Abstract:

The first-order resonant Raman spectra of monolayer MoS₂ are calculated under the circularly polarized photoexcitation. The anomalously nonzero Raman intensity of the in-plane E mode under the $\bar{Z}\left( {{\sigma }_{+}}{{\sigma }_{+}} \right)Z$ or $\bar{Z}\left( {{\sigma }_{-}}{{\sigma }_{-}} \right)Z$ geometry, which goes against the conventional selection rule, appears under some circumstances when optical absorption occurs at some special reciprocal points between the zone-center Γ and the zone-edge-center M points. At that moment, the valley selectivity to the circular polarization is lifted. The analysis shows that the anomalous Raman intensity of the E mode for the same circularly polarized incident and scattered light is consistent with the pseudo-angular-momentum conservation law. The calculated E Raman tensor of monolayer MoS₂ is found to vary with laser energy. The two diagonal terms of the Raman tensor change their signs from mutually opposite to the same when the relative intensity of the in-plane E mode to the out-of-plane $A_{1}^{\prime }$ mode increases, indicating the increasingly important role played by the Frölich-type electron-phonon interaction over the deformation potential. Our study may shed new light on the understanding of the novel electron-photon process and assist in the design of new type of optoelectronic devices.

Key words: First principles calculation, Resonant Raman, Helicity selection rule, MoS₂ monolayer