Brillouin Scattering Study on Thermal Phonon Resonance

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In our previous studies, we had already succeeded in introducing the optical beating spectroscopy technique to Brillouin scattering experiment. The resolution 100 to 1000 times higher than the previous experiments was realized The technique provided us with a very effective means to investigate various phenomena of molecular dynamics in soft materials including (a) the first observation of the true shape of Brillouin triplet, (b) anomalous spectrum shape of depolarized light scattering in highly anisotropic moleculeds, and (c) the observation of asymmetry between the Stokes and the anti-Stokes components of Brillouin peaks in materials out of equilibrium.The purpose of the present study was to apply the new technique for the thermal-phonon resonance, the strange phenomenon that the natural phonons confined in a limited space reveal resonance effect. We intended to describe the physical mechanism of the phenomenon and find its applications, for instance, Brillouin scattering in liquids which show characteristic structures at molecular level near boundary or in specimen of very small amount.In the present study we established the method, "Brillouin scattering of thermal-phonon resonance" which enabled us to measure the phonons excited in a small volume-100mu m in size with 0.1% precision. The most serious problem in the light scattering experiment near boundaries would be the stray light which is must stronger than the weak signal light. To avoid this harmful effect, we developed a technique of polarization modulation for lock-in detection by which the spectrum of low-angle scattering(rheta-1ﾟ)was clearly observed with sufficient accuracy.The new method established in this study is being applied for studying dynamics of soft materials, yielding several interesting results. The method made a breakthrough to the Brillouin scattering which has more than 30 years history.