  The coupling between strain gradients and polarization, known as
flexoelectricity, offers a new mechanism to control the functionality of
dielectric materials. However, for the effect to be practically attractive,
dynamic control of the strain gradient with magnitudes far exceeding those
achievable 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 range
that greatly enhances the steady-state flexoelectric effect. Here we report a
giant, optically initiated dynamic enhancement of the strain gradient, also on
the order of $10^5 -10^6 m^{-1}$, in ferroelectric BiFeO3 epitaxial thin films
via time-dependent coherence analysis of X-ray diffractions. The finding opens
the door for dynamic coupling of the flexoelectric effect with light, making
optical switching of polarization, and thus application such as direct optical
writing of non-volatile ferroelectric memory, possible. A combination of
time-resolved X-ray scattering and optical spectroscopy shows that the
enhancement of the strain gradient is due to a piezoelectric effect driven by a
transient screening electric field, opening the opportunity for new ways of
studying flexoelectric effect in strain engineered ferroelectric thin films.
