Anomalous phonon vibrations of monolayer MoS₂ revealed by polarization degree of freedom of light

Light of linear and circular polarization is broadly used to explore interesting light-matter interaction in layered two-dimensional transition metal dichalcogenides (TMDs) such through Raman scattering and photoluminescence. Little attention has been given to the interaction of TMD materials and light with the elliptical polarization state. We applied elliptically polarized light in micro-Raman spectroscopy to observe variations in the relative intensity of the two most dominant Raman peaks of MoS₂. This provides an additional degree of freedom affecting the relationship between the two orthogonal components of the material’s in-plane lattice vibrations. Different magnitudes of the in-plane lattice vibrations along two orthogonal directions were induced by varying the light from circularly polarized to elliptically polarized states. Besides, with changing the incident linear polarization angle, the out-of-plane A_1g phonon mode red shifts, while the in-plane E_2g phonon mode blue shifts. The anomalous lattice vibrations of monolayer MoS2 originate from the built-in strain introduced by the SiO₂/Si substrate. This work investigates the material's unexplored fundamental phonon property which may enlighten past and future studies.