Optically Controlled Phase Transitions in 2D Se Allotropes

Due to the advantages in high response speed and nonvolatility, materials with optically controlled phase transitions are promising candidates for data storage applications. In this work, we use real-time time-dependent density functional theory (rt-TDDFT) simulations to demonstrate that phase transitions can be precisely manipulated in two-dimensional (2D) ferroelectric α-Se through the application of an appropriate laser pulse. Unlike thermal activation, selected coherent phonon modes are activated optically. Specifically, the activations of A-1 and A-2 phonon modes at the Γ-point result in the transition to a state with an opposite electrical polarization. Through momentum scattering, these two phonon modes coherently evolve into phonon modes at the S-point. Then resultant nonequilibrium atomic dynamics, originating from the activation of three dominant B-1(S), B-2(S), and B-3(S) at S-point, facilitate the formation of a previously unexplored 2D selenium allotrope, which we refer to as r-Se.