IMPULSIVE STIMULATED SCATTERING STUDY OF THE COUPLED ACOUSTIC AND SOFT MODES IN KD2PO4

The temperature and wave vector dependence of the soft shear acoustic mode and the soft polarization mode of KD2PO4 are examined in the high-temperature phase through impulsive stimulated Brillouin and Raman scattering. With different scattering wave vectors the regimes omegatau < 1, omegatau almost-equal-to 1 and omegatau > 1 are probed where omega is the acoustic frequency and tau is the polarization relaxation time. The results illustrate how the coupled acoustic and polarization response may be observed in the time domain, and indicate advantages for the time-domain approach in the omegatau almost-equal-to 1 regime. In this regime the softening of the acoustic mode is followed by hardening (increasing of the frequency) as the transition temperature is approached from above. On femtosecond time scales, it is possible to test critically for any inertial component to the polarization response, which would indicate an overdamped oscillator rather than relaxational description of the polar optic phonon mode.