Phase transition dynamics studied by coherent phonon excitation with ultrashort laser pulses

The low-frequency light scattering spectroscopy always encounters experimentally a difficulty near the phase transition point due to the overlapping between the soft mode spectra and the strong elastic-scattering component originated from the surface and/or imperfections irradiated by the laser light. A time-resolved phonon spectroscopy, the coherent phonon excitation method , has been introduced as a new method to compliment the light scattering method. It observe directly the time dependence of the soft phonons excited artificially in terms of pico- or femtosecond laser pulses. Several cases of the studies on TGS, TGSe, KDP, DKDP and LiNbO 3 are briefly mentioned.