title: Helicity-resolved Raman scattering of MoS₂, MoSe₂, WS₂, and WSe₂ atomic layers.
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 manipulation 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. In addition to providing a useful tool for characterization of TMDC atomic layers, these experimental observations shed new light on the connection between photon helicity and valley polarization.
authors.affiliation: †Department of Physics, University of Massachusetts, Amherst, Massachusetts 01003, United States.
authors.forename: Shao-Yu
authors.initials: SY
authors.lastname: Chen
authors.forename: Changxi
authors.initials: C
authors.lastname: Zheng
authors.forename: Michael S
authors.initials: MS
authors.lastname: Fuhrer
authors.affiliation: †Department of Physics, University of Massachusetts, Amherst, Massachusetts 01003, United States.
authors.forename: Jun
authors.initials: J
authors.lastname: Yan
keywords: Raman scattering; Valleytronics; breathing mode; pseudospin; shear mode; transition metal dichalcogenide