Coherent Excitation of Ferroelectric Soft B ₂ Mode of KDP by Impulsive Stimulated Raman Scattering

Coherent excitation of the ferroelectric soft modes of KDP has been done in the time domain by femtosecond impulsive stimulated Raman scattering. The observed time dependence shows clearly a relaxational character of the soft mode in a wide temperature range above T-c. A critical slowing-down is directly revealed in the temperature dependence of the relaxation time tau = tau(0)T(0)/(T - T-0) as tau(0) is 0.11 x 10(-12) s, where T-0 is the phase transition point of the clamped crystal. The order-disorder phase transition mechanism is reasonable within the limit of the time resolution of the present study.