstract

The interactions between the spin, orbital, and charge degrees of freedom of electrons give rise to a variety of exotic phenomena and emergent phases of matter in quantum materials. However, understanding these phenomena is often hindered by the lack of experimental probes that are sensitive to multiple degrees of freedom. Linear response probes, such as scattering and electron transport measurements, are sensitive to a single electronic degree of freedom and cannot provide insights into electron-electron coupling, which can result in hidden orders and spectral features still under debate.

Recent advances in ultrafast laser technology have made it possible to study the dynamical behavior of solids that remains hidden to equilibrium probes. Novel spectroscopy techniques, such as coherent phonon spectroscopy, transient absorption spectroscopy, second harmonic generation microscopy, and high field THz excitation spectroscopy, are capable of probing the properties of quantum materials that are not accessible with linear response techniques. By employing these techniques, this thesis visualizes different types of collective excitations in quantum materials and unveils new physics that only exists under non-equilibrium conditions. The results presented in this thesis contribute to the understanding of the mechanisms underlying exotic states of matter and have the potential for applications in the development of novel devices.

This thesis focuses on the use of novel ultrafast spectroscopy techniques to study the collective behavior and emergent phases of matter in quantum materials. In Chapter 2, we unravel a exceptionally strong coupling between lattice vibrations and orbitals in a van der Waals antiferromagnet, NiPS3. In Chapter 3, we show that this exceptionally strong phonon-orbital coupling can lead to a strong effective coupling between spins and lattice in FePS3, which is not detectible with conventional probes, such as Raman and X-ray scattering. In Chapter 4, by using nonlinear optical microscopy techniques, we discover a two-dimensional multiferroic, NiI2. In Chapter 5, using high electric field excitations in THz domain, we observe the phononic analog of Cherenkov effect, where electrons propagating faster than the speed of sound directionally emit acoustic phonons. Finally, we provide a comprehensive and insightful outlook towards the possibilities that lie ahead and elucidate the prospective trajectories and viable experiments that could potentially pave the way for further advancements and breakthroughs in the realm of ultrafast spectroscopy of quantum materials.