Visualizing Optical Phase Anisotropy in Black Phosphorus

Layered black phosphorus has triggered enormous interest since its recent emergence. Compared to most other two-dimensional materials, black phosphorus features a moderate band gap and pronounced in-plane anisotropy, which stems from the unique atomic-puckering crystal structure. The future potential of black phosphorus in optoelectronics demands a deeper understanding of its unique anisotropic behavior. In particular, the phase information on light when interacting with the material is imperative for many applications in the optical regime. In this work we have comprehensively studied a wide range of optical anisotropic properties of black phosphorus, including the Raman scattering, extinction spectra, and phase retardance by utilizing conventional spectral measurements and a uniquely developed interferometric spectroscopy and imaging technique. The phase retardance of light passed through black phosphorus is exploited in conjunction with polarization interferometric techniques to demonstrate an optical contrast an order of magnitude higher than a purely polarization-based measurement could offer.