Study on Acoustic Soft-Mode Dispersion by Impulsive Stimulated Brillouin Scattering

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We have developed a new phonon spectroscopy using impulsive stimulated Brillouin scattering. This method is based on the phonon generation by two crossing light pulses and the diffraction of a probe light by the phonon. Unlike recent measurements, we have employed a cw laser as a probe. This enables us to measure an extremely slow response near the phase transition point, indicating a high-resolution phonon spectroscopy in frequency regime. Furthermore, coherent nature of the generated phonons offers a measurement at extremely low phonon wave number. Combining with a spherical Fabry-Perot interferometer, we can perform directly the frequency-domain measurement for the generated non-equilibrium phonons. To confirm the advantage of this method, we have investigated the mechanisms of the phonon generation, the propagation of the generated phonon, the resonance effect on the phonon generation by the pulse train. Several applications have been performed, which include the generation of acoustic waves in liquids, metal surfaces, and liquid crystals. For the measurement of the acoustic soft-mode, we have employed a ferroelastic crystal KD_3(SeO_3)_2 as a sample, which is known to show a C_<66> soft phonon mode. We have performed the spatial intensity profile of the laser light transmitted through the crystal, which is influenced by the domain formation below the transition point. Particularly, the intesity of the refracted light at the domain wall gives a new observation method of the elastic fluctuation near the transition point. Indeed the refracted light at the domain wall is observable even above the transition point, suggesting the fluctuation of the domain formation takes place at those temperatures. Combining the ISBS experiment with the above transmission experiment, we have obtained the soft-mode behavior near the transition point. The detailed analysis of the result is now in progress.