On the importance of the optical anisotropy in the detection of coherent acoustic phonons in a time-domain Brillouin scattering experiment

In this perspectives paper, we discuss the role of optical anisotropy in detecting coherent acoustic phonons in a time-domain Brillouin experiment. First, we present the general theoretical background to the optical detection of acoustic phonons in a uniaxial material. We then illustrate this theory experimentally by studying highly birefringent BiFeO3 material as a case study. We demonstrate how the amplitude of the time-domain Brillouin signal of both longitudinal and shear phonons can be controlled by selecting the polarization of the probe beam (ordinary or extraordinary). Using classical electrodynamics combined with first-principles calculation of photoelastic coefficients, we find a good qualitative agreement with experiments. Besides the general demonstration of the importance of optical anisotropy, we demonstrate that BiFeO3 exhibits comparable photoelastic coefficients to the technologically significant LiNbO3 and LiTaO3 materials. These findings could lead to the development of BiFeO3-based sub-THz acousto-optic devices. In addition to ferroelectrics, our comprehensive description of the coherent acoustic phonon detection process could, in the future, be extended to other birefringent functional oxides such as TiO2, ZnO, and distorted perovskite systems (ABO3), as well as polar semiconductors such as GaN and two-dimensional and van der Waals materials.