Photoinduced twist and untwist of moir\'e superlattices in TMDC heterobilayers
Two-dimensional moir\'e materials are formed by artificially stacking atomically thin monolayers. A wealth of correlated and topological quantum phases can be engineered via precise choice of stacking geometry. These designer electronic properties depend crucially on interlayer coupling and atomic registry. An important open question is how atomic registry responds on ultrafast timescales to optical excitation and whether the moir\'e geometry can be dynamically reconfigured to tune emergent phenomena in real time. Here we show that femtosecond photoexcitation drives a coherent twist-untwist motion of the moir\'e superlattice in $2^\circ$ and $57^\circ$ twisted WSe$_2$/MoSe$_2$ heterobilayers, resolved directly by ultrafast electron diffraction. Upon above-band-gap photoexcitation, the moir\'e superlattice diffraction features are enhanced within 1 ps and subsequently suppressed several picoseconds after, deviating markedly from typical photoinduced lattice heating. Kinetic diffraction analysis, supported by simulations of the sample dynamics, indicates a peak-to-trough local twist angle modulation of $0.6^\circ$, correlated with a sub-THz frequency moir\'e phonon. This motion is driven by ultrafast charge transfer that transiently increases interlayer attraction. Our results could lead to ultrafast control of moir\'e periodic lattice distortions and, by extension, the local moir\'e potential that shapes excitons, polarons, and correlation-driven behaviors