Visible Out-of-plane Polarized Luminescence and Electronic Resonance from Black Phosphorus

TL;DR

This study uncovers out-of-plane polarized luminescence and electronic resonance in black phosphorus, revealing significant anisotropic optical properties through polarization-resolved spectroscopy and theoretical calculations.

Contribution

It provides the first detailed experimental and theoretical analysis of out-of-plane optical responses in bulk black phosphorus, highlighting its anisotropic electronic and vibrational behavior.

Findings

Observation of b-polarized emission at 1.75 eV

Identification of electronic resonance at 2.3 eV

Enhanced Raman scattering of Ag phonons due to anisotropy

Abstract

Black Phosphorus (BP) is unique among layered materials owing to its homonuclear lattice and strong structural anisotropy. While recent investigations on few layers BP have extensively explored the in-plane (a,c) anisotropy, much less attention has been given to the out-of-plane direction (b). Here, the optical response from bulk BP is probed using polarization-resolved photoluminescence (PL), photoluminescence excitation (PLE) and resonant Raman scattering along the zigzag, out-of-plane, and armchair directions. PL reveals an unexpected b-polarized emission occurring in the visible at 1.75 eV, far above the fundamental gap (0.3 eV). PLE indicates that this emission is generated through b-polarized excitation at 2.3 eV. The same electronic resonance is observed in resonant Raman scans, where the scattering efficiency of both Ag phonon modes is enhanced. These experimental results are fully consistent with DFT calculations of the permittivity tensor elements and demonstrate the remarkable extent to which the anisotropy influences the optical properties and carrier dynamics in black phosphorus.