Ultrafast Electron Microscopy and Diffraction

In the first part I will discuss a recent study on photo driven structural dynamics in Td-WTe₂, a material where subtle changes in stacking order have profound influence on the electronic and optical properties. A shear phonon, involving layer displacement along the b axis, may be excited by a 515 nm laser pulse. Pump fluences in excess of a threshold of ~1 mJ/cm² results in formation, ~4 ps time constant, of a new stacking order by layer displacement along the b axis in the direction towards the centro-symmetric 1T* phase. The shear displacement of the layers is observed to increase with pump fluence until saturation at ~8 pm. We demonstrate that the excitation of the shear-phonon and the stabilization of the metastable phase are decoupled when using an optical pump as evidenced by the formation of the metastable phase also in samples with structurally pinned shear phonons. The results are compared to dynamic first-principles simulations and the transition is interpreted in terms of a mechanism where the material transiently explore a large phase space before settling at the atomic positions of the metastable phase. This interpretation is corroborated by results from diffuse scattering. The correlation between excitation of intralayer vibrations and interlayer interaction demonstrates the importance of including both short- and long-range interactions in an accurate description of how optical fields can be employed to manipulate the stacking order in 2-dimensional transition metal dichalcogenides.

In the second part, I will describe the application of a novel combination of transient grating excitation with Lorentz ultrafast electron microscopy to control and detect magnetization dynamics with combined nanometer and picosecond resolutions. Excitation of Ni₈₀Fe₂₀ thin film samples results in the formation of transient coherently precessing magnetic gratings. From the time-resolved results, we extract detailed real space information of the magnetic precession, including local magnetization, precession frequency, and relevant decay factors. The Lorentz contrast of the dynamics is sensitive to the alignment of the in-plane components of the applied field. The experimental results are rationalized by a model considering local demagnetization and the phase of the precessing magnetic moments. We envision that this technique can be extended to the study of spin waves and dynamic behavior in ferrimagnetic and antiferromagnetic systems.

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