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.