Rydberg Excitons and Doubly Resonant Raman Scattering in Transition-Metal Dichalcogenides

Monolayer transition-metal dichalcogenides (TMDs) are emergent semiconductor materials with a wide range of potential applications. Rydberg excitons are similar to the Rydberg atomic states having a large principal quantum number n. The huge binding energies found in TMD semiconductors, up to 900 meV, facilitate studies of the Rydberg excitons, opening a new perspective of research by employing their long lifetimes, strong dipolar interactions, and the potential for coherent effects and quantum chaos. In the framework of a microscopic theory, we provide a complete description of the first-order resonant Raman scattering (RRS) valid for two-dimensional (2D) TMD semiconductors. Assuming as electronic intermediate states the high-n exciton states, we present explicit expressions for the RRS intensity, which are valid for incident laser energies close to the excitonic resonances. The intravalley Pekar–Fröhlich polar longitudinal optical mode and A1-homopolar mode deformation potential coupling mechanisms are considered. We report a large enhancement of the Raman efficiency due to the simultaneous incoming and outgoing resonances with bound exciton states, occurring for an appropriate choice of the excitation photon energy. We show that the 2D semiconductors guarantee the necessary conditions for the doubly RRS (DRRS) when intravalley transitions occur between two different Rydberg states separated by an optical phonon energy. The observation of the intravalley DRRS process can open new perspectives for deeper studies of the role of Rydberg excitons in solid-state physics.