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 stim ulated Raman scattering (ISRS) experiments are reported for both the o rthorhombic phase of KNbO3 and the tetragonal phase of BaTiO3. The dat a from potassium niobate clearly show a heavily damped soft mode of B2 symmetry and relaxational modes of A1 symmetry but not B2 symmetry. S imilarly, the data from barium titanate clearly show a heavily damped E-symmetry soft mode and no relaxational modes of this symmetry. The a bsence of relaxational modes of the same symmetries as the soft modes is consistent with an eight-site order-disorder model of the phase tra nsitions in this crystal class. The present results demonstrate signif icant advantages of ISRS over conventional Raman spectroscopy of low-f requency, heavily damped soft modes.