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The coupling between strain gradients and polarization, known asflexoelectricity, offers a new mechanism to control the functionality ofdielectric materials. However, for the effect to be practically attractive,dynamic control of the strain gradient with magnitudes far exceeding thoseachievable via mechanical deformation (~10 $m^{-1}$) is needed.Strain-engineered thin films exhibit extraordinary strain gradients of$10^5-10^6 m{-1}$ arising from structural relaxation within a short space rangethat greatly enhances the steady-state flexoelectric effect. Here we report agiant, optically initiated dynamic enhancement of the strain gradient, also onthe order of $10^5 -10^6 m^{-1}$, in ferroelectric BiFeO3 epitaxial thin filmsvia time-dependent coherence analysis of X-ray diffractions. The finding opensthe door for dynamic coupling of the flexoelectric effect with light, makingoptical switching of polarization, and thus application such as direct opticalwriting of non-volatile ferroelectric memory, possible. A combination oftime-resolved X-ray scattering and optical spectroscopy shows that theenhancement of the strain gradient is due to a piezoelectric effect driven by atransient screening electric field, opening the opportunity for new ways ofstudying flexoelectric effect in strain engineered ferroelectric thin films.

Giant optical enhancement of strain gradient in ferroelectric thin films and its physics origin