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.
@article{arxiv.1503.02549,
title = {Anisotropic Particle-Hole Excitations in Black Phosphorus},
author = {R. Schuster and J. Trinckauf and M. Knupfer and B. Büchner},
journal= {arXiv preprint arXiv:1503.02549},
year = {2015}
}