  Despite extensive research interests in perovskite oxides, low energy
consumption, non-destructive and maneuverable methods for phase transition in
perovskite oxides are still the under its exploration, and the underlying
mechanisms remain ambiguous. Here, optical susceptibility including electronic
and anharmonic phononic contributions is used to evaluate Gibbs free energy
variations of $\mathrm{PbTiO}_3$ and $\mathrm{BaTiO}_3$ under terahertz
irradiation. This corresponds to an off-resonant light-controlled phase
transition, rather than the resonant approaches that excites hot carriers over
electronic band or infrared-active vibrations in the phonon band. We show that
intermediate terahertz light can trigger polarization change between
ferroelectric orientation variants of $\mathrm{PbTiO}_3$ at room temperature
300 K. Furthermore, the phase transition from low symmetric ferroelectric phase
to high symmetric paraelectric structure in $\mathrm{PbTiO}_3$ can be driven by
changing the direction and intensity of the incident light under the same
conditions. Similar results are observed in $\mathrm{BaTiO}_3$. In detail,
phonon spectrum and optical susceptibility are obviously modified and show
temperature dependence, in which we show the significant effects of anharmonic
vibration. In order to show its nonlinear optical nature, we perform an
alternating electric field dressed ab initio molecular dynamics simulation,
which maps the Raman-active phonon excitation under off-resonant terahertz
irradiation.

Materials Science