Excitation of Soft Acoustic Mode by Stimulated Brillouim Scattering and Phase Transition Dynamics

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the purpose of this research is to excite phonon in material directly by impulsive stimulated Brillouin scattering and to clarify the role of the phonon in phase transition phenomena. In the present work, we have done the following exeriment : 1) Measurement of quasi-longitudinal acoustic mode in TGSe (triglycine selenate) near the ferroelectric phase transition, 2) Development of femtosecond stimulated light scattering system, 3) Direct comparison of femtosecond optical Kerr effect spectroscopy with high resolution light scattering spectrum, 4) Experimental verification of the breakdown of Debye model.1) TGSe is a crystal which undergoes ferroelectric phase transition at 22? A1C.Below the transition point, the anomaly in the velocity of the quasi-longitudinal phonon mode C_<33> has been found from the light scattering and ultrasonic experiments. This anomaly comes from the coupling of the phonon modes with the polarization fluctuation. Since the characteristic frequency of the polariz … More ation fluctuation is temperature-dependent, the velocity anomaly occurs at different temperatures dependent on the frequency of the phonon measured. In the present work, we have measured the temperature dependence of the phonon velocity in the intermediate frequency region (450 MHz) and have confirmed the proposed mechanism of this anomaly experimentally.2) We have developed the femtosecond version of the stimulated light scattering. This system provides the excitation of THz phonon.3) Optical Kerr effect spectroscopy and spontaneous light scattering in off-resonance excitation region are connected through fluctuation-dissipation theorem and we have confirmed this theorem experimentally by use of the high-resolution ligh scattering and femtosecond optical Kerr effect measurements.4) Debye model has been often used to express the fluctuational behavior in phase transition studies. However, this model accompanies essential difficulty in itself such as breakdown of the momentum sum rule. Since the breakdown of this model is clearly observed in the time-domain by the finite initial rise in the response function, we have performed the experiment using typical liquids as samples. The response function obtained by the femtosecond optical Kerr effect and femtosecond polarization-selective four wave mixing experiment shows a clear initial rise in every sample. It is suggested that the breakdown of Debye model has serious effect when the fluctuation in a wide frequency range is discussed such as in glass transtions. Less