Helicity-resolved resonant Raman spectroscopy of layered WS2

When excited by circularly polarized light, the Raman scattered light may have the same or opposite helicity as the incident light determined by the helicity selection rule. For two-dimensional (2D) transition metal dichalcogonides (TMDCs), the helicity selection rule can be broken down due to the strong Frohlich exciton-phonon interaction. However, how the helicity selection rule changes with excitation energy has not been reported yet. Here, we study the helicity-resolved Raman scattering of layered WS(2)excited by circularly polarized light for excitations off-excitonic resonance and near resonance to different excitons. We find that for off-resonance excitation, the helicity ofE2g1mode of WS(2)obeys the helicity selection rule determined by the symmetry of crystal structure and vibration modes. When excited near resonance to the B exciton, the breakdown of the helicity selection rule is observed, which is attributed to the appearance of nonzero diagonal elements of the Raman tensor resulted from the Frohlich exciton-phonon interaction. The layer number dependence of the helicity polarization ratio of theE2g1mode shows that the proportion of helicity-conserved component decreases with the increase of layer number. When the excitation energy is near resonance to the A exciton, the different helicity polarization ratio is found and remains unchanged with increasing layer number, which may be attributed to the different coupling strengths of exciton-phonon interactions.

Automated summary

The research found that the helicity of the E2g1 mode of WS(2) follows or breaks the helicity selection rule under different excitation conditions, which can be attributed to the influences of crystal structure, vibration modes, and exciton-phonon interactions.