Anisotropic Coherent Acoustic Phonon Dynamics of Black Phosphorus Studied by Transient Absorption Microscopy

Due to its corrugated hexagonal lattice structure, Black phosphorus (BP) has unique anisotropic physical properties, which provides an additional freedom for designing devices. Many body interactions, including interactions with phonon, is crucial for heat dissipation and charge carrier mobility in device. However, the rich properties of the coherent acoustic phonon, including anisotropy, propagation and generation, were not fully interrogated. In this paper, the polarization-resolved transient absorption microscopy was conducted on BP flakes to study the anisotropic coherent acoustic phonon dynamics. It is found that the damping of the coherent acoustic phonon oscillation was anisotropic, which was revealed to be originated from the anisotropic absorption by pump power dependent experiment. The initial oscillation amplitude of coherent acoustic phonon was found to be anisotropic, from which we believed that the direct photoelastic mechanism dominated the generation of coherent acoustic phonons in our experiment. Besides, we calculated the sound velocity of the coherent acoustic phonon from the oscillation frequency and the acoustic echo, respectively, which well agreed with each other. These findings provide significant insights into the rich acoustic phonon properties of BP, and promise important application for BP in polarization-sensitive optical and optoelectronic devices.