Young Ho Ko

Pressure Dependence of Acoustic Properties of Liquid Ethanol by using High-pressure Brillouin Spectroscopy

Brillouin spectroscopy has been widely used for the investigation of acoustic properties of condensed matters in the hypersonic region. A high-pressure Brillouin spectrometer was set up by combining a diamond anvil cell and a tandem multi-pass Fabry-Perot interferometer. It was successfully applied to liquid ethanol, and the pressure dependence of the sound velocity, the refractive index and other acoustic properties were derived based on the measurements. The detailed optical setup and experimental procedure are described.

Acoustic anisotropy of oriented polyethylene terephthalate films studied through Brillouin light scattering

The polyethylene terephthalate (PET) substrate is one of the key materials comprising optical films, which are essential components in various optical devices, such as displays. The elastic properties of oriented PET were investigated via Brillouin light scattering spectroscopy. The high-frequency longitudinal and transverse sound velocities in the hypersonic region were determined using a special right-angle scattering geometry, where the information of the refractive index was not included in the calculation. When the propagation direction of the acoustic modes was changed in the plane with respect to the stretching directions, the longitudinal and transverse sound velocities exhibited periodic modulations, clearly revealing the acoustic anisotropy of the film arising from the stretching condition.

Enhanced Polarization Fluctuations in PbZr0.72Sn0.28O3 Compared to PbZrO3 Single Crystals Studied by Brillouin Light Scattering

Acoustic properties of antiferroelectric (AFE) PbZr0.72Sn0.28O3 single crystals were investigated in a wide temperature range by Brillouin light scattering spectroscopy. The Brillouin frequency shift of the longitudinal acoustic (LA) mode exhibited three abrupt changes corresponding to the successive phase transitions between the paraelectric, intermediate, antiferroelectric (AFE2) and antiferroelectric (AFE1) phases upon cooling. The change in the LA mode frequency and the damping maximum of PbZr0.72Sn0.28O3 were much higher than those of pure PbZrO3. This was attributed to enhanced polarization fluctuations caused by increased ferroelectric distortions due to Sn substitution and the proximity to the tricritical point on the phase diagram of PbZr1-xSnxO3.

Acoustic behaviour and equation of state of amorphous ethylene–vinyl acetate copolymer studied by means of high-pressure Brillouin scattering spectroscopy

The determination of the equation of state (EOS) of amorphous materials is very important for fundamental understanding of the glass transition and applications as well. Simultaneous observation of both longitudinal and transverse acoustic modes by Brillouin scattering spectroscopy has been one of the major methods to obtain EOS of amorphous materials. However, the transverse acoustic mode is hardly seen from some of the amorphous polymers, which makes it difficult to derive EOS. The temperature and pressure dependences of the acoustic properties of amorphous ethylene–vinyl acetate (EVA) copolymer were measured by using high-pressure Brillouin scattering spectroscopy. The temperature variation induced large changes in the frequency shift and linewidth of the longitudinal acoustic mode due to strong coupling between the structural relaxation process and the propagating density fluctuations. The residual linewidth in the glassy state was attributed to the remnant intramolecular motions of EVA, the activation energy of which was estimated to be ∼3.30 ± 0.27 kcal/mol. The pressure–density relationship of EVA could be obtained for the first time by measuring the refractive index and using the Lorentz–Lorenz equation. The density and the refractive index exhibited monotonic increase up to approximately 12 GPa. The strong reduction of the acoustic damping at low pressures below ∼3 GPa was attributed to the collapsing free volume in EVA. The present study clearly shows that measuring the refractive index by high-pressure Brillouin spectroscopy may be an alternative method to get the EOS of polymeric materials whose transverse acoustic mode is too weak to be observed.