  The competition between the electron-hole Coulomb attraction and the
three-dimensional dielectric screening dictates the optical properties of
layered semiconductors. In low-dimensional materials, the equilibrium
dielectric environment can be significantly altered by the ultrafast excitation
of photo-carriers, leading to renormalized band gap and exciton binding
energies. Recently, black phosphorus emerged as a 2D material with strongly
layer-dependent electronic properties. Here, we resolve the coherent response
of screening to sub-gap photo-excitation in bulk black phosphorus and find that
mid-infrared pulses tuned across the band gap drive a transient non-thermal
suppression of the dielectric screening, which is revealed by the emergence of
the single-layer exciton resonance. Our work exposes the role of interlayer
interactions in determining the electronic properties of 2D materials and
discloses the possibility of optically manipulate them, which is of great
relevance for the engineering of versatile van der Waals low-dimensional
materials.
