Controlling and probing nonlinear collective mode dynamics in quantum materials

Abstract

Tailored laser pulses offer a powerful means of driving materials out of equilibrium by selectively addressing specific degrees of freedom. In particular, the excitation of low-energy collective modes in solids—such as lattice vibrations (phonons) and spin precessions (magnons)—to large amplitudes opens fundamentally new pathways for controlling and probing material properties that are otherwise inaccessible under thermal equilibrium conditions. In this regime, both the nonlinear interactions between light and matter and the intrinsic nonlinear dynamics of the driven modes present significant challenges for understanding the underlying mechanisms and for realizing potential applications.

This dissertation centers on two major themes: (1) probing equilibrium properties of materials via nonlinear light-matter interactions; and (2) unveiling emergent phenomena hidden in equilibrium by driving collective modes far from equilibrium.

I begin by providing an overview of recent advances in controlling and probing quantum materials out of equilibrium, followed by a discussion of the theoretical frameworks and experimental methodologies used to interrogate collective excitations. Building on this foundation, I present two studies demonstrating how terahertz Raman excitation can reveal distinct spectroscopic signatures of material states.

Subsequently, I focus on coherent nonlinear magnon-magnon interactions in canted antiferromagnets, induced by tailored terahertz fields. In these experiments, we demonstrate a unidirectional magnon upconversion process and identify correlated magnonic responses at both the sum and difference frequencies of the interacting modes. We achieve parametric amplification of magnon coherence by tuning the magnonic difference-frequency generation into resonance with a low-frequency magnon. Furthermore, by increasing the driving field strength to access a far-from-equilibrium regime, we uncover spectroscopic signatures of non-perturbative dynamics marked by strong magnon self-interactions.

Finally, I present an example in which spatially heterogeneous responses of electromagnon modes in a van der Waals multiferroic are revealed through terahertz photon echo measurements. Together, these results highlight how tailored light-matter interactions can be leveraged to probe, control, and manipulate material degrees of freedom, both in and out of equilibrium.