Ultrafast Switchable Polar and Magnetic Orders by Nonlinear Light-Matter Interaction.

An outstanding challenge in materials science and physics is the harnessing of light for switching ferroic orders in, e.g., ferroelectrics. Here, we propose a mechanism through which electrons in few-layer 2D materials excited with above-bandgap light cause ionic structural transitions. Using perturbation theory within a many-body formalism, we show that the ionic coupling is mediated by a resonant change in electronic occupation functions, ultimately governed by the quantum geometric tensor (QGT) of the ground state. Furthermore, we show that such transitions are generally accompanied by multiferroic order switching. We demonstrate three examples of light-induced structural and polarization switching under this mechanism using first-principles calculations on bilayer CrI3, MoTe2, and trilayer 3R-MoSe2. We show that the three materials can switch between atomic stackings with a light intensity threshold of only ∼101 GW/cm2, a value 2-3 orders of magnitude lower than that required by direct light-ion coupling thanks to the superior efficiency of resonant light-electron coupling. Since such switching is fast, highly controllable, contactless, and reversible, it is promising for use in optically controlled nonvolatile memory, nanophotonics, and polar electronic devices.