Würzburg ToCoTronics Colloquium

Vibrational motion is a powerful means to interrogate the properties of molecules and materials, providing insights into chemical reactions, phase transitions, and electron-phonon coupling mechanisms. Excitation by femtosecond light pulses can drive structural motion coherently, that is, launching phase-locked collective atomic displacements (coherent phonons) across the lattice. This capability has enabled the investigation of material structures far from equilibrium.

Recently, the focus of the field has shifted in two substantial ways. First, from the mere observation of structural dynamics to the active, all-optical driving of lattice motion into novel states of matter. Second, from unspecific excitation of many lattice degrees of freedom to the selective driving of targeted vibrational modes – in other words, from hammers to tweezers.

In this talk, I will present how selective optical excitation schemes, combined with structure-sensitive probes such as ultrafast low-energy electron diffraction and time-resolved (non)linear optical spectroscopy, can be harnessed to control physical phenomena including metal-insulator transitions, ferroelectric order, and topological properties. Based on recent experiments on atomic indium wires on Si(111) and the ferroelectric Weyl semimetal WTe2, I will demonstrate that pushing atoms multiple times like a child on a swing provides remarkable leverage over the order parameter, enabling coherent control of phase transitions and symmetry-breaking pathways.