Giant Photostriction and Optically Modulated Ferroelectricity in BiFeO₃

BiFeO₃ thin films, with their intertwined lattice, charge, and spin orders, hold immense potential for next-generation optomechanical applications. However, their photostrictive response remains underexplored and typically demands high optical power. Here, we demonstrate a strong photostriction effect in nanocrystalline BiFeO₃ thin films synthesized via scalable chemical spray pyrolysis─activated under relatively low optical powers (∼1.7 × 10⁴ W m^-2). This phenomenon is accompanied by light-driven enhancements in piezoelectricity and polarization switching together with a dense network of domain walls promoting efficient exciton separation in unconstrained nanocrystalline BiFeO₃ films. The nanostructured films exhibit a photostriction coefficient of ∼4.5 × 10^-7 m² W^-1─five times higher than bulk BiFeO₃ single crystals and rivaling state-of-the-art halide perovskites. These findings offer valuable insights and provide a way forward for integrating solution-processed bismuth ferrite films into advanced photosensors, wireless optomechanical and multifunctional devices.