Nanoscale polarization anisotropy in black phosphorus

Black phosphorus (BP) supports anisotropic charge, phonon, plasmonic, and polaritonic behavior, making it well-suited for the development of polarization-dependent photodetectors and thermoelectrics, as well as directional waveguides and light emitters. However, BP is also rich in nanoscale morphologies such as edges, strain, and wrinkles, which strongly modulate its electronic properties. Determining the interplay of morphology-mediated and intrinsic electronic behavior requires 10-100 nm-scale resolution, beyond the reach of conventional optical microscopy (> 200 nm), and a large field of view, beyond that of TEM and STM (< 20 nm). We use polarization-dependent photoemission electron microscopy (PEEM) to probe the morphology-electronic structure relationship of few-layer BP with 54 nm resolution. At the edges of BP we find the polarization dichroism is phase-shifted by 10 - 20^○ with respect to the armchair direction of the bulk lattice in flakes. This suggests an edge modification to optical transitions and the symmetries of the conduction and valence bands, which may manifest from edge reconstructions and interlayer effects, enabling selective excitation and manipulation of black phosphorus edge states.