Optical excitation perturbs the balance of phenomena selecting the tilt
orientation of domain walls within ferroelectric thin films. The high carrier
density induced in a low-strain BaTiO3 thin film by an above-bandgap ultrafast
optical pulse changes the tilt angle that 90{\deg} a/c domain walls form with
respect to the substrate-film interface. The dynamics of the changes are
apparent in time-resolved synchrotron x-ray scattering studies of the domain
diffuse scattering. Tilting occurs at 298 K, a temperature at which the a/b and
a/c domain phases coexist but is absent at 343 K in the better ordered
single-phase a/c regime. Phase coexistence at 298 K leads to increased
domain-wall charge density, and thus a larger screening effect than in the
single-phase regime. The screening mechanism points to new directions for the
manipulation of nanoscale ferroelectricity