Femtosecond time-resolved spectroscopy of soft modes in structural phase transitions of perovskites

Femtosecond time-domain observations of soft-mode dynamics in crystals near structural phase transitions have been conducted. Impulsive stimulated Raman scattering (ISRS) experiments are reported for both the orthorhombic phase of ${\mathrm{KNbO}}_3$ and the tetragonal phase of ${\mathrm{BaTiO}}_3$. The data from potassium niobate clearly show a heavily damped soft mode of ${\mathit{B}}_2$ symmetry and relaxational modes of ${\mathit{A}}_1$ symmetry but not ${\mathit{B}}_2$ symmetry. Similarly, the data from barium titanate clearly show a heavily damped E-symmetry soft mode and no relaxational modes of this symmetry. The absence of relaxational modes of the same symmetries as the soft modes is consistent with an eight-site order-disorder model of the phase transitions in this crystal class. The present results demonstrate significant advantages of ISRS over conventional Raman spectroscopy of low-frequency, heavily damped soft modes.