Understanding angle-resolved polarized Raman scattering from black phosphorus at normal and oblique laser incidences

TL;DR

This paper develops a BLD model to accurately predict angle-resolved polarized Raman scattering in anisotropic layered materials like black phosphorus, accounting for birefringence and dichroism effects without fitting parameters.

Contribution

The authors introduce a parameter-free BLD model that incorporates birefringence and linear dichroism to understand ARPR intensity at various angles and incidences in anisotropic layered materials.

Findings

The BLD model accurately predicts ARPR intensities in black phosphorus.

Experimental methods to determine Raman tensors and refractive indexes are proposed.

The model explains ARPR behavior in ultrathin flakes considering multilayer interference effects.

Abstract

The selection rule for angle-resolved polarized Raman (ARPR) intensity of phonons from standard group-theoretical method in isotropic materials would break down in anisotropic layered materials (ALMs) due to birefringence and linear dichroism effects. The two effects result in depth-dependent polarization and intensity of incident laser and scattered signal inside ALMs and thus make a challenge to predict ARPR intensity at any laser incidence direction. Herein, taking in-plane anisotropic black phosphorus as a prototype, we developed a so-called birefringence-linear-dichroism (BLD) model to quantitatively understand its ARPR intensity at both normal and oblique laser incidences by the same set of real Raman tensors for certain laser excitation. No fitting parameter is needed, once the birefringence and linear dichroism effects are considered with the complex refractive indexes. An approach was proposed to experimentally determine real Raman tensor and complex refractive indexes, respectively, from the relative Raman intensity along its principle axes and incident-angle resolved reflectivity by Fresnel$'$s law. The results suggest that the previously reported ARPR intensity of ultrathin ALM flakes deposited on a multilayered substrate at normal laser incidence can be also understood based on the BLD model by considering the depth-dependent polarization and intensity of incident laser and scattered Raman signal induced by both birefringence and linear dichroism effects within ALM flakes and the interference effects in the multilayered structures, which are dependent on the excitation wavelength, thickness of ALM flakes and dielectric layers of the substrate. This work can be generally applicable to any opaque anisotropic crystals, offering a promising route to predict and manipulate the polarized behaviors of related phonons.