Anisotropic Charge Carrier and 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 provide an additional freedom for designing devices. Many-body interactions, including interactions with phonons, are crucial for heat dissipation and charge carrier mobility in devices. However, the rich properties of the coherent acoustic phonon, including anisotropy, propagation, and generation, were not fully interrogated. In this paper, polarization-resolved transient absorption microscopy was conducted on BP flakes to study the dynamics of photoexcited charge carriers and coherent acoustic phonons. Polarization-resolved transient absorption images and traces were recorded and showed anisotropic and thickness-dependent charge carrier decay dynamics. The damping of the coherent acoustic phonon oscillation was found to be anisotropic, which was attributed to the polarization-dependent absorption length of the probe pulse. From the analysis of initial oscillation amplitude and phase of coherent acoustic phonon oscillation, we proposed that the direct deformation potential mechanism dominated the generation of coherent acoustic phonons in our experiment. Besides, we obtained the sound velocity of the coherent acoustic phonon from the oscillation frequency and the "acoustic echo", respectively, which agreed well with each other. These findings provide significant insight into the rich acoustic phonon properties of BP and promise important applications for BP in polarization-sensitive optical and optoelectronic devices.