FEMTOSECOND TIME-RESOLVED SPECTROSCOPY OF POLARIZATION DYNAMICS IN KNBO3

Femtosecond time-resolved impulsive stimulated scattering experiments were conducted on KNbO3 at temperatures spanning the orthorhombic phase. A heavily damped soft optic phonon mode associated with the displacive character of the transition and relaxational modes associated with order-disorder character were examined. Previous frequency-domain light scattering experiments had yielded conflicting results for the dynamics of the soft mode and the symmetry of the order-disorder motions. We find the relaxational motion to be of A, symmetry and the soft mode to be the only lowfrequency exictation of B, symmetry. The soft mode is well described by a damped polariton model without any coupling to a relaxational mode. These results are consistent with a previously proposed eight-site model for the phase transitions. © 1991, Taylor & Francis Group, LLC. All rights reserved.