  The spontaneously formed striped polarization nanodomain configuration of a
PbTiO${_3}$/SrTiO${_3}$ superlattice transforms to a uniform polarization state
under above-bandgap illumination with a time dependence varying with the
intensity of optical illumination and a well-defined threshold intensity.
Recovery after the end of illumination occurs over a temperature-dependent
period of tens of seconds at room temperature and shorter times at elevated
temperatures. A model in which the screening of the depolarization field
depends on the population of trapped electrons correctly predicts the observed
temperature and optical intensity dependence.
