Ultrafast Optical Probing of Temperature-Induced Antiferrodistortive Phase Transition in SrTiO₃

SrTiO₃ (STO) is a highly versatile perovskite renowned for its wide array of ionic transport, dielectric and ferroelectric properties, making it a subject of great technological interest, particularly in energy, information processing, and data storage. In this study, we monitor the ultrafast dynamics of the antiferrodistortive (AFD) phase transition in STO via time-domain Brillouin scattering (TDBS). Monitoring the depth-resolved Brillouin oscillation (BO) frequency (associated with longitudinal coherent acoustic phonons), we demonstrate the presence of fingerprint frequencies of cubic and tetragonal phases across the high and low temperature regions of the sample, respectively. Coupled with a time-resolved heat diffusion model, the BO frequency profiles provide evidence of a ultrafast transformation of the low temperature tetragonal phase into cubic phase driven by thermal energy deposition^[1]. The initial rapid phase change is followed by a slower reverse cubic-to-tetragonal phase transformation occurring on a time scale of hundreds of picoseconds. The observed phase transformation can be attributed to a structural resemblance between atomic displacements of the R-point soft optic mode of the cubic phase and the tetragonal phase of the perovskites, both exhibiting an anti-phase rotation of oxygen octahedra. The evidence of such a fast structural transition in STO offers opportunities for developing novel ultrafast switching and phase-changing devices.<br/>1. https://doi.org/10.48550/arXiv.2309.03172