  We report about the energy and momentum resolved optical response of black
phosphorus (BP) in its bulk form. Along the armchair direction of the puckered
layers we find a highly dispersive mode that is trongly suppressed in the
perpendicular (zig-zag) direction. This mode emerges out of the single-particle
continuum for finite values of momentum and is therefore interpreted as an
exciton. We argue that this exciton, which has already been predicted
theoretically for phosphorene -- the monolayer form of BP -- can be detected by
conventional optical spectroscopy in the two-dimensional case and might pave
the way for optoelectronic applications of this emerging material.

Mesoscale and Nanoscale Physics;Materials Science