  We calculate the excitonic spectrum of few-layer black phosphorus by direct
diagonalization of the effective mass Hamiltonian in the presence of an applied
in-plane electric field. The strong attractive interaction between electrons
and holes in this system allows one to investigate the Stark effect up to very
high ionizing fields, including also the excited states. Our results show that
the band anisotropy in black phosphorus becomes evident in the direction
dependent field induced polarizability of the exciton.
