  Black phosphorus (BP) exhibits highly anisotropic properties and dynamical
behavior that are unique even among two-dimensional and van der Waals (vdW)
layered materials. Here, we show that an interlayer lattice contraction and
concerted, symmetric intralayer vibrations take place concurrently within few
picoseconds following the photoinjection and relaxation of carriers, using
ultrafast electron diffraction in the reflection geometry to probe the
out-of-plane motions. A strong coupling between the photocarriers and BP's
puckered structure, with the alignment of the electronic band structure, is at
work for such directional atomic motions without a photoinduced phase
transition. Three temporal regimes can be identified for the phonon
thermalization dynamics where a quasi-equilibrium without anisotropy is reached
in about 50 ps, followed by propagation of coherent acoustic phonons and heat
diffusion into the bulk. The early-time out-of-plane dynamics reported here
have important implications for single- and few-layer BP and other vdW
materials with strong electronic-lattice correlations.
