Anomalous polarization dependence of Raman scattering and crystallographic orientation of black phosphorus

TL;DR

This study explores the polarization dependence of Raman scattering in black phosphorus, revealing unique behaviors of specific modes and proposing a reliable method to determine crystal orientation combining Raman and optical microscopy.

Contribution

It uncovers anomalous polarization behaviors in black phosphorus Raman modes and introduces a combined optical and Raman technique for crystal orientation determination.

Findings

B2g mode shows consistent polarization dependence across wavelengths and thicknesses.

Ag1 and Ag2 modes exhibit wavelength- and thickness-dependent polarization behaviors.

Thickness dependence explained by anisotropic interference effects, wavelength dependence remains unexplained.

Abstract

We investigated polarization dependence of the Raman modes in black phosphorus (BP) using five different excitation wavelengths. The crystallographic orientation was determined by comparing polarized optical microscopy with high-resolution transmission electron microscope analysis. In polarized Raman spectroscopy, the B2g mode shows the same polarization dependence regardless of the excitation wavelength or the sample thickness. On the other hand, the Ag1 and Ag2 modes show a peculiar polarization behavior that depends on the excitation wavelength and the sample thickness. The thickness dependence can be explained by considering the anisotropic interference effect due to birefringence and dichroism of the BP crystal, but the wavelength dependence cannot be explained. We propose a simple and fail-proof procedure to determine the orientation of a BP crystal by combining polarized Raman scattering with polarized optical microscopy.