  Black Phosphorus (BP) is unique among layered materials owing to its
homonuclear lattice and strong structural anisotropy. While recent
investigations on few layers BP have extensively explored the in-plane (a,c)
anisotropy, much less attention has been given to the out-of-plane direction
(b). Here, the optical response from bulk BP is probed using
polarization-resolved photoluminescence (PL), photoluminescence excitation
(PLE) and resonant Raman scattering along the zigzag, out-of-plane, and
armchair directions. PL reveals an unexpected b-polarized emission occurring in
the visible at 1.75 eV, far above the fundamental gap (0.3 eV). PLE indicates
that this emission is generated through b-polarized excitation at 2.3 eV. The
same electronic resonance is observed in resonant Raman scans, where the
scattering efficiency of both Ag phonon modes is enhanced. These experimental
results are fully consistent with DFT calculations of the permittivity tensor
elements and demonstrate the remarkable extent to which the anisotropy
influences the optical properties and carrier dynamics in black phosphorus.
