The proposed ring transducer array and SA ultrasound imaging method were shown to be capable of producing isotropic high-resolution images whose quality was independent of depth.

Thermal phonon resonance was observed in a stiff solid material of fused silica, from which a lower intensity of scattered light is expected than from liquids. The improved detection sensitivity of the optical beating Brillouin spectroscopy technique enabled us to observe the spontaneous elastic resonance of the solid sample attributable to thermal density fluctuation with a frequency resolution of 1 kHz. The accuracy in determining phonon velocity was then increased up to 10 À5 . The high frequency resolution also revealed the fine structure of the resonance peak train brought about by the lateral mode vibration, and the eigen frequencies of the compound mode are in good agreement with those theoretically estimated. Volume and shear elasticities were uniquely determined from the resonance spectra of longitudinal and shear phonons. Phonon absorption at about 100 MHz was also determined from the resonance peak width of the Brillouin component.

It has recently become possible to observe Brillouin scattering spectra of gases at high temperatures as well as their rotational–translational (R–T) relaxation at room temperatures with optical beating Brillouin scattering spectroscopy. In this study, the sound velocity in nitrogen gas was measured at high temperature and a R–T relaxation in the frequency region of 100 MHz was observed. The obtained relaxation frequency value was found to be experimentally reasonable when compared to theoretical values. To our knowledge, optical beating Brillouin spectroscopy is the only method used to observe the total change in sound velocity in R–T relaxation at high temperature. Employing the spectroscopy allowed for a discussion of the temperature and frequency dependence on mechanical properties such as the specific ratio; that is, the internal energy degrees of freedom of the molecules.

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