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Abstract
The ferroelectric modes in KDP and LiNbO3 are coherently excited by the femtosecond impulsive stimulated Raman scattering (ISRS). The time dependence of the mode excited artificially shows a characteristic behavior in real time; the soft B2 modes of KDP and its deuterated isomorphous DKDP show a single-exponential time decay in contrast to the A1 phonon of LiNbO3 which behaves like a single damped harmonic oscillator. A critical-slowing down of the B2 modes has been observed above Tc for both KDP and DKDP. The isotope effect appears as the difference between the characteristic relaxation time τ0equals;1·1×10−11 s of KDP and 8·2×10−11s of DKDP. The dispersion relation of the A1phonon-polariton in LiNbO3 is studied by an improved ISRS method and confirmed to be a simple one without any resonance point in contrast to the result reported previously.