Highly polarized optical responses in bulk black phosphorus from high energy states

Black phosphorus (BP) is unique among other layered materials owing to its homonuclear lattice and strong structural anisotropy. While recent investigations on few layers BP have extensively explored the in-plane anisotropy, much less attention has been given to the out-of-plane direction of the bulk crystal. Here, polarization-resolved spectroscopy measurements are performed on bulk BP along the zigzag, out-of-plane and armchair directions under visible light excitation to determine the complete optical response of the crystal. Under out-of-plane polarized excitation, an unexpected PL emission is detected near 1.75 eV, which is far above the ∼0.3 eV band gap energy. Photoluminescence excitation and Raman energy maps indicate the presence of an unreported resonance at ∼2.3 eV in the out-of-plane direction, which gives rise to the above-bandgap luminescence and an enhancement of the A_g phonon modes intensity. This intriguing resonance is analyzed using the complex dielectric permittivity of bulk BP obtained by DFT calculations. The calculated strongly polarized dipolar transitions in bulk crystals are fully consistent with the optical anisotropy observed here.