Collective octahedral tilting in ultrathin Ruddlesden-Popper perovskite under terahertz light

Perovskites have been applied in a wide range of fields such as solar cells and non-volatile memories due to their multiferroic nature and excellent photo-electric conversion capabilities. Recently, two-dimensional (2D) perovskites with a few atomic layers have been successfully synthesized, attracting significant attention for potential applications. In this work, we perform first-principles calculations to investigate an ultrathin prototypical Ruddlesden-Popper phase, $\mathrm{Bi}_2\mathrm{FeO}_4$, with its thickness down to one unit cell. We show that this compound could exist in two (meta-)stable octahedral tilting phases, belonging to $P2_1/c$ and $C2/m$ space groups, respectively. Using the optomechanical theory, we suggest that reversible and non-volatile phase switching can be triggered using non-destructive terahertz light. In addition, the two phases show distinct optical reflectance spectrum in the visible light range, which can be used as an optical probe for phase transformation. This enables both "write" and "read" in an all-optical route.