Gate-tunable exciton-polaron Rydberg series with roton effect

Electronic exciton polaron is a hypothetical many-body quasiparticle formed by an exciton dressed with a polarized electron-hole cloud in the Fermi sea (FS). It is predicted to display rich many-body physics and unusual roton-like dispersion. The roton-like dispersion has the energy minima at finite momenta for a doped 2D semiconductor because of the Fermi-sea blocking effect. Here, by using reflectance contrast and photoluminescence (PL) spectroscopy, we observe optical signatures of the exciton-polaron Rydberg series and characterize their gate-dependent optical properties in monolayer MoSe₂ and WSe₂. We also established a comprehensive exciton-polaron theory to quantitatively explain our results. Remarkably, the observed exciton polarons exhibit a significant energy gap between the absorption and emission bands, with the gap size increasing from ground to excited state. Such an absorption-emission gap can be attributed to the roton-like exciton-polaron dispersion. Our calculations for the absorption and emission with the roton-effect match very well with the experiments. Such exciton polarons with strong and roton effect shall much enrich the fundamental physics in 2D excitonic systems.