Helicity resolved Raman scattering of MoS2, MoSe2, WS2 and WSe2 atomic layers

TL;DR

This study investigates how helicity-resolved Raman scattering in TMDC monolayers reveals the relationship between photon helicity and valley polarization, offering new insights beyond luminescence-based methods.

Contribution

It demonstrates that Raman scattering can preserve or switch photon helicity, providing a novel characterization tool for TMDC monolayers and advancing understanding of valley polarization.

Findings

Raman modes can maintain or switch photon helicity.

Raman scattering offers a new way to characterize valley polarization.

Contrast with luminescence indicates different helicity conservation behaviors.

Abstract

The two-fold valley degeneracy in two-dimensional (2D) semiconducting transition metal dichalcogenides (TMDCs) (Mo,W)(S,Se)2 is suitable for "valleytronics", the storage and manupulation of information utilizing the valley degree of freedom. The conservation of luminescent photon helicity in these 2D crystal monolayers has been widely regarded as a benchmark indicator for charge carrier valley polarization. Here we perform helicity-resolved Raman scattering of the TMDC atomic layers. In drastic contrast to luminescence, the dominant first-order zone-center Raman bands, including the low energy breathing and shear modes as well as the higher energy optical phonons, are found to either maintain or completely switch the helicity of incident photons. These experimental observations, in addition to providing a useful tool for characterization of TMDC atomic layers, shed new light on the connection between photon helicity and valley polarization.