  Complex oxides offer a wide range of functional properties, and recent
advances in fabrication of freestanding membranes of these oxides are adding
new mechanical degrees of freedom to this already rich functional ecosystem.
Here, we demonstrate photoactuation in freestanding thin film resonators of
ferroelectric Barium Titanate (BaTiO3) and paraelectric Strontium Titanate
(SrTiO3). The free-standing films, transferred onto perforated supports, act as
nano-drums, oscillating at their natural resonance frequency when illuminated
by a frequency-modulated laser. The light-induced deflections in the
ferroelectric BaTiO3 membranes are two orders of magnitude larger than in the
paraelectric SrTiO3 ones. Time-resolved X-ray micro-diffraction under
illumination and temperature-dependent and holographic interferometry provide
combined evidence for the photostrictive strain in BaTiO3 originating from
partial screening of ferroelectric polarization by photo-excited carriers,
which decreases the tetragonality of the unit cell. These findings showcase the
potential of photostrictive freestanding ferroelectric films as wireless
actuators operated by light.
