Light-Induced Transient Polarization Reversal in Rhombohedrally Stacked Bilayer Transition Metal Dichalcogenides via an Electronic Mechanism.

Light-induced sliding ferroelectricity in two-dimensional van der Waals materials enables polarization control via relative layer motion. However, polarization switching occurs on the timescale of shear modes (tens of picoseconds) and requires very large fluences, potentially damaging the samples. Here, using constrained density functional theory and many-body real-time simulations, we demonstrate an ultrafast electronic reversal of the total out-of-plane polarization sign in the photoexcited state, without requiring interlayer sliding, in rhombohedrally stacked transition metal dichalcogenide bilayers. The polarization changes sign relative to its initial ground-state value at moderate fluences and within ∼200  fs, about 50 times faster than the typical shear-mode period. The ultrafast switching is driven by a rearrangement of localized dipoles around the tungsten sites and can be probed by phase-sensitive transient second-harmonic generation measurements. We establish a novel general mechanism for electronic control of low-dimensional ferroelectrics common to all polar multilayers having type II band alignment. Our Letter has direct implications for ultrahigh-speed volatile optical memory operating on subpicosecond timescales.