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 KNbO3 and the tetragonal phase of BaTiO3. The data from potassium niobate clearly show a heavily damped soft mode of B2 symmetry and relaxational modes of A1 symmetry but not B2 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.