Ultrafast Optical Control of Rashba Effect in Monolayer WSi ₂ N ₄ Family

ABSTRACT The Rashba effect is of central importance for spintronic and quantum technologies, and its controllability enables efficient and tunable spin manipulation. Using first‐principles calculations and molecular dynamics with machine‐learned force fields, we reveal a laser‐pulse‐triggered reversible phase transition from a nonpolar phase lacking spin splitting to a polar phase characterized by Rashba spin splitting in monolayer . This nonpolar‐to‐polar phase transition provides an ultrafast route for switching spin states between a spin‐splitting‐vanishing state and Rashba‐like splitting state. The transition is mainly associated with resonant excitation of infrared‐active shear phonon modes and their subsequent nonlinear coupling to Raman‐active modes under a THz light field. Our findings establish an effective strategy for manipulating Rashba‐like spin splitting in two‐dimensional systems and highlight the potential of ultrafast optical control in spintronics and phase transitions.