Accessing the Anisotropic Nonthermal Phonon Populations in Black Phosphorus

We combine femtosecond electron diffuse scattering experiments and first-principles calculations of the coupled electron-phonon dynamics to provide a detailed momentum-resolved picture ultrafast lattice thermalization in thin film black phosphorus. The measurements reveal emergence highly anisotropic non-thermal phonon populations which persist for several picoseconds following excitation electrons with light pulse. Combining simulations based on time-dependent Boltzmann formalism structure factor, we reproduce experimental data identify vibrational modes primarily responsible carrier relaxation via coupling subsequent phonon-phonon scattering. In particular, attribute non-equilibrium phosphorus highly-anisotropic phonon-assisted processes, are mediated by high-energy optical phonons. Our approach paves way towards unravelling controlling microscopic energy-flow pathways two-dimensional materials van der Waals heterostructures, may also be extended other phenomena involving such as superconductivity, phase transitions or polaron physics.