Development of a High-pressure Brillouin Spectrometer and Its Application to an Ethylene-vinyl Acetate Copolymer

A high-pressure Brillouin spectrometer with which the elastic properties of condensed matter could be measured over a wide pressure range was developed. A wide-angle, symmetric-type diamond anvil cell and an optical microscope were combined with a tandem Fabry-Perot interferometer and an optical stage to carry out both symmetric forward scattering and backscattering experiments under the same conditions. The constructed high-pressure Brillouin spectrometer was applied to ethylene-vinyl acetate (EVA) over a pressure range from ambient pressure to similar to 13 GPa. The observed Brillouin spectra consisted of one Brillouin doublet caused by longitudinal acoustic waves. The Brillouin shift and the corresponding longitudinal sound velocity increased with increasing pressure. The combination of the two scattering geometries enabled us to obtain the refractive index of EVA at ambient pressure. This experimental technique will be useful in obtaining the equation of states of various types of condensed matter.