The B2 polariton mode in KDP studied by impulsive stimulated Raman scattering

Impulsive stimulated Raman scattering experiment has been performed in KDP (KH2PO4) to directly observe the time-dependent Feature of the ferroelectric soft B-2 mode in the polariton regime. The underdamped soft oscillatory mode has been observed and its squared natural frequency omega(0)(2) shows the linear temperature dependence, w(0)(2) proportional to T - T-0 where T-0 is the phase transition point of the clamped crystal. The present results are consistent with the previous study of the forward Raman scattering at room temperature. It is pointed out that the polariton behavior is explained by the order-disorder type mechanism of the phase transition as well as in the phonon-polariton scheme expected for the displacive type.