  Monolayer transition metal dichalcogenide semiconductors, with versatile
experimentally accessible exciton species, offer an interesting platform for
investigating the interaction between excitons and a Fermi sea of charges.
Using hexagonal boron nitride encapsulated monolayer MoSe2, we study the impact
of charge density tuning on the ground and excited Rydberg states in the atomic
layer. Consistent excitonpolaron behavior is revealed in both photoluminescence
and reflection spectra of the A exciton 1s (A:1s) Rydberg state, in contrast to
previous studies. The A:2s Rydberg state provides an opportunity to understand
such interactions with greatly reduced exciton binding energy. We found that
the impact of the Fermi sea becomes much more dramatic. With a
photoluminescence upconversion technique, we further verify the 2s polaron-like
behavior for the repulsive branch of B:2s exciton whose energy is well above
the bare bandgap. Our studies show that the polaron-like interaction features
are quite generic and highly robust, offering key insights into the dressed
manybody state in a Fermi sea.
