Raman-like resonant secondary emission causes valley coherence in CVD-grown monolayer MoS2

Monolayer transition metal dichalcogenides are promising materials for valleytronics. They have band gaps at energy-degenerate K and K' valleys with opposite spins. Due to the lack of inversion symmetry, electron-hole pairs can be selectively created at K or K' valleys by circularly polarized photons. In addition, linearly polarized light excitation creates the coherent superposition of exciton valley states, referred to as the generation of valley coherence. In this study we performed polarization resolved photoluminescence and resonant Raman spectroscopy of CVD-grown monolayer MoS2. We found that the lowest exciton photoluminescence becomes polarized, indicating the effective generation of valley polarization and valley coherence due to the resonant effect, accompanied by a drastic change of the polarization selection rule of Raman scattering. These results were theoretically explained from the viewpoint of the selection rules of resonant Raman scattering. We conclude that the Raman-like resonant second-order optical process should be the main mechanism of valley coherence.