Ultrafast optical manipulation of the stacking order in Td-WTe₂

Subtle changes in stacking order of layered transition metal dichalcogenides may have profound influence on the electronic and optical properties. The intriguing electronic properties of Td-WTe2 can be traced to the break of inversion symmetry resulting from the ground-state stacking sequence. Strategies for perturbation of the stacking order are actively pursued for intentional tuning of material properties, where optical excitation is of specific interest since it holds the potential for integration of ultrafast switches in future device designs. Here we investigate the structural response in Td-WTe2 following ultrafast photoexcitation by time-resolved electron diffraction. Using a combination of pump-probe ultrafast electron diffraction and first-principles modeling, we show that a 515 nm laser pump activates a 0.23 THz shear phonon, involving layer displacement along the b-axis. Pump fluences in excess of a threshold of ∼1 mJ/cm2 result in formation of a new stacking order by layer displacement along the b axis in the direction toward the centrosymmetric 1T* phase. The shear displacement of the layers increases with pump fluence until saturation at ∼8 pm.