Ultrafast structural dynamics in quantum materials

by Jianyu Wu (KTH Applied Physics)

Friday 13 Feb 2026, 10:00 → 13:00 Europe/Stockholm

FA31 (AlbaNova Main Building)

Opponent: Professor Peter Baum, University Konstanz.

Supervisor: professor Jonas Weissenrieder, Ljus och materiens fysik; professor Oscar Tjernberg, Nordic Institute for Theoretical Physics NORDITA, Ljus och materiens fysik

Abstract

Strain provides a powerful route for manipulating quantum states in topological materials by coupling lattice distortions to spin, charge, and topological degrees of freedom. While static approaches such as strain engineering, stacking, and twisting have expanded access to new quantum phases, reversible and programmable control of such states on ultrafast timescales remains a central challenge. This thesis addresses this gap by investigating ultrafast structural dynamics and strain modulation in van der Waals materials using ultrafast electron microscopy (UEM), which combines nanometer spatial resolution with picosecond temporal resolution.

We begin by studying strain wave propagation and interference in defective samples. The interaction between static strain fields and photoexcited coherent acoustic phonons is studied in the Weyl semimetal WTe₂. Local standing waves are generated at defect sites where static and dynamic strain couple, enabling deterministic modulation of strain at tens of nanometer scales. Finite-element simulations complement UEM imaging and elucidate the mechanisms of light-induced transient strain engineering. By employing spatially structured femtosecond optical fields, we further confine lattice oscillations into programmable phonon cavities with tunable dimensions, symmetries, and frequencies. UEM directly images these phonon dynamics, including confined out-of-plane oscillations and in-plane Lamb waves, establishing a platform for coherent phonon engineering and programmable transient lattice control at the nanoscale.

We then extend the study into the metastable structural phase control. We demonstrate a photoinduced ultrafast structural transition in PdSe2, occurring within tens of picoseconds and followed by long-lived metastable states persisting on nanosecond timescales. This behavior highlights the potential of pentagonal-layered materials as optically switchable platforms for phase control. We further realize femtosecond laser-driven topological phase patterning in WTe₂ by engineering transient optical gratings. This approach enables selective and reversible transitions between topological and trivial phases, with real-space imaging revealing strain-mediated interface dynamics. These studies of metastable structure control pave the way for optically addressable quantum and topological devices.

Finally, we report the direct observation of moiré pattern dynamics under transient strain in twisted van der Waals homostructure. Time-resolved dark-field imaging reveals coherent oscillations of moiré contrast at phonon frequencies, spatially correlated with bent regions. These results demonstrate the coupling between lattice vibrations and emergent functionality in van der Waals junctions.

Together, this work establishes a general framework for spatiotemporal ultrafast structural control in layered materials. By bridging structured light excitation with lattice dynamics, it opens new pathway toward optically reconfigurable quantum phases, topological textures, and novel device functionalities.

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