  The ultrafast dynamics of the antiferrodistortive (AFD) phase transition in
perovskite SrTiO3 is monitored via time-domain Brillouin scattering. Using
femtosecond optical pulses, we induce a thermally driven tetragonal-to-cubic
structural transformation and detect notable changes in the frequency of
Brillouin oscillations (BO) induced by propagating acoustic phonons. First, we
establish a fingerprint frequency of different regions across the temperature
phase diagram of the AFD transition characterized by tetragonal and cubic
phases in the low and high temperature sides, respectively. Then, we
demonstrate that in a sample nominally kept in tetragonal phase, deposition of
sufficient thermal energy induces an instantaneous transformation of the
heat-affected region to the cubic phase. Coupling the measured depth-resolved
BO frequency with a time and depth-resolved heat diffusion model, we detect a
reverse cubic-to-tetragonal phase transformation occurring on a time scale of
hundreds of picoseconds. We attribute this ultrafast phase transformation in
the perovskite to a structural resemblance between atomic displacements of the
R-point soft optic mode of the cubic phase and the tetragonal phase, both
characterized by anti-phase rotation of oxygen octahedra. Evidence of such a
fast structural transition in perovskites can open up new avenues in the field
of information processing and energy storage.
