  The electronic structure and functionality of 2D materials is highly
sensitive to structural morphology, opening the possibility for manipulating
material properties, but also making predictable and reproducible functionality
challenging. Black phosphorus (BP), a corrugated orthorhombic 2D material, has
in-plane optical absorption anisotropy critical for applications such as
directional photonics, plasmonics, and waveguides. Here, we use
polarization-dependent photoemission electron microscopy to visualize the
anisotropic optical absorption of BP with 54 nm spatial resolution. We find the
edges of BP flakes have a shift in their optical polarization anisotropy from
the flake interior due to the 1D confinement and symmetry reduction at flake
edges that alter the electronic charge distributions and transition dipole
moments of edge electronic states, confirmed with first-principles
calculations. These results uncover previously hidden modification of the
polarization-dependent absorbance at the edges of BP, highlighting the
opportunity for selective excitation of edge states of 2D materials with
polarized light.
