Anisotropic Photoinduced Lattice Dynamics of Black Phosphorus Revealed by Reflection Ultrafast Electron Diffraction

Black phosphorus (BP) is emerging as a fascinating two-dimensional (2D) material given its anisotropic structure and optical, electrical, and mechanical properties, which have been readily observed through steady-state measurements of, e.g., absorption, carrier and thermal transports. Such anisotropic behavior is also prominent on ultrafast time scales following photoexcitation, which is distinct among 2D materials. Detailed knowledge of photoinduced responses in BP is important to the understanding of the material’s relaxation pathways in various degrees of freedom. However, to date, most reports of ultrafast dynamics have been focused on the in-plane components, whereas the out-of-plane counterpart remains unclear. In this presentation, we shed light on the photoinduced BP dynamics in the out-of-plane direction using ultrafast electron diffraction in reflection geometry as a direct structure-probing technique, whose results are cross-examined with those by optical transient reflectivity. It is found that following an optical excitation by 515-nm photons, the large excess energy of photoinjected carriers lead to an interlayer lattice compression at ultrashort times, which appears to be somewhat independent of the laser fluence used. We further show that the electron-phonon coupling takes place within 1 ps, followed by a slower relaxation process on the order of few tens of picoseconds to reach a thermalized phonon bath. A schematic of the different temporal regimes and the corresponding physical processes, as well as the production of coherent acoustic phonons moving into the bulk, will also be discussed.