Modulating van der Waals Bonding in 2D Transition Metal Dichalcogenides with Light

Modulation of weak interlayer interactions between quasi-two-dimensional atomic planes in the transition metal dichalcogenides (TMDCs) provides avenues for tuning their functional properties. Here we show that above-gap optical excitation in the TMDCs leads to a large-amplitude, ultrafast compressive force between the two-dimensional layers, as probed by in-situ measurements of the atomic layer spacing at femtosecond time resolution. We show that this compressive response arises from a dynamic modulation of the interlayer van der Waals interaction and that this represents the dominant light-induced stress at low excitation densities. A simple analytic model predicts the magnitude and carrier density dependence of the measured strains. Probing in-plane lattice dynamics of the semi-metallic WTe2, we observe large amplitude interlayer shear oscillations in response to optical or THz excitation. This shear motion occurs along the direction of the phase transition pathway between the monoclinic and orthorhombic phases of the material, the latter of which has been predicted to be a type II Weyl semimetal. This work establishes new methods for dynamic tuning of van der Waals bonding and of the optomechanical functionality of TMDC quasi-two-dimensional materials.