Nano-imaging of the edge-dependent optical polarization anisotropy of black phosphorus

TL;DR

This study visualizes the edge-dependent optical polarization anisotropy in black phosphorus using nano-imaging, revealing how edges modify electronic states and optical properties, which could enable targeted edge state excitation in 2D materials.

Contribution

It demonstrates polarization-dependent photoemission microscopy of black phosphorus at nanoscale resolution, uncovering edge-specific optical anisotropy modifications due to symmetry reduction.

Findings

Edges of BP show shifted polarization anisotropy

Edge electronic states have altered transition dipole moments

Edge modifications influence optical absorption properties

Abstract

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.